Use bindey for databinding instead of hand rolling ui updates

2023-09-28 08:29:55 +10:00
parent f168bfab76
commit f09ba5ffd5
36 changed files with 1557 additions and 307 deletions
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@@ -0,0 +1,44 @@
+name: ci
+
+on: [pull_request]
+
+env:
+  # Customize the CMake build type here (Release, Debug, RelWithDebInfo, etc.)
+  BUILD_TYPE: Release
+
+jobs:
+  Build-And-Test:
+    runs-on: macos-latest
+
+    steps:
+      - uses: actions/checkout@v2
+        with:
+          submodules: true
+
+      - name: Create Build Environment
+        # Some projects don't allow in-source building, so create a separate build directory
+        # We'll use this as our working directory for all subsequent commands
+        run: cmake -E make_directory ${{runner.workspace}}/build
+
+      - name: Configure
+        shell: bash
+        working-directory: ${{runner.workspace}}/build
+        run: cmake $GITHUB_WORKSPACE -GXcode -DCMAKE_BUILD_TYPE=$BUILD_TYPE -DBINDEY_BUILD_TESTS=ON
+        env:
+          CC: clang
+          CXX: clang
+
+      - name: Build
+        working-directory: ${{runner.workspace}}/build
+        shell: bash
+        run: cmake --build . --config $BUILD_TYPE
+        env:
+          CC: clang
+          CXX: clang
+
+      - name: Test
+        working-directory: ${{runner.workspace}}/build
+        shell: bash
+        # Execute tests defined by the CMake configuration.  
+        # See https://cmake.org/cmake/help/latest/manual/ctest.1.html for more detail
+        run: ctest -C $BUILD_TYPE
@@ -0,0 +1,5 @@
+*.a
+*.lib
+*.o
+*.pdb
+.DS_Store
@@ -0,0 +1,6 @@
+[submodule "lib/Catch2"]
+	path = lib/Catch2
+	url = git@github.com:catchorg/Catch2.git
+[submodule "lib/nod"]
+	path = lib/nod
+	url = git@github.com:fr00b0/nod.git
@@ -0,0 +1,6 @@
+# Copyright 2023 jacqueline <me@jacqueline.id.au>
+#
+# SPDX-License-Identifier: GPL-3.0-only
+idf_component_register(
+  INCLUDE_DIRS "include"
+)
@@ -0,0 +1,21 @@
+## The MIT License (MIT)
+
+Copyright (c) 2021 Kevin Dixon
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+SOFTWARE.
@@ -0,0 +1,116 @@
+# bindey
+
+Everyone knows Model-View-ViewModel is the best architecture, but how can we realize it in C++ applications with minimal overhead, and no complicated framework impositions?
+
+`bindey` provides the basic building block of MVVM -- an observable "Property" and a databinding mechanism.
+
+## Property Usage
+
+At minimum, `bindey::property` can allow you to avoid writing getters and setters. Consider this example:
+
+```
+#include <bindey/property.h>
+
+using namespace bindey;
+
+class Person
+{
+public:
+    property<std::string> name;
+    property<int> age;
+};
+```
+Then we can use it like this:
+```
+Person p;
+p.name("Kevin");
+p.age(666);
+
+auto thatDudesName = p.name();
+auto ageIsJustANumber = p.age();
+```
+
+`property` default initializes its value with `{}`, and of course allows initialization.
+```
+Person::Person()
+: name("Default Name")
+, age(0)
+{}
+```
+## Data Binding
+`bindey` provides a simple binding mechanism to connect a "source" `property` to an arbitrary object. This base signature is
+```
+template <typename T, typename To>
+binding bind( property<T>& from, To& to );
+```
+And a specialization for `property` to `property` binding of the same type is provided.
+```
+template<typename T>
+binding bind( property<T>& from, property<T>& to )
+{
+    return from.onChanged( [&]( const auto& newValue ) { to( newValue ); } );
+}
+```
+
+### Writing Your Own Bindings
+Where this becomes fun is when you get to reduce boilerplate. For example, assume a `Button` class from some UI Framework.
+```
+struct Button
+{
+    void setText(const std::string& text)
+    {
+        this->text = text;
+    }
+
+    std::string text;
+};
+```
+To make your life better, simply implement a template speciailization in the `bindey` namespace.
+```
+namespace bindey
+{
+template <>
+binding bind( property<std::string>& from, Button& to )
+{
+    return from.onChanged( [&]( const auto& newValue ){ to.setText( newValue ); } );
+}
+} // namespace bindey
+```
+Then, bind your property to the button as needed:
+```
+bindey::property<std::string> name;
+...
+Button someButton;
+...
+bindey::bind( name, someButton );
+```
+
+### Binding Lifetimes
+The result of a call to `bind` is a `bindey::binding` object. If this return value is discarded, then the binding's lifetime is coupled to the `property`'s.
+
+Otherwise, this token can be used to disconnect the binding as needed, the easiest way is to capture it in a `scoped_binding` object.
+
+For example, if your binding involves objects who's lifetime you do not control, you should certainly capture the binding to avoid crashes.
+```
+struct GreatObj
+{
+    GreatObj(Button* b)
+    {
+        mSomeButton = b;
+        mButtonBinding = bindey::bind( name, *mSomeButton );
+    }
+
+    void updateButton(Button* newB)
+    {
+        mSomeButton = nullptr;
+        mButtonBinding = {}; // disconnect from old button
+        if( newB != nullptr )
+        {
+            mSomeButton = newB;
+            mButtonBinding = bindey::bind( name, *mSomeButton );
+        }
+    }
+
+    bindey::scoped_binding mButtonBinding;
+};
+```
@@ -0,0 +1,47 @@
+#pragma once
+
+#include "property.h"
+
+#include <nod/nod.hpp>
+
+#include <functional>
+#include <type_traits>
+
+namespace bindey
+{
+
+using binding        = nod::connection;
+using scoped_binding = nod::scoped_connection;
+
+/**
+ * base binding signature
+ */
+template <typename T, typename To>
+binding bind( property<T>& from, To& to );
+
+/**
+ * binds two properties of the same type
+ */
+template <typename T>
+binding bind( property<T>& from, property<T>& to )
+{
+    return from.onChanged( [&]( const auto& newValue ) { to( newValue ); } );
+}
+
+/**
+ * binds two properties of differing types using a Converter callable
+ * @param   from  property to observe
+ * @param   to  property to write to
+ * @param   bindingConverter    a callable to invoke to convert between the types
+ */
+template <typename TFrom, typename TTo, typename Converter>
+binding bind( property<TFrom>& from, property<TTo>& to, Converter&& bindingConverter )
+{
+    static_assert( std::is_convertible<Converter&&, std::function<TTo( const TFrom& )>>::value,
+                   "Wrong Signature for binding converter!" );
+
+    return from.onChanged(
+        [&to, converter = bindingConverter]( const auto& newValue ) { to( converter( newValue ) ); } );
+}
+
+} // namespace bindey
@@ -0,0 +1,137 @@
+#pragma once
+
+#include <nod/nod.hpp>
+
+#include <functional>
+
+namespace bindey
+{
+
+/**
+ * Optional always_update policy to notify subscribers everytime the property value is set, not just when it changes
+ */
+class always_update
+{
+public:
+    template <typename T>
+    bool operator()( const T&, const T& ) const
+    {
+        return true;
+    }
+};
+
+template <typename T,
+          typename UpdatePolicy = std::not_equal_to<T>,
+          typename Signal       = nod::unsafe_signal<void( const T& )>>
+class property
+{
+public:
+    property()
+    {
+    }
+    property( T&& value )
+    : mStorage( std::move( value ) )
+    {
+    }
+    property( const property& ) = delete;
+    property& operator=(const property&) = delete;
+
+
+    /**
+     * gets the current value
+     * @return const reference to the value
+     */
+    const T& get() const
+    {
+        return mStorage;
+    }
+
+    /**
+     * gets the current value
+     * @return mutable reference to the value
+     */
+    T& get()
+    {
+        return mStorage;
+    }
+
+    const T& operator()() const
+    {
+        return get();
+    }
+
+    T& operator()()
+    {
+        return get();
+    }
+
+    /**
+     * sets the value of the property.
+     * @param value the new value
+     * @discussion the value will only be updated if the UpdatePolicy's critera is met.
+     * if the value is changed, then the @ref changed event will be fired.
+     */
+    void set( const T& value )
+    {
+        if ( UpdatePolicy{}( mStorage, value ) )
+        {
+            mStorage = value;
+            changed( mStorage );
+        }
+    }
+
+    void set( T&& value )
+    {
+        if ( UpdatePolicy{}( mStorage, value ) )
+        {
+            mStorage = std::move( value );
+            changed( mStorage );
+        }
+    }
+
+    void operator()( const T& value )
+    {
+        set( value );
+    }
+
+    void operator()( T&& value )
+    {
+        set( std::move( value ) );
+    }
+
+    /**
+     * this signal is invoked whenever the the value changes per the UpdatePolicy
+     * @discussion nod::unsafe_signal is used here for speed. Take care of your own threading.
+     */
+    Signal changed;
+
+    /**
+     * convience function to attach a change listener to this property
+     */
+    auto onChanged( typename decltype( changed )::slot_type&& c )
+    {
+        return changed.connect( std::move( c ) );
+    }
+
+    /**
+     * convience function to attach a change listener to this property and call it right away
+     */
+    auto onChangedAndNow( typename decltype( changed )::slot_type&& c )
+    {
+        auto connection = onChanged( std::move( c ) );
+        changed( mStorage );
+        return connection;
+    }
+
+
+private:
+    T mStorage{};
+};
+
+/**
+ * thread safe property type based on nod::signal
+ */
+template <typename T, typename UpdatePolicy = std::not_equal_to<T>>
+using safe_property = property<T, UpdatePolicy, nod::signal<void( const T& )>>;
+
+} // namespace bindey
@@ -0,0 +1,681 @@
+#ifndef IG_NOD_INCLUDE_NOD_HPP
+#define IG_NOD_INCLUDE_NOD_HPP
+
+#include <vector>       // std::vector
+#include <functional>   // std::function
+#include <mutex>        // std::mutex, std::lock_guard
+#include <memory>       // std::shared_ptr, std::weak_ptr
+#include <algorithm>    // std::find_if()
+#include <cassert>      // assert()
+#include <thread>       // std::this_thread::yield()
+#include <type_traits>  // std::is_same
+#include <iterator>     // std::back_inserter
+
+namespace nod {
+	// implementational details
+	namespace detail {
+		/// Interface for type erasure when disconnecting slots
+		struct disconnector {
+			virtual ~disconnector() {}
+			virtual void operator()( std::size_t index ) const = 0;
+		};
+		/// Deleter that doesn't delete
+		inline void no_delete(disconnector*){
+		};
+	} // namespace detail
+
+	/// Base template for the signal class
+	template <class P, class T>
+	class signal_type;
+
+
+	/// Connection class.
+	///
+	/// This is used to be able to disconnect slots after they have been connected.
+	/// Used as return type for the connect method of the signals.
+	///
+	/// Connections are default constructible.
+	/// Connections are not copy constructible or copy assignable.
+	/// Connections are move constructible and move assignable.
+	///
+	class connection {
+		public:
+			/// Default constructor
+			connection() :
+				_index()
+			{}
+
+			// Connection are not copy constructible or copy assignable
+			connection( connection const& ) = delete;
+			connection& operator=( connection const& ) = delete;
+
+			/// Move constructor
+			/// @param other   The instance to move from.
+			connection( connection&& other ) :
+				_weak_disconnector( std::move(other._weak_disconnector) ),
+				_index( other._index )
+			{}
+
+			/// Move assign operator.
+			/// @param other   The instance to move from.
+			connection& operator=( connection&& other ) {
+				_weak_disconnector = std::move( other._weak_disconnector );
+				_index = other._index;
+				return *this;
+			}
+
+			/// @returns `true` if the connection is connected to a signal object,
+			///          and `false` otherwise.
+			bool connected() const {
+				return !_weak_disconnector.expired();
+			}
+
+			/// Disconnect the slot from the connection.
+			///
+			/// If the connection represents a slot that is connected to a signal object, calling
+			/// this method will disconnect the slot from that object. The result of this operation
+			/// is that the slot will stop receiving calls when the signal is invoked.
+			void disconnect();
+
+		private:
+			/// The signal template is a friend of the connection, since it is the
+			/// only one allowed to create instances using the meaningful constructor.
+			template<class P,class T> friend class signal_type;
+
+			/// Create a connection.
+			/// @param shared_disconnector   Disconnector instance that will be used to disconnect
+			///                              the connection when the time comes. A weak pointer
+			///                              to the disconnector will be held within the connection
+			///                              object.
+			/// @param index                 The slot index of the connection.
+			connection( std::shared_ptr<detail::disconnector> const& shared_disconnector, std::size_t index ) :
+				_weak_disconnector( shared_disconnector ),
+				_index( index )
+			{}
+
+			/// Weak pointer to the current disconnector functor.
+			std::weak_ptr<detail::disconnector> _weak_disconnector;
+			/// Slot index of the connected slot.
+			std::size_t _index;
+	};
+
+	/// Scoped connection class.
+	///
+	/// This type of connection is automatically disconnected when
+	/// the connection object is destructed.
+	///
+	class scoped_connection
+	{
+		public:
+			/// Scoped are default constructible
+			scoped_connection() = default;
+			/// Scoped connections are not copy constructible
+			scoped_connection( scoped_connection const& ) = delete;
+			/// Scoped connections are not copy assingable
+			scoped_connection& operator=( scoped_connection const& ) = delete;
+
+			/// Move constructor
+			scoped_connection( scoped_connection&& other ) :
+				_connection( std::move(other._connection) )
+			{}
+
+			/// Move assign operator.
+			/// @param other   The instance to move from.
+			scoped_connection& operator=( scoped_connection&& other ) {
+				reset( std::move( other._connection ) );
+				return *this;
+			}
+
+			/// Construct a scoped connection from a connection object
+			/// @param connection   The connection object to manage
+			scoped_connection( connection&& c ) :
+				_connection( std::forward<connection>(c) )
+			{}
+
+			/// destructor
+			~scoped_connection() {
+				disconnect();
+			}
+
+			/// Assignment operator moving a new connection into the instance.
+			/// @note If the scoped_connection instance already contains a
+			///       connection, that connection will be disconnected as if
+			///       the scoped_connection was destroyed.
+			/// @param c   New connection to manage
+			scoped_connection& operator=( connection&& c ) {
+				reset( std::forward<connection>(c) );
+				return *this;
+			}
+
+			/// Reset the underlying connection to another connection.
+			/// @note The connection currently managed by the scoped_connection
+			///       instance will be disconnected when resetting.
+			/// @param c   New connection to manage
+			void reset( connection&& c = {} ) {
+				disconnect();
+				_connection = std::move(c);
+			}
+
+			/// Release the underlying connection, without disconnecting it.
+			/// @returns The newly released connection instance is returned.
+			connection release() {
+				connection c = std::move(_connection);
+				_connection = connection{};
+				return c;
+			}
+
+			///
+			/// @returns `true` if the connection is connected to a signal object,
+			///          and `false` otherwise.
+			bool connected() const {
+				return _connection.connected();
+			}
+
+			/// Disconnect the slot from the connection.
+			///
+			/// If the connection represents a slot that is connected to a signal object, calling
+			/// this method will disconnect the slot from that object. The result of this operation
+			/// is that the slot will stop receiving calls when the signal is invoked.
+			void disconnect() {
+				_connection.disconnect();
+			}
+
+		private:
+			/// Underlying connection object
+			connection _connection;
+	};
+
+	/// Policy for multi threaded use of signals.
+	///
+	/// This policy provides mutex and lock types for use in
+	/// a multithreaded environment, where signals and slots
+	/// may exists in different threads.
+	///
+	/// This policy is used in the `nod::signal` type provided
+	/// by the library.
+	struct multithread_policy
+	{
+		using mutex_type = std::mutex;
+		using mutex_lock_type = std::unique_lock<mutex_type>;
+		/// Function that yields the current thread, allowing
+		/// the OS to reschedule.
+		static void yield_thread() {
+			std::this_thread::yield();
+		}
+		/// Function that defers a lock to a lock function that prevents deadlock
+		static mutex_lock_type defer_lock(mutex_type & m){
+			return mutex_lock_type{m, std::defer_lock};
+		}
+		/// Function that locks two mutexes and prevents deadlock
+		static void lock(mutex_lock_type & a,mutex_lock_type & b) {
+			std::lock(a,b);
+		}
+	};
+
+	/// Policy for single threaded use of signals.
+	///
+	/// This policy provides dummy implementations for mutex
+	/// and lock types, resulting in that no synchronization
+	/// will take place.
+	///
+	/// This policy is used in the `nod::unsafe_signal` type
+	/// provided by the library.
+	struct singlethread_policy
+	{
+		/// Dummy mutex type that doesn't do anything
+		struct mutex_type{};
+		/// Dummy lock type, that doesn't do any locking.
+		struct mutex_lock_type
+		{
+			/// A lock type must be constructible from a
+			/// mutex type from the same thread policy.
+			explicit mutex_lock_type( mutex_type const& ) {
+			}
+		};
+		/// Dummy implementation of thread yielding, that
+		/// doesn't do any actual yielding.
+		static void yield_thread() {
+		}
+		/// Dummy implemention of defer_lock that doesn't
+		/// do anything
+		static mutex_lock_type defer_lock(mutex_type &m){
+			return mutex_lock_type{m};
+		}
+		/// Dummy implemention of lock that doesn't
+		/// do anything
+		static void lock(mutex_lock_type &,mutex_lock_type &) {
+		}
+	};
+
+	/// Signal accumulator class template.
+	///
+	/// This acts sort of as a proxy for triggering a signal and
+	/// accumulating the slot return values.
+	///
+	/// This class is not really intended to instantiate by client code.
+	/// Instances are aquired as return values of the method `accumulate()`
+	/// called on signals.
+	///
+	/// @tparam S      Type of signal. The signal_accumulator acts
+	///                as a type of proxy for a signal instance of
+	///                this type.
+	/// @tparam T      Type of initial value of the accumulate algorithm.
+	///                This type must meet the requirements of `CopyAssignable`
+	///                and `CopyConstructible`
+	/// @tparam F      Type of accumulation function.
+	/// @tparam A...   Argument types of the underlying signal type.
+	///
+	template <class S, class T, class F, class...A>
+	class signal_accumulator
+	{
+		public:
+			/// Result type when calling the accumulating function operator.
+			using result_type = typename std::result_of<F(T, typename S::slot_type::result_type)>::type;
+
+			/// Construct a signal_accumulator as a proxy to a given signal
+			//
+			/// @param signal   Signal instance.
+			/// @param init     Initial value of the accumulate algorithm.
+			/// @param func     Binary operation function object that will be
+			///                 applied to all slot return values.
+			///                 The signature of the function should be
+			///                 equivalent of the following:
+			///                   `R func( T1 const& a, T2 const& b )`
+			///                  - The signature does not need to have `const&`.
+			///                  - The initial value, type `T`, must be implicitly
+			///                    convertible to `R`
+			///                  - The return type `R` must be implicitly convertible
+			///                    to type `T1`.
+			///                  - The type `R` must be `CopyAssignable`.
+			///                  - The type `S::slot_type::result_type` (return type of
+			///                    the signals slots) must be implicitly convertible to
+			///                    type `T2`.
+			signal_accumulator( S const& signal, T init, F func ) :
+				_signal( signal ),
+				_init( init ),
+				_func( func )
+			{}
+
+			/// Function call operator.
+			///
+			/// Calling this will trigger the underlying signal and accumulate
+			/// all of the connected slots return values with the current
+			/// initial value and accumulator function.
+			///
+			/// When called, this will invoke the accumulator function will
+			/// be called for each return value of the slots. The semantics
+			/// are similar to the `std::accumulate` algorithm.
+			///
+			/// @param args   Arguments to propagate to the slots of the
+			///               underlying when triggering the signal.
+			result_type operator()( A const& ... args ) const {
+				return _signal.trigger_with_accumulator( _init, _func, args... );
+			}
+
+		private:
+
+			/// Reference to the underlying signal to proxy.
+			S const& _signal;
+			/// Initial value of the accumulate algorithm.
+			T _init;
+			/// Accumulator function.
+			F _func;
+
+	};
+
+	/// Signal template specialization.
+	///
+	/// This is the main signal implementation, and it is used to
+	/// implement the observer pattern whithout the overhead
+	/// boilerplate code that typically comes with it.
+	///
+	/// Any function or function object is considered a slot, and
+	/// can be connected to a signal instance, as long as the signature
+	/// of the slot matches the signature of the signal.
+	///
+	/// @tparam P      Threading policy for the signal.
+	///                A threading policy must provide two type definitions:
+	///                 - P::mutex_type, this type will be used as a mutex
+	///                   in the signal_type class template.
+	///                 - P::mutex_lock_type, this type must implement a
+	///                   constructor that takes a P::mutex_type as a parameter,
+	///                   and it must have the semantics of a scoped mutex lock
+	///                   like std::lock_guard, i.e. locking in the constructor
+	///                   and unlocking in the destructor.
+	///
+	/// @tparam R      Return value type of the slots connected to the signal.
+	/// @tparam A...   Argument types of the slots connected to the signal.
+	template <class P, class R, class... A >
+	class signal_type<P,R(A...)>
+	{
+		public:
+			/// signals are not copy constructible
+			signal_type( signal_type const& ) = delete;
+			/// signals are not copy assignable
+			signal_type& operator=( signal_type const& ) = delete;
+			/// signals are move constructible
+			signal_type(signal_type&& other)
+			{
+				mutex_lock_type lock{other._mutex};
+				_slot_count = std::move(other._slot_count);
+				_slots = std::move(other._slots);
+				if(other._shared_disconnector != nullptr)
+				{
+					_disconnector = disconnector{ this };
+					_shared_disconnector = std::move(other._shared_disconnector);
+					// replace the disconnector with our own disconnector
+					*static_cast<disconnector*>(_shared_disconnector.get()) = _disconnector;
+				}
+			}
+			/// signals are move assignable
+			signal_type& operator=(signal_type&& other)
+			{
+				auto lock = thread_policy::defer_lock(_mutex);
+				auto other_lock = thread_policy::defer_lock(other._mutex);
+				thread_policy::lock(lock,other_lock);
+
+				_slot_count = std::move(other._slot_count);
+				_slots = std::move(other._slots);
+				if(other._shared_disconnector != nullptr)
+				{
+					_disconnector = disconnector{ this };
+					_shared_disconnector = std::move(other._shared_disconnector);
+					// replace the disconnector with our own disconnector
+					*static_cast<disconnector*>(_shared_disconnector.get()) = _disconnector;
+				}
+				return *this;
+			}
+
+			/// signals are default constructible
+			signal_type() :
+				_slot_count(0)
+			{}
+
+			// Destruct the signal object.
+			~signal_type() {
+				invalidate_disconnector();
+			}
+
+			/// Type that will be used to store the slots for this signal type.
+			using slot_type = std::function<R(A...)>;
+			/// Type that is used for counting the slots connected to this signal.
+			using size_type = typename std::vector<slot_type>::size_type;
+
+
+			/// Connect a new slot to the signal.
+			///
+			/// The connected slot will be called every time the signal
+			/// is triggered.
+			/// @param slot   The slot to connect. This must be a callable with
+			///               the same signature as the signal itself.
+			/// @return       A connection object is returned, and can be used to
+			///               disconnect the slot.
+			template <class T>
+			connection connect( T&& slot ) {
+				mutex_lock_type lock{ _mutex };
+				_slots.push_back( std::forward<T>(slot) );
+				std::size_t index = _slots.size()-1;
+				if( _shared_disconnector == nullptr ) {
+					_disconnector = disconnector{ this };
+					_shared_disconnector = std::shared_ptr<detail::disconnector>{&_disconnector, detail::no_delete};
+				}
+				++_slot_count;
+				return connection{ _shared_disconnector, index };
+			}
+
+			/// Function call operator.
+			///
+			/// Calling this is how the signal is triggered and the
+			/// connected slots are called.
+			///
+			/// @note The slots will be called in the order they were
+			///       connected to the signal.
+			///
+			/// @param args   Arguments that will be propagated to the
+			///               connected slots when they are called.
+			void operator()( A const&... args ) const {
+				for( auto const& slot : copy_slots() ) {
+					if( slot ) {
+						slot( args... );
+					}
+				}
+			}
+
+			/// Construct a accumulator proxy object for the signal.
+			///
+			/// The intended purpose of this function is to create a function
+			/// object that can be used to trigger the signal and accumulate
+			/// all the slot return values.
+			///
+			/// The algorithm used to accumulate slot return values is similar
+			/// to `std::accumulate`. A given binary function is called for
+			/// each return value with the parameters consisting of the
+			/// return value of the accumulator function applied to the
+			/// previous slots return value, and the current slots return value.
+			/// A initial value must be provided for the first slot return type.
+			///
+			/// @note This can only be used on signals that have slots with
+			///       non-void return types, since we can't accumulate void
+			///       values.
+			///
+			/// @tparam T      The type of the initial value given to the accumulator.
+			/// @tparam F      The accumulator function type.
+			/// @param init    Initial value given to the accumulator.
+			/// @param op      Binary operator function object to apply by the accumulator.
+			///                The signature of the function should be
+			///                equivalent of the following:
+			///                  `R func( T1 const& a, T2 const& b )`
+			///                 - The signature does not need to have `const&`.
+			///                 - The initial value, type `T`, must be implicitly
+			///                   convertible to `R`
+			///                 - The return type `R` must be implicitly convertible
+			///                   to type `T1`.
+			///                 - The type `R` must be `CopyAssignable`.
+			///                 - The type `S::slot_type::result_type` (return type of
+			///                   the signals slots) must be implicitly convertible to
+			///                   type `T2`.
+			template <class T, class F>
+			signal_accumulator<signal_type, T, F, A...> accumulate( T init, F op ) const {
+				static_assert( std::is_same<R,void>::value == false, "Unable to accumulate slot return values with 'void' as return type." );
+				return { *this, init, op };
+			}
+
+
+			/// Trigger the signal, calling the slots and aggregate all
+			/// the slot return values into a container.
+			///
+			/// @tparam C     The type of container. This type must be
+			///               `DefaultConstructible`, and usable with
+			///               `std::back_insert_iterator`. Additionally it
+			///               must be either copyable or moveable.
+			/// @param args   The arguments to propagate to the slots.
+			template <class C>
+			C aggregate( A const&... args ) const {
+				static_assert( std::is_same<R,void>::value == false, "Unable to aggregate slot return values with 'void' as return type." );
+				C container;
+				auto iterator = std::back_inserter( container );
+				for( auto const& slot : copy_slots() ) {
+					if( slot ) {
+						(*iterator) = slot( args... );
+					}
+				}
+				return container;
+			}
+
+			/// Count the number of slots connected to this signal
+			/// @returns   The number of connected slots
+			size_type slot_count() const {
+				return _slot_count;
+			}
+
+			/// Determine if the signal is empty, i.e. no slots are connected
+			/// to it.
+			/// @returns   `true` is returned if the signal has no connected
+			///            slots, and `false` otherwise.
+			bool empty() const {
+				return slot_count() == 0;
+			}
+
+			/// Disconnects all slots
+			/// @note This operation invalidates all scoped_connection objects
+			void disconnect_all_slots() {
+				mutex_lock_type lock{ _mutex };
+				_slots.clear();
+				_slot_count = 0;
+				invalidate_disconnector();
+			}
+
+		private:
+			template<class, class, class, class...> friend class signal_accumulator;
+			/// Thread policy currently in use
+			using thread_policy = P;
+			/// Type of mutex, provided by threading policy
+			using mutex_type = typename thread_policy::mutex_type;
+			/// Type of mutex lock, provided by threading policy
+			using mutex_lock_type = typename thread_policy::mutex_lock_type;
+
+			/// Invalidate the internal disconnector object in a way
+			/// that is safe according to the current thread policy.
+			///
+			/// This will effectively make all current connection objects to
+			/// to this signal incapable of disconnecting, since they keep a
+			/// weak pointer to the shared disconnector object.
+			void invalidate_disconnector() {
+				// If we are unlucky, some of the connected slots
+				// might be in the process of disconnecting from other threads.
+				// If this happens, we are risking to destruct the disconnector
+				// object managed by our shared pointer before they are done
+				// disconnecting. This would be bad. To solve this problem, we
+				// discard the shared pointer (that is pointing to the disconnector
+				// object within our own instance), but keep a weak pointer to that
+				// instance. We then stall the destruction until all other weak
+				// pointers have released their "lock" (indicated by the fact that
+				// we will get a nullptr when locking our weak pointer).
+				std::weak_ptr<detail::disconnector> weak{_shared_disconnector};
+				_shared_disconnector.reset();
+				while( weak.lock() != nullptr )	{
+					// we just yield here, allowing the OS to reschedule. We do
+					// this until all threads has released the disconnector object.
+					thread_policy::yield_thread();
+				}
+			}
+
+			/// Retrieve a copy of the current slots
+			///
+			/// It's useful and necessary to copy the slots so we don't need
+			/// to hold the lock while calling the slots. If we hold the lock
+			/// we prevent the called slots from modifying the slots vector.
+			/// This simple "double buffering" will allow slots to disconnect
+			/// themself or other slots and connect new slots.
+			std::vector<slot_type> copy_slots() const
+			{
+				mutex_lock_type lock{ _mutex };
+				return _slots;
+			}
+
+			/// Implementation of the signal accumulator function call
+			template <class T, class F>
+			typename signal_accumulator<signal_type, T, F, A...>::result_type trigger_with_accumulator( T value, F& func, A const&... args ) const {
+				for( auto const& slot : copy_slots() ) {
+					if( slot ) {
+						value = func( value, slot( args... ) );
+					}
+				}
+				return value;
+			}
+
+			/// Implementation of the disconnection operation.
+			///
+			/// This is private, and only called by the connection
+			/// objects created when connecting slots to this signal.
+			/// @param index   The slot index of the slot that should
+			///                be disconnected.
+			void disconnect( std::size_t index ) {
+				mutex_lock_type lock( _mutex );
+				assert( _slots.size() > index );
+				if( _slots[ index ] != nullptr ) {
+					--_slot_count;
+				}
+				_slots[ index ] = slot_type{};
+				while( _slots.size()>0 && !_slots.back() ) {
+					_slots.pop_back();
+				}
+			}
+
+			/// Implementation of the shared disconnection state
+			/// used by all connection created by signal instances.
+			///
+			/// This inherits the @ref detail::disconnector interface
+			/// for type erasure.
+			struct disconnector :
+				detail::disconnector
+			{
+				/// Default constructor, resulting in a no-op disconnector.
+				disconnector() :
+					_ptr(nullptr)
+				{}
+
+				/// Create a disconnector that works with a given signal instance.
+				/// @param ptr   Pointer to the signal instance that the disconnector
+				///              should work with.
+				disconnector( signal_type<P,R(A...)>* ptr ) :
+					_ptr( ptr )
+				{}
+
+				/// Disconnect a given slot on the current signal instance.
+				/// @note If the instance is default constructed, or created
+				///       with `nullptr` as signal pointer this operation will
+				///       effectively be a no-op.
+				/// @param index   The index of the slot to disconnect.
+				void operator()( std::size_t index ) const override {
+					if( _ptr ) {
+						_ptr->disconnect( index );
+					}
+				}
+
+				/// Pointer to the current signal.
+				signal_type<P,R(A...)>* _ptr;
+			};
+
+			/// Mutex to synchronize access to the slot vector
+			mutable mutex_type _mutex;
+			/// Vector of all connected slots
+			std::vector<slot_type> _slots;
+			/// Number of connected slots
+			size_type _slot_count;
+			/// Disconnector operation, used for executing disconnection in a
+			/// type erased manner.
+			disconnector _disconnector;
+			/// Shared pointer to the disconnector. All connection objects has a
+			/// weak pointer to this pointer for performing disconnections.
+			std::shared_ptr<detail::disconnector> _shared_disconnector;
+	};
+
+	// Implementation of the disconnect operation of the connection class
+	inline void connection::disconnect() {
+		auto ptr = _weak_disconnector.lock();
+		if( ptr ) {
+			(*ptr)( _index );
+		}
+		_weak_disconnector.reset();
+	}
+
+	/// Signal type that is safe to use in multithreaded environments,
+	/// where the signal and slots exists in different threads.
+	/// The multithreaded policy provides mutexes and locks to synchronize
+	/// access to the signals internals.
+	///
+	/// This is the recommended signal type, even for single threaded
+	/// environments.
+	template <class T> using signal = signal_type<multithread_policy, T>;
+
+	/// Signal type that is unsafe in multithreaded environments.
+	/// No synchronizations are provided to the signal_type for accessing
+	/// the internals.
+	///
+	/// Only use this signal type if you are sure that your environment is
+	/// single threaded and performance is of importance.
+	template <class T> using unsafe_signal = signal_type<singlethread_policy, T>;
+} // namespace nod
+
+#endif // IG_NOD_INCLUDE_NOD_HPP
@@ -187,8 +187,8 @@ int CmdDbTracks(int argc, char** argv) {
  std::unique_ptr<database::Result<database::Track>> res(
      db->GetTracks(20).get());
  while (true) {
-    for (database::Track s : res->values()) {
-      std::cout << s.tags()[database::Tag::kTitle].value_or("[BLANK]")
+    for (const auto& s : res->values()) {
+      std::cout << s->tags()[database::Tag::kTitle].value_or("[BLANK]")
                << std::endl;
    }
    if (res->next_page()) {
@@ -256,12 +256,12 @@ int CmdDbIndex(int argc, char** argv) {
      std::cout << "choice out of range" << std::endl;
      return -1;
    }
-    if (res->values().at(choice).track()) {
+    if (res->values().at(choice)->track()) {
      AppConsole::sServices->track_queue().IncludeLast(
          std::make_shared<playlist::IndexRecordSource>(
              AppConsole::sServices->database(), res, 0, res, choice));
    }
-    auto cont = res->values().at(choice).Expand(20);
+    auto cont = res->values().at(choice)->Expand(20);
    if (!cont) {
      std::cout << "more choices than levels" << std::endl;
      return 0;
@@ -270,10 +270,10 @@ int CmdDbIndex(int argc, char** argv) {
    choice_index++;
  }

-  for (database::IndexRecord r : res->values()) {
-    std::cout << r.text().value_or("<unknown>");
-    if (r.track()) {
-      std::cout << "\t(id:" << *r.track() << ")";
+  for (const auto& r : res->values()) {
+    std::cout << r->text().value_or("<unknown>");
+    if (r->track()) {
+      std::cout << "\t(id:" << *r->track() << ")";
    }
    std::cout << std::endl;
  }
@@ -311,8 +311,8 @@ int CmdDbDump(int argc, char** argv) {

  std::unique_ptr<database::Result<std::pmr::string>> res(db->GetDump(5).get());
  while (true) {
-    for (const std::pmr::string& s : res->values()) {
-      std::cout << s << std::endl;
+    for (const auto& s : res->values()) {
+      std::cout << *s << std::endl;
    }
    if (res->next_page()) {
      auto continuation = res->next_page().value();
@@ -103,6 +103,7 @@ void Decoder::Main() {
  for (;;) {
    if (source_->HasNewStream() || !stream_) {
      std::shared_ptr<codecs::IStream> new_stream = source_->NextStream();
+      ESP_LOGI(kTag, "decoder has new stream");
      if (new_stream && BeginDecoding(new_stream)) {
        stream_ = new_stream;
      } else {
@@ -34,6 +34,7 @@
 #include "future_fetcher.hpp"
 #include "tag_parser.hpp"
 #include "tasks.hpp"
+#include "track.hpp"
 #include "types.hpp"

 static const char* kTag = "SRC";
@@ -118,13 +119,13 @@ auto FatfsAudioInput::NextStream() -> std::shared_ptr<codecs::IStream> {
 auto FatfsAudioInput::OpenFile(const std::pmr::string& path) -> bool {
  ESP_LOGI(kTag, "opening file %s", path.c_str());

-  database::TrackTags tags;
-  if (!tag_parser_.ReadAndParseTags(path, &tags)) {
+  auto tags = tag_parser_.ReadAndParseTags(path);
+  if (!tags) {
    ESP_LOGE(kTag, "failed to read tags");
    return false;
  }

-  auto stream_type = ContainerToStreamType(tags.encoding());
+  auto stream_type = ContainerToStreamType(tags->encoding());
  if (!stream_type.has_value()) {
    ESP_LOGE(kTag, "couldn't match container to stream");
    return false;
@@ -19,6 +19,8 @@

 namespace audio {

+static constexpr char kTag[] = "tracks";
+
 TrackQueue::TrackQueue() {}

 auto TrackQueue::GetCurrent() const -> std::optional<database::TrackId> {
@@ -202,6 +204,9 @@ auto TrackQueue::Previous() -> void {

 auto TrackQueue::Clear() -> void {
  const std::lock_guard<std::mutex> lock(mutex_);
+  if (enqueued_.empty() && played_.empty()) {
+    return;
+  }
  QueueUpdate ev{.current_changed = !enqueued_.empty()};
  played_.clear();
  enqueued_.clear();
@@ -26,6 +26,10 @@ class Battery {
  struct BatteryState {
    uint_fast8_t percent;
    bool is_charging;
+
+    bool operator==(const BatteryState& other) const {
+      return percent == other.percent && is_charging == other.is_charging;
+    }
  };

  auto State() -> std::optional<BatteryState>;
@@ -144,7 +144,7 @@ auto Database::Update() -> std::future<void> {
      OwningSlice prefix = EncodeDataPrefix();
      it->Seek(prefix.slice);
      while (it->Valid() && it->key().starts_with(prefix.slice)) {
-        std::optional<TrackData> track = ParseDataValue(it->value());
+        std::shared_ptr<TrackData> track = ParseDataValue(it->value());
        if (!track) {
          // The value was malformed. Drop this record.
          ESP_LOGW(kTag, "dropping malformed metadata");
@@ -159,9 +159,9 @@ auto Database::Update() -> std::future<void> {
          continue;
        }

-        TrackTags tags{};
-        if (!tag_parser_.ReadAndParseTags(track->filepath(), &tags) ||
-            tags.encoding() == Container::kUnsupported) {
+        std::shared_ptr<TrackTags> tags =
+            tag_parser_.ReadAndParseTags(track->filepath());
+        if (!tags || tags->encoding() == Container::kUnsupported) {
          // We couldn't read the tags for this track. Either they were
          // malformed, or perhaps the file is missing. Either way, tombstone
          // this record.
@@ -174,7 +174,7 @@ auto Database::Update() -> std::future<void> {
        // At this point, we know that the track still exists in its original
        // location. All that's left to do is update any metadata about it.

-        uint64_t new_hash = tags.Hash();
+        uint64_t new_hash = tags->Hash();
        if (new_hash != track->tags_hash()) {
          // This track's tags have changed. Since the filepath is exactly the
          // same, we assume this is a legitimate correction. Update the
@@ -185,7 +185,9 @@ auto Database::Update() -> std::future<void> {
          dbPutHash(new_hash, track->id());
        }

-        dbCreateIndexesForTrack({*track, tags});
+        Track t{track, tags};
+
+        dbCreateIndexesForTrack(t);

        it->Next();
      }
@@ -197,15 +199,14 @@ auto Database::Update() -> std::future<void> {
        .stage = event::UpdateProgress::Stage::kScanningForNewTracks,
    });
    file_gatherer_.FindFiles("", [&](const std::pmr::string& path) {
-      TrackTags tags;
-      if (!tag_parser_.ReadAndParseTags(path, &tags) ||
-          tags.encoding() == Container::kUnsupported) {
+      std::shared_ptr<TrackTags> tags = tag_parser_.ReadAndParseTags(path);
+      if (!tags || tags->encoding() == Container::kUnsupported) {
        // No parseable tags; skip this fiile.
        return;
      }

      // Check for any existing record with the same hash.
-      uint64_t hash = tags.Hash();
+      uint64_t hash = tags->Hash();
      OwningSlice key = EncodeHashKey(hash);
      std::optional<TrackId> existing_hash;
      std::string raw_entry;
@@ -219,33 +220,36 @@ auto Database::Update() -> std::future<void> {
        TrackId id = dbMintNewTrackId();
        ESP_LOGI(kTag, "recording new 0x%lx", id);

-        TrackData data(id, path, hash);
-        dbPutTrackData(data);
+        auto data = std::make_shared<TrackData>(id, path, hash);
+        dbPutTrackData(*data);
        dbPutHash(hash, id);
-        dbCreateIndexesForTrack({data, tags});
+        auto t = std::make_shared<Track>(data, tags);
+        dbCreateIndexesForTrack(*t);
        return;
      }

-      std::optional<TrackData> existing_data = dbGetTrackData(*existing_hash);
+      std::shared_ptr<TrackData> existing_data = dbGetTrackData(*existing_hash);
      if (!existing_data) {
        // We found a hash that matches, but there's no data record? Weird.
-        TrackData new_data(*existing_hash, path, hash);
-        dbPutTrackData(new_data);
-        dbCreateIndexesForTrack({*existing_data, tags});
+        auto new_data = std::make_shared<TrackData>(*existing_hash, path, hash);
+        dbPutTrackData(*new_data);
+        auto t = std::make_shared<Track>(new_data, tags);
+        dbCreateIndexesForTrack(*t);
        return;
      }

      if (existing_data->is_tombstoned()) {
        ESP_LOGI(kTag, "exhuming track %lu", existing_data->id());
        dbPutTrackData(existing_data->Exhume(path));
-        dbCreateIndexesForTrack({*existing_data, tags});
+        auto t = std::make_shared<Track>(existing_data, tags);
+        dbCreateIndexesForTrack(*t);
      } else if (existing_data->filepath() != path) {
        ESP_LOGW(kTag, "tag hash collision for %s and %s",
                 existing_data->filepath().c_str(), path.c_str());
        ESP_LOGI(kTag, "hash components: %s, %s, %s",
-                 tags.at(Tag::kTitle).value_or("no title").c_str(),
-                 tags.at(Tag::kArtist).value_or("no artist").c_str(),
-                 tags.at(Tag::kAlbum).value_or("no album").c_str());
+                 tags->at(Tag::kTitle).value_or("no title").c_str(),
+                 tags->at(Tag::kArtist).value_or("no artist").c_str(),
+                 tags->at(Tag::kAlbum).value_or("no album").c_str());
      }
    });
    events::Ui().Dispatch(event::UpdateFinished{});
@@ -264,26 +268,27 @@ auto Database::GetTrackPath(TrackId id)
      });
 }

-auto Database::GetTrack(TrackId id) -> std::future<std::optional<Track>> {
-  return worker_task_->Dispatch<std::optional<Track>>(
-      [=, this]() -> std::optional<Track> {
-        std::optional<TrackData> data = dbGetTrackData(id);
+auto Database::GetTrack(TrackId id) -> std::future<std::shared_ptr<Track>> {
+  return worker_task_->Dispatch<std::shared_ptr<Track>>(
+      [=, this]() -> std::shared_ptr<Track> {
+        std::shared_ptr<TrackData> data = dbGetTrackData(id);
        if (!data || data->is_tombstoned()) {
          return {};
        }
-        TrackTags tags;
-        if (!tag_parser_.ReadAndParseTags(data->filepath(), &tags)) {
+        std::shared_ptr<TrackTags> tags =
+            tag_parser_.ReadAndParseTags(data->filepath());
+        if (!tags) {
          return {};
        }
-        return Track(*data, tags);
+        return std::make_shared<Track>(data, tags);
      });
 }

 auto Database::GetBulkTracks(std::vector<TrackId> ids)
-    -> std::future<std::vector<std::optional<Track>>> {
-  return worker_task_->Dispatch<std::vector<std::optional<Track>>>(
-      [=, this]() -> std::vector<std::optional<Track>> {
-        std::map<TrackId, Track> id_to_track{};
+    -> std::future<std::vector<std::shared_ptr<Track>>> {
+  return worker_task_->Dispatch<std::vector<std::shared_ptr<Track>>>(
+      [=, this]() -> std::vector<std::shared_ptr<Track>> {
+        std::map<TrackId, std::shared_ptr<Track>> id_to_track{};

        // Sort the list of ids so that we can retrieve them all in a single
        // iteration through the database, without re-seeking.
@@ -299,16 +304,16 @@ auto Database::GetBulkTracks(std::vector<TrackId> ids)
            // This id wasn't found at all. Skip it.
            continue;
          }
-          std::optional<Track> track =
+          std::shared_ptr<Track> track =
              ParseRecord<Track>(it->key(), it->value());
          if (track) {
-            id_to_track.insert({id, *track});
+            id_to_track.insert({id, track});
          }
        }

        // We've fetched all of the ids in the request, so now just put them
        // back into the order they were asked for in.
-        std::vector<std::optional<Track>> results;
+        std::vector<std::shared_ptr<Track>> results;
        for (const TrackId& id : ids) {
          if (id_to_track.contains(id)) {
            results.push_back(id_to_track.at(id));
@@ -426,7 +431,7 @@ auto Database::dbPutTrackData(const TrackData& s) -> void {
  }
 }

-auto Database::dbGetTrackData(TrackId id) -> std::optional<TrackData> {
+auto Database::dbGetTrackData(TrackId id) -> std::shared_ptr<TrackData> {
  OwningSlice key = EncodeDataKey(id);
  std::string raw_val;
  if (!db_->Get(leveldb::ReadOptions(), key.slice, &raw_val).ok()) {
@@ -454,7 +459,7 @@ auto Database::dbGetHash(const uint64_t& hash) -> std::optional<TrackId> {
  return ParseHashValue(raw_val);
 }

-auto Database::dbCreateIndexesForTrack(Track track) -> void {
+auto Database::dbCreateIndexesForTrack(const Track& track) -> void {
  for (const IndexInfo& index : GetIndexes()) {
    leveldb::WriteBatch writes;
    if (Index(index, track, &writes)) {
@@ -481,7 +486,7 @@ auto Database::dbGetPage(const Continuation<T>& c) -> Result<T>* {

  // Grab results.
  std::optional<std::pmr::string> first_key;
-  std::vector<T> records;
+  std::vector<std::shared_ptr<T>> records;
  while (records.size() < c.page_size && it->Valid()) {
    if (!it->key().starts_with({c.prefix.data(), c.prefix.size()})) {
      break;
@@ -489,9 +494,9 @@ auto Database::dbGetPage(const Continuation<T>& c) -> Result<T>* {
    if (!first_key) {
      first_key = it->key().ToString();
    }
-    std::optional<T> parsed = ParseRecord<T>(it->key(), it->value());
+    std::shared_ptr<T> parsed = ParseRecord<T>(it->key(), it->value());
    if (parsed) {
-      records.push_back(*parsed);
+      records.push_back(parsed);
    }
    if (c.forward) {
      it->Next();
@@ -577,7 +582,7 @@ template auto Database::dbGetPage<std::pmr::string>(
 template <>
 auto Database::ParseRecord<IndexRecord>(const leveldb::Slice& key,
                                        const leveldb::Slice& val)
-    -> std::optional<IndexRecord> {
+    -> std::shared_ptr<IndexRecord> {
  std::optional<IndexKey> data = ParseIndexKey(key);
  if (!data) {
    return {};
@@ -588,28 +593,29 @@ auto Database::ParseRecord<IndexRecord>(const leveldb::Slice& key,
    title = val.ToString();
  }

-  return IndexRecord(*data, title, data->track);
+  return std::make_shared<IndexRecord>(*data, title, data->track);
 }

 template <>
 auto Database::ParseRecord<Track>(const leveldb::Slice& key,
                                  const leveldb::Slice& val)
-    -> std::optional<Track> {
-  std::optional<TrackData> data = ParseDataValue(val);
+    -> std::shared_ptr<Track> {
+  std::shared_ptr<TrackData> data = ParseDataValue(val);
  if (!data || data->is_tombstoned()) {
    return {};
  }
-  TrackTags tags;
-  if (!tag_parser_.ReadAndParseTags(data->filepath(), &tags)) {
+  std::shared_ptr<TrackTags> tags =
+      tag_parser_.ReadAndParseTags(data->filepath());
+  if (!tags) {
    return {};
  }
-  return Track(*data, tags);
+  return std::make_shared<Track>(data, tags);
 }

 template <>
 auto Database::ParseRecord<std::pmr::string>(const leveldb::Slice& key,
                                             const leveldb::Slice& val)
-    -> std::optional<std::pmr::string> {
+    -> std::shared_ptr<std::pmr::string> {
  std::ostringstream stream;
  stream << "key: ";
  if (key.size() < 3 || key.data()[1] != '\0') {
@@ -634,7 +640,7 @@ auto Database::ParseRecord<std::pmr::string>(const leveldb::Slice& key,
    }
  }
  std::pmr::string res{stream.str(), &memory::kSpiRamResource};
-  return res;
+  return std::make_shared<std::pmr::string>(res);
 }

 IndexRecord::IndexRecord(const IndexKey& key,
@@ -48,12 +48,14 @@ struct Continuation {
 template <typename T>
 class Result {
 public:
-  auto values() const -> const std::vector<T>& { return values_; }
+  auto values() const -> const std::vector<std::shared_ptr<T>>& {
+    return values_;
+  }

  auto next_page() -> std::optional<Continuation<T>>& { return next_page_; }
  auto prev_page() -> std::optional<Continuation<T>>& { return prev_page_; }

-  Result(const std::vector<T>&& values,
+  Result(const std::vector<std::shared_ptr<T>>&& values,
         std::optional<Continuation<T>> next,
         std::optional<Continuation<T>> prev)
      : values_(values), next_page_(next), prev_page_(prev) {}
@@ -62,7 +64,7 @@ class Result {
  Result& operator=(const Result&) = delete;

 private:
-  std::vector<T> values_;
+  std::vector<std::shared_ptr<T>> values_;
  std::optional<Continuation<T>> next_page_;
  std::optional<Continuation<T>> prev_page_;
 };
@@ -102,14 +104,14 @@ class Database {

  auto GetTrackPath(TrackId id) -> std::future<std::optional<std::pmr::string>>;

-  auto GetTrack(TrackId id) -> std::future<std::optional<Track>>;
+  auto GetTrack(TrackId id) -> std::future<std::shared_ptr<Track>>;

  /*
   * Fetches data for multiple tracks more efficiently than multiple calls to
   * GetTrack.
   */
  auto GetBulkTracks(std::vector<TrackId> id)
-      -> std::future<std::vector<std::optional<Track>>>;
+      -> std::future<std::vector<std::shared_ptr<Track>>>;

  auto GetIndexes() -> std::vector<IndexInfo>;
  auto GetTracksByIndex(const IndexInfo& index, std::size_t page_size)
@@ -145,30 +147,30 @@ class Database {
  auto dbEntomb(TrackId track, uint64_t hash) -> void;

  auto dbPutTrackData(const TrackData& s) -> void;
-  auto dbGetTrackData(TrackId id) -> std::optional<TrackData>;
+  auto dbGetTrackData(TrackId id) -> std::shared_ptr<TrackData>;
  auto dbPutHash(const uint64_t& hash, TrackId i) -> void;
  auto dbGetHash(const uint64_t& hash) -> std::optional<TrackId>;
-  auto dbCreateIndexesForTrack(Track track) -> void;
+  auto dbCreateIndexesForTrack(const Track& track) -> void;

  template <typename T>
  auto dbGetPage(const Continuation<T>& c) -> Result<T>*;

  template <typename T>
  auto ParseRecord(const leveldb::Slice& key, const leveldb::Slice& val)
-      -> std::optional<T>;
+      -> std::shared_ptr<T>;
 };

 template <>
 auto Database::ParseRecord<IndexRecord>(const leveldb::Slice& key,
                                        const leveldb::Slice& val)
-    -> std::optional<IndexRecord>;
+    -> std::shared_ptr<IndexRecord>;
 template <>
 auto Database::ParseRecord<Track>(const leveldb::Slice& key,
                                  const leveldb::Slice& val)
-    -> std::optional<Track>;
+    -> std::shared_ptr<Track>;
 template <>
 auto Database::ParseRecord<std::pmr::string>(const leveldb::Slice& key,
                                             const leveldb::Slice& val)
-    -> std::optional<std::pmr::string>;
+    -> std::shared_ptr<std::pmr::string>;

 }  // namespace database
@@ -53,7 +53,7 @@ auto EncodeDataValue(const TrackData& track) -> OwningSlice;
 * Parses bytes previously encoded via EncodeDataValue back into a TrackData.
 * May return nullopt if parsing fails.
 */
-auto ParseDataValue(const leveldb::Slice& slice) -> std::optional<TrackData>;
+auto ParseDataValue(const leveldb::Slice& slice) -> std::shared_ptr<TrackData>;

 /* Encodes a hash key for the specified hash. */
 auto EncodeHashKey(const uint64_t& hash) -> OwningSlice;
@@ -16,21 +16,21 @@ namespace database {
 class ITagParser {
 public:
  virtual ~ITagParser() {}
-  virtual auto ReadAndParseTags(const std::pmr::string& path, TrackTags* out)
-      -> bool = 0;
+  virtual auto ReadAndParseTags(const std::pmr::string& path)
+      -> std::shared_ptr<TrackTags> = 0;
 };

 class GenericTagParser : public ITagParser {
 public:
-  auto ReadAndParseTags(const std::pmr::string& path, TrackTags* out)
-      -> bool override;
+  auto ReadAndParseTags(const std::pmr::string& path)
+      -> std::shared_ptr<TrackTags> override;
 };

 class TagParserImpl : public ITagParser {
 public:
  TagParserImpl();
-  auto ReadAndParseTags(const std::pmr::string& path, TrackTags* out)
-      -> bool override;
+  auto ReadAndParseTags(const std::pmr::string& path)
+      -> std::shared_ptr<TrackTags> override;

 private:
  std::map<std::pmr::string, std::unique_ptr<ITagParser>> extension_to_parser_;
@@ -41,7 +41,7 @@ class TagParserImpl : public ITagParser {
   * cache should be slightly larger than any page sizes in the UI.
   */
  std::mutex cache_mutex_;
-  util::LruCache<16, std::pmr::string, TrackTags> cache_;
+  util::LruCache<16, std::pmr::string, std::shared_ptr<TrackTags>> cache_;

  // We could also consider keeping caches of artist name -> std::pmr::string
  // and similar. This hasn't been done yet, as this isn't a common workload in
@@ -50,8 +50,8 @@ class TagParserImpl : public ITagParser {

 class OpusTagParser : public ITagParser {
 public:
-  auto ReadAndParseTags(const std::pmr::string& path, TrackTags* out)
-      -> bool override;
+  auto ReadAndParseTags(const std::pmr::string& path)
+      -> std::shared_ptr<TrackTags> override;
 };

 }  // namespace database
@@ -61,12 +61,17 @@ enum class Tag {
 */
 class TrackTags {
 public:
-  auto encoding() const -> Container { return encoding_; };
-  auto encoding(Container e) -> void { encoding_ = e; };
-
  TrackTags()
      : encoding_(Container::kUnsupported), tags_(&memory::kSpiRamResource) {}

+  TrackTags(const TrackTags& other) = delete;
+  TrackTags& operator=(TrackTags& other) = delete;
+
+  bool operator==(const TrackTags&) const = default;
+
+  auto encoding() const -> Container { return encoding_; };
+  auto encoding(Container e) -> void { encoding_ = e; };
+
  std::optional<int> channels;
  std::optional<int> sample_rate;
  std::optional<int> bits_per_sample;
@@ -85,10 +90,6 @@ class TrackTags {
   */
  auto Hash() const -> uint64_t;

-  bool operator==(const TrackTags&) const = default;
-  TrackTags& operator=(const TrackTags&) = default;
-  TrackTags(const TrackTags&) = default;
-
 private:
  Container encoding_;
  std::pmr::unordered_map<Tag, std::pmr::string> tags_;
@@ -139,6 +140,11 @@ class TrackData {
        play_count_(play_count),
        is_tombstoned_(is_tombstoned) {}

+  TrackData(TrackData&& other) = delete;
+  TrackData& operator=(TrackData& other) = delete;
+
+  bool operator==(const TrackData&) const = default;
+
  auto id() const -> TrackId { return id_; }
  auto filepath() const -> std::pmr::string { return filepath_; }
  auto play_count() const -> uint32_t { return play_count_; }
@@ -158,8 +164,6 @@ class TrackData {
   * new location.
   */
  auto Exhume(const std::pmr::string& new_path) const -> TrackData;
-
-  bool operator==(const TrackData&) const = default;
 };

 /*
@@ -172,23 +176,22 @@ class TrackData {
 */
 class Track {
 public:
-  Track(const TrackData& data, const TrackTags& tags)
+  Track(std::shared_ptr<TrackData>& data, std::shared_ptr<TrackTags> tags)
      : data_(data), tags_(tags) {}
-  Track(const Track& other) = default;

-  auto data() const -> const TrackData& { return data_; }
-  auto tags() const -> const TrackTags& { return tags_; }
+  Track(Track& other) = delete;
+  Track& operator=(Track& other) = delete;
+
+  bool operator==(const Track&) const = default;
+
+  auto data() const -> const TrackData& { return *data_; }
+  auto tags() const -> const TrackTags& { return *tags_; }

  auto TitleOrFilename() const -> std::pmr::string;

-  bool operator==(const Track&) const = default;
-  Track operator=(const Track& other) const { return Track(other); }
-
 private:
-  const TrackData data_;
-  const TrackTags tags_;
+  std::shared_ptr<TrackData> data_;
+  std::shared_ptr<TrackTags> tags_;
 };

-void swap(Track& first, Track& second);
-
 }  // namespace database
@@ -149,7 +149,7 @@ auto EncodeDataValue(const TrackData& track) -> OwningSlice {
  return OwningSlice(as_str);
 }

-auto ParseDataValue(const leveldb::Slice& slice) -> std::optional<TrackData> {
+auto ParseDataValue(const leveldb::Slice& slice) -> std::shared_ptr<TrackData> {
  CborParser parser;
  CborValue container;
  CborError err;
@@ -211,7 +211,7 @@ auto ParseDataValue(const leveldb::Slice& slice) -> std::optional<TrackData> {
    return {};
  }

-  return TrackData(id, path, hash, play_count, is_tombstoned);
+  return std::make_shared<TrackData>(id, path, hash, play_count, is_tombstoned);
 }

 /* 'H/ 0xBEEF' */
@@ -130,14 +130,13 @@ TagParserImpl::TagParserImpl() {
  extension_to_parser_["opus"] = std::make_unique<OpusTagParser>();
 }

-auto TagParserImpl::ReadAndParseTags(const std::pmr::string& path,
-                                     TrackTags* out) -> bool {
+auto TagParserImpl::ReadAndParseTags(const std::pmr::string& path)
+    -> std::shared_ptr<TrackTags> {
  {
    std::lock_guard<std::mutex> lock{cache_mutex_};
-    std::optional<TrackTags> cached = cache_.Get(path);
+    std::optional<std::shared_ptr<TrackTags>> cached = cache_.Get(path);
    if (cached) {
-      *out = *cached;
-      return true;
+      return *cached;
    }
  }

@@ -152,41 +151,43 @@ auto TagParserImpl::ReadAndParseTags(const std::pmr::string& path,
    }
  }

-  if (!parser->ReadAndParseTags(path, out)) {
-    return false;
+  std::shared_ptr<TrackTags> tags = parser->ReadAndParseTags(path);
+  if (!tags) {
+    return {};
  }

  // There wasn't a track number found in the track's tags. Try to synthesize
  // one from the filename, which will sometimes have a track number at the
  // start.
-  if (!out->at(Tag::kAlbumTrack)) {
+  if (!tags->at(Tag::kAlbumTrack)) {
    auto slash_pos = path.find_last_of("/");
    if (slash_pos != std::pmr::string::npos && path.size() - slash_pos > 1) {
-      out->set(Tag::kAlbumTrack, path.substr(slash_pos + 1));
+      tags->set(Tag::kAlbumTrack, path.substr(slash_pos + 1));
    }
  }

  // Normalise track numbers; they're usually treated as strings, but we would
  // like to sort them lexicographically.
-  out->set(Tag::kAlbumTrack,
-           convert_track_number(out->at(Tag::kAlbumTrack).value_or("0")));
+  tags->set(Tag::kAlbumTrack,
+            convert_track_number(tags->at(Tag::kAlbumTrack).value_or("0")));

  {
    std::lock_guard<std::mutex> lock{cache_mutex_};
-    cache_.Put(path, *out);
+    cache_.Put(path, tags);
  }

-  return true;
+  return tags;
 }

-auto GenericTagParser::ReadAndParseTags(const std::pmr::string& path,
-                                        TrackTags* out) -> bool {
+auto GenericTagParser::ReadAndParseTags(const std::pmr::string& path)
+    -> std::shared_ptr<TrackTags> {
  libtags::Aux aux;
-  aux.tags = out;
+  auto out = std::make_shared<TrackTags>();
+  aux.tags = out.get();
  if (f_stat(path.c_str(), &aux.info) != FR_OK ||
      f_open(&aux.file, path.c_str(), FA_READ) != FR_OK) {
    ESP_LOGW(kTag, "failed to open file %s", path.c_str());
-    return false;
+    return {};
  }
  // Fine to have this on the stack; this is only called on tasks with large
  // stacks anyway, due to all the string handling.
@@ -205,7 +206,7 @@ auto GenericTagParser::ReadAndParseTags(const std::pmr::string& path,
  if (res != 0) {
    // Parsing failed.
    ESP_LOGE(kTag, "tag parsing for %s failed, reason %d", path.c_str(), res);
-    return false;
+    return {};
  }

  switch (ctx.format) {
@@ -240,25 +241,26 @@ auto GenericTagParser::ReadAndParseTags(const std::pmr::string& path,
  if (ctx.duration > 0) {
    out->duration = ctx.duration;
  }
-  return true;
+  return out;
 }

-auto OpusTagParser::ReadAndParseTags(const std::pmr::string& path,
-                                     TrackTags* out) -> bool {
+auto OpusTagParser::ReadAndParseTags(const std::pmr::string& path)
+    -> std::shared_ptr<TrackTags> {
  std::pmr::string vfs_path = "/sdcard" + path;
  int err;
  OggOpusFile* f = op_test_file(vfs_path.c_str(), &err);
  if (f == NULL) {
    ESP_LOGE(kTag, "opusfile tag parsing failed: %d", err);
-    return false;
+    return {};
  }
  const OpusTags* tags = op_tags(f, -1);
  if (tags == NULL) {
    ESP_LOGE(kTag, "no tags in opusfile");
    op_free(f);
-    return false;
+    return {};
  }

+  auto out = std::make_shared<TrackTags>();
  out->encoding(Container::kOpus);
  for (const auto& pair : kVorbisIdToTag) {
    const char* tag = opus_tags_query(tags, pair.first, 0);
@@ -268,7 +270,7 @@ auto OpusTagParser::ReadAndParseTags(const std::pmr::string& path,
  }

  op_free(f);
-  return true;
+  return out;
 }

 }  // namespace database
@@ -64,12 +64,6 @@ auto TrackData::Exhume(const std::pmr::string& new_path) const -> TrackData {
  return TrackData(id_, new_path, tags_hash_, play_count_, false);
 }

-void swap(Track& first, Track& second) {
-  Track temp = first;
-  first = second;
-  second = temp;
-}
-
 auto Track::TitleOrFilename() const -> std::pmr::string {
  auto title = tags().at(Tag::kTitle);
  if (title) {
@@ -51,7 +51,7 @@ auto IndexRecordSource::Current() -> std::optional<database::TrackId> {
    return {};
  }

-  return current_page_->values().at(current_item_).track();
+  return current_page_->values().at(current_item_)->track();
 }

 auto IndexRecordSource::Advance() -> std::optional<database::TrackId> {
@@ -128,7 +128,7 @@ auto IndexRecordSource::Peek(std::size_t n, std::vector<database::TrackId>* out)
      working_item = 0;
    }

-    out->push_back(working_page->values().at(working_item).track().value());
+    out->push_back(working_page->values().at(working_item)->track().value());
    n--;
    items_added++;
    working_item++;
@@ -66,6 +66,7 @@ auto Booting::entry() -> void {

  ESP_LOGI(kTag, "installing remaining drivers");
  sServices->samd(std::unique_ptr<drivers::Samd>(drivers::Samd::Create()));
+  vTaskDelay(pdMS_TO_TICKS(1000));
  sServices->nvs(
      std::unique_ptr<drivers::NvsStorage>(drivers::NvsStorage::OpenSync()));
  sServices->touchwheel(
@@ -7,10 +7,11 @@ idf_component_register(
  "wheel_encoder.cpp" "screen_track_browser.cpp" "screen_playing.cpp"
  "themes.cpp" "widget_top_bar.cpp" "screen.cpp" "screen_onboarding.cpp"
  "modal_progress.cpp" "modal.cpp" "modal_confirm.cpp" "screen_settings.cpp"
+  "event_binding.cpp"
  "splash.c" "font_fusion.c" "font_symbols.c"
  "icons/battery_empty.c" "icons/battery_full.c" "icons/battery_20.c"
  "icons/battery_40.c" "icons/battery_60.c" "icons/battery_80.c" "icons/play.c"
  "icons/pause.c" "icons/bluetooth.c"
  INCLUDE_DIRS "include"
-  REQUIRES "drivers" "lvgl" "tinyfsm" "events" "system_fsm" "database" "esp_timer" "battery")
+  REQUIRES "drivers" "lvgl" "tinyfsm" "events" "system_fsm" "database" "esp_timer" "battery" "bindey")
 target_compile_options(${COMPONENT_LIB} PRIVATE ${EXTRA_WARNINGS})
@@ -0,0 +1,23 @@
+/*
+ * Copyright 2023 jacqueline <me@jacqueline.id.au>
+ *
+ * SPDX-License-Identifier: GPL-3.0-only
+ */
+
+#include "event_binding.hpp"
+
+#include "core/lv_event.h"
+
+namespace ui {
+
+static auto event_cb(lv_event_t* ev) -> void {
+  EventBinding* binding =
+      static_cast<EventBinding*>(lv_event_get_user_data(ev));
+  binding->signal()(lv_event_get_target(ev));
+}
+
+EventBinding::EventBinding(lv_obj_t* obj, lv_event_code_t ev) {
+  lv_obj_add_event_cb(obj, event_cb, ev, this);
+}
+
+}  // namespace ui
@@ -0,0 +1,30 @@
+/*
+ * Copyright 2023 jacqueline <me@jacqueline.id.au>
+ *
+ * SPDX-License-Identifier: GPL-3.0-only
+ */
+
+#pragma once
+
+#include <cstdint>
+
+#include "lvgl.h"
+
+#include "core/lv_event.h"
+#include "core/lv_obj.h"
+#include "nod/nod.hpp"
+
+namespace ui {
+
+class EventBinding {
+ public:
+  EventBinding(lv_obj_t* obj, lv_event_code_t ev);
+
+  auto signal() -> nod::signal<void(lv_obj_t*)>& { return signal_; }
+
+ private:
+  lv_obj_t* obj_;
+  nod::signal<void(lv_obj_t*)> signal_;
+};
+
+}  // namespace ui
@@ -0,0 +1,26 @@
+/*
+ * Copyright 2023 jacqueline <me@jacqueline.id.au>
+ *
+ * SPDX-License-Identifier: GPL-3.0-only
+ */
+
+#pragma once
+
+#include "bindey/property.h"
+
+#include "track.hpp"
+
+namespace ui {
+namespace models {
+
+struct Playback {
+  bindey::property<bool> is_playing;
+  bindey::property<std::optional<database::TrackId>> current_track;
+  bindey::property<std::vector<database::TrackId>> upcoming_tracks;
+
+  bindey::property<uint32_t> current_track_position;
+  bindey::property<uint32_t> current_track_duration;
+};
+
+}  // namespace models
+}  // namespace ui
@@ -8,11 +8,15 @@

 #include <memory>
 #include <optional>
+#include <vector>

+#include "bindey/binding.h"
 #include "core/lv_group.h"
 #include "core/lv_obj.h"
 #include "core/lv_obj_tree.h"
+#include "event_binding.hpp"
 #include "lvgl.h"
+#include "nod/nod.hpp"
 #include "widget_top_bar.hpp"

 namespace ui {
@@ -51,6 +55,16 @@ class Screen {
  auto CreateTopBar(lv_obj_t* parent, const widgets::TopBar::Configuration&)
      -> widgets::TopBar*;

+  std::pmr::vector<bindey::scoped_binding> data_bindings_;
+  std::pmr::vector<std::unique_ptr<EventBinding>> event_bindings_;
+
+  template <typename T>
+  auto lv_bind(lv_obj_t* obj, lv_event_code_t ev, T fn) -> void {
+    auto binding = std::make_unique<EventBinding>(obj, ev);
+    binding->signal().connect(fn);
+    event_bindings_.push_back(std::move(binding));
+  }
+
  lv_obj_t* const root_;
  lv_obj_t* content_;
  lv_obj_t* modal_content_;
@@ -11,10 +11,13 @@
 #include <memory>
 #include <vector>

+#include "bindey/property.h"
+#include "esp_log.h"
 #include "lvgl.h"

 #include "database.hpp"
 #include "future_fetcher.hpp"
+#include "model_playback.hpp"
 #include "screen.hpp"
 #include "track.hpp"
 #include "track_queue.hpp"
@@ -28,48 +31,36 @@ namespace screens {
 */
 class Playing : public Screen {
 public:
-  explicit Playing(std::weak_ptr<database::Database> db,
+  explicit Playing(models::Playback& playback_model,
+                   std::weak_ptr<database::Database> db,
                   audio::TrackQueue& queue);
  ~Playing();

  auto Tick() -> void override;

-  // Callbacks invoked by the UI state machine in response to audio events.
-
-  auto OnTrackUpdate() -> void;
-  auto OnPlaybackUpdate(uint32_t, uint32_t) -> void;
-  auto OnQueueUpdate() -> void;
-
  auto OnFocusAboveFold() -> void;
  auto OnFocusBelowFold() -> void;

+  Playing(const Playing&) = delete;
+  Playing& operator=(const Playing&) = delete;
+
 private:
  auto control_button(lv_obj_t* parent, char* icon) -> lv_obj_t*;
  auto next_up_label(lv_obj_t* parent, const std::pmr::string& text)
      -> lv_obj_t*;

-  auto BindTrack(const database::Track& track) -> void;
-  auto ApplyNextUp(const std::vector<database::Track>& tracks) -> void;
-
  std::weak_ptr<database::Database> db_;
  audio::TrackQueue& queue_;

-  std::optional<database::Track> track_;
-  std::vector<database::Track> next_tracks_;
+  bindey::property<std::shared_ptr<database::Track>> current_track_;
+  bindey::property<std::vector<std::shared_ptr<database::Track>>> next_tracks_;

-  std::unique_ptr<database::FutureFetcher<std::optional<database::Track>>>
+  std::unique_ptr<database::FutureFetcher<std::shared_ptr<database::Track>>>
      new_track_;
  std::unique_ptr<
-      database::FutureFetcher<std::vector<std::optional<database::Track>>>>
+      database::FutureFetcher<std::vector<std::shared_ptr<database::Track>>>>
      new_next_tracks_;

-  lv_obj_t* artist_label_;
-  lv_obj_t* album_label_;
-  lv_obj_t* title_label_;
-
-  lv_obj_t* scrubber_;
-  lv_obj_t* play_pause_control_;
-
  lv_obj_t* next_up_header_;
  lv_obj_t* next_up_label_;
  lv_obj_t* next_up_hint_;
@@ -7,13 +7,16 @@
 #pragma once

 #include <stdint.h>
+#include <sys/_stdint.h>
 #include <memory>
 #include <stack>

 #include "audio_events.hpp"
 #include "battery.hpp"
+#include "bindey/property.h"
 #include "gpios.hpp"
 #include "lvgl_task.hpp"
+#include "model_playback.hpp"
 #include "nvs.hpp"
 #include "relative_wheel.hpp"
 #include "screen_playing.hpp"
@@ -27,6 +30,7 @@
 #include "storage.hpp"
 #include "system_events.hpp"
 #include "touchwheel.hpp"
+#include "track.hpp"
 #include "track_queue.hpp"
 #include "ui_events.hpp"
 #include "wheel_encoder.hpp"
@@ -49,11 +53,11 @@ class UiState : public tinyfsm::Fsm<UiState> {
  /* Fallback event handler. Does nothing. */
  void react(const tinyfsm::Event& ev) {}

-  virtual void react(const system_fsm::BatteryStateChanged&);
-  virtual void react(const audio::PlaybackStarted&);
-  virtual void react(const audio::PlaybackFinished&);
-  virtual void react(const audio::PlaybackUpdate&) {}
-  virtual void react(const audio::QueueUpdate&) {}
+  void react(const system_fsm::BatteryStateChanged&);
+  void react(const audio::PlaybackStarted&);
+  void react(const audio::PlaybackFinished&);
+  void react(const audio::PlaybackUpdate&);
+  void react(const audio::QueueUpdate&);

  virtual void react(const system_fsm::KeyLockChanged&);

@@ -88,6 +92,10 @@ class UiState : public tinyfsm::Fsm<UiState> {
  static std::stack<std::shared_ptr<Screen>> sScreens;
  static std::shared_ptr<Screen> sCurrentScreen;
  static std::shared_ptr<Modal> sCurrentModal;
+
+  static models::Playback sPlaybackModel;
+
+  static bindey::property<battery::Battery::BatteryState> sPropBatteryState;
 };

 namespace states {
@@ -96,7 +104,6 @@ class Splash : public UiState {
 public:
  void exit() override;
  void react(const system_fsm::BootComplete&) override;
-  void react(const system_fsm::BatteryStateChanged&) override{};
  using UiState::react;
 };

@@ -140,10 +147,6 @@ class Playing : public UiState {

  void react(const internal::BackPressed&) override;

-  void react(const audio::PlaybackStarted&) override;
-  void react(const audio::PlaybackUpdate&) override;
-  void react(const audio::PlaybackFinished&) override;
-  void react(const audio::QueueUpdate&) override;
  using UiState::react;
 };

@@ -9,6 +9,7 @@
 #include <memory>

 #include "audio_events.hpp"
+#include "bindey/binding.h"
 #include "core/lv_event.h"
 #include "core/lv_obj.h"
 #include "core/lv_obj_scroll.h"
@@ -35,6 +36,7 @@
 #include "misc/lv_area.h"
 #include "misc/lv_color.h"
 #include "misc/lv_txt.h"
+#include "model_playback.hpp"
 #include "track.hpp"
 #include "ui_events.hpp"
 #include "ui_fsm.hpp"
@@ -46,8 +48,6 @@
 namespace ui {
 namespace screens {

-static constexpr std::size_t kMaxUpcoming = 10;
-
 static void above_fold_focus_cb(lv_event_t* ev) {
  if (ev->user_data == NULL) {
    return;
@@ -64,10 +64,6 @@ static void below_fold_focus_cb(lv_event_t* ev) {
  instance->OnFocusBelowFold();
 }

-static void play_pause_cb(lv_event_t* ev) {
-  events::Audio().Dispatch(audio::TogglePlayPause{});
-}
-
 static lv_style_t scrubber_style;

 auto info_label(lv_obj_t* parent) -> lv_obj_t* {
@@ -105,13 +101,42 @@ auto Playing::next_up_label(lv_obj_t* parent, const std::pmr::string& text)
  return button;
 }

-Playing::Playing(std::weak_ptr<database::Database> db, audio::TrackQueue& queue)
+Playing::Playing(models::Playback& playback_model,
+                 std::weak_ptr<database::Database> db,
+                 audio::TrackQueue& queue)
    : db_(db),
      queue_(queue),
-      track_(),
+      current_track_(),
      next_tracks_(),
      new_track_(),
      new_next_tracks_() {
+  data_bindings_.emplace_back(playback_model.current_track.onChangedAndNow(
+      [=, this](const std::optional<database::TrackId>& id) {
+        if (!id) {
+          return;
+        }
+        if (current_track_.get() && current_track_.get()->data().id() == *id) {
+          return;
+        }
+        auto db = db_.lock();
+        if (!db) {
+          return;
+        }
+        new_track_.reset(
+            new database::FutureFetcher<std::shared_ptr<database::Track>>(
+                db->GetTrack(*id)));
+      }));
+  data_bindings_.emplace_back(playback_model.upcoming_tracks.onChangedAndNow(
+      [=, this](const std::vector<database::TrackId>& ids) {
+        auto db = db_.lock();
+        if (!db) {
+          return;
+        }
+        new_next_tracks_.reset(new database::FutureFetcher<
+                               std::vector<std::shared_ptr<database::Track>>>(
+            db->GetBulkTracks(ids)));
+      }));
+
  lv_obj_set_layout(content_, LV_LAYOUT_FLEX);
  lv_group_set_wrap(group_, false);

@@ -143,20 +168,40 @@ Playing::Playing(std::weak_ptr<database::Database> db, audio::TrackQueue& queue)
  lv_obj_set_flex_align(info_container, LV_FLEX_ALIGN_CENTER,
                        LV_FLEX_ALIGN_CENTER, LV_FLEX_ALIGN_CENTER);

-  artist_label_ = info_label(info_container);
-  album_label_ = info_label(info_container);
-  title_label_ = info_label(info_container);
+  lv_obj_t* artist_label = info_label(info_container);
+  lv_obj_t* album_label = info_label(info_container);
+  lv_obj_t* title_label = info_label(info_container);

-  scrubber_ = lv_slider_create(above_fold_container);
-  lv_obj_set_size(scrubber_, lv_pct(100), 5);
-  lv_slider_set_range(scrubber_, 0, 100);
-  lv_slider_set_value(scrubber_, 0, LV_ANIM_OFF);
+  data_bindings_.emplace_back(current_track_.onChangedAndNow(
+      [=](const std::shared_ptr<database::Track>& t) {
+        if (!t) {
+          return;
+        }
+        lv_label_set_text(
+            artist_label,
+            t->tags().at(database::Tag::kArtist).value_or("").c_str());
+        lv_label_set_text(
+            album_label,
+            t->tags().at(database::Tag::kAlbum).value_or("").c_str());
+        lv_label_set_text(title_label, t->TitleOrFilename().c_str());
+      }));
+
+  lv_obj_t* scrubber = lv_slider_create(above_fold_container);
+  lv_obj_set_size(scrubber, lv_pct(100), 5);

  lv_style_init(&scrubber_style);
  lv_style_set_bg_color(&scrubber_style, lv_color_black());
-  lv_obj_add_style(scrubber_, &scrubber_style, LV_PART_INDICATOR);
+  lv_obj_add_style(scrubber, &scrubber_style, LV_PART_INDICATOR);

-  lv_group_add_obj(group_, scrubber_);
+  lv_group_add_obj(group_, scrubber);
+
+  data_bindings_.emplace_back(
+      playback_model.current_track_duration.onChangedAndNow([=](uint32_t d) {
+        lv_slider_set_range(scrubber, 0, std::max<uint32_t>(1, d));
+      }));
+  data_bindings_.emplace_back(
+      playback_model.current_track_position.onChangedAndNow(
+          [=](uint32_t p) { lv_slider_set_value(scrubber, p, LV_ANIM_OFF); }));

  lv_obj_t* controls_container = lv_obj_create(above_fold_container);
  lv_obj_set_size(controls_container, lv_pct(100), 20);
@@ -164,15 +209,25 @@ Playing::Playing(std::weak_ptr<database::Database> db, audio::TrackQueue& queue)
  lv_obj_set_flex_align(controls_container, LV_FLEX_ALIGN_SPACE_EVENLY,
                        LV_FLEX_ALIGN_CENTER, LV_FLEX_ALIGN_CENTER);

-  play_pause_control_ = control_button(controls_container, LV_SYMBOL_PLAY);
-  lv_obj_add_event_cb(play_pause_control_, play_pause_cb, LV_EVENT_CLICKED,
-                      NULL);
-  lv_group_add_obj(group_, play_pause_control_);
+  lv_obj_t* play_pause_control =
+      control_button(controls_container, LV_SYMBOL_PLAY);
+  lv_group_add_obj(group_, play_pause_control);
+  lv_bind(play_pause_control, LV_EVENT_CLICKED, [=](lv_obj_t*) {
+    events::Audio().Dispatch(audio::TogglePlayPause{});
+  });
+
+  lv_obj_t* track_prev = control_button(controls_container, LV_SYMBOL_PREV);
+  lv_group_add_obj(group_, track_prev);
+  lv_bind(track_prev, LV_EVENT_CLICKED, [=](lv_obj_t*) { queue_.Previous(); });
+
+  lv_obj_t* track_next = control_button(controls_container, LV_SYMBOL_NEXT);
+  lv_group_add_obj(group_, track_next);
+  lv_bind(track_next, LV_EVENT_CLICKED, [=](lv_obj_t*) { queue_.Next(); });
+
+  lv_obj_t* shuffle = control_button(controls_container, LV_SYMBOL_SHUFFLE);
+  lv_group_add_obj(group_, shuffle);
+  // lv_bind(shuffle, LV_EVENT_CLICKED, [=](lv_obj_t*) { queue_ });

-  lv_group_add_obj(group_, control_button(controls_container, LV_SYMBOL_PREV));
-  lv_group_add_obj(group_, control_button(controls_container, LV_SYMBOL_NEXT));
-  lv_group_add_obj(group_,
-                   control_button(controls_container, LV_SYMBOL_SHUFFLE));
  lv_group_add_obj(group_, control_button(controls_container, LV_SYMBOL_LOOP));

  next_up_header_ = lv_obj_create(above_fold_container);
@@ -198,111 +253,56 @@ Playing::Playing(std::weak_ptr<database::Database> db, audio::TrackQueue& queue)
  lv_obj_set_flex_align(next_up_container_, LV_FLEX_ALIGN_START,
                        LV_FLEX_ALIGN_CENTER, LV_FLEX_ALIGN_CENTER);

-  OnTrackUpdate();
-  OnQueueUpdate();
+  data_bindings_.emplace_back(next_tracks_.onChangedAndNow(
+      [=](const std::vector<std::shared_ptr<database::Track>>& tracks) {
+        // TODO(jacqueline): Do a proper diff to maintain selection.
+        int children = lv_obj_get_child_cnt(next_up_container_);
+        while (children > 0) {
+          lv_obj_del(lv_obj_get_child(next_up_container_, 0));
+          children--;
+        }
+
+        if (tracks.empty()) {
+          lv_label_set_text(next_up_label_, "Nothing queued");
+          lv_label_set_text(next_up_hint_, "");
+          return;
+        } else {
+          lv_label_set_text(next_up_label_, "Next up");
+          lv_label_set_text(next_up_hint_, "");
+        }
+
+        for (const auto& track : tracks) {
+          lv_group_add_obj(group_, next_up_label(next_up_container_,
+                                                 track->TitleOrFilename()));
+        }
+      }));
 }

 Playing::~Playing() {}

-auto Playing::OnTrackUpdate() -> void {
-  auto current = queue_.GetCurrent();
-  if (!current) {
-    return;
-  }
-  if (track_ && track_->data().id() == *current) {
-    return;
-  }
-  auto db = db_.lock();
-  if (!db) {
-    return;
-  }
-  new_track_.reset(new database::FutureFetcher<std::optional<database::Track>>(
-      db->GetTrack(*current)));
-}
-
-auto Playing::OnPlaybackUpdate(uint32_t pos_seconds, uint32_t new_duration)
-    -> void {
-  if (!track_) {
-    return;
-  }
-  lv_slider_set_range(scrubber_, 0, new_duration);
-  lv_slider_set_value(scrubber_, pos_seconds, LV_ANIM_ON);
-}
-
-auto Playing::OnQueueUpdate() -> void {
-  OnTrackUpdate();
-  auto current = queue_.GetUpcoming(kMaxUpcoming);
-  auto db = db_.lock();
-  if (!db) {
-    return;
-  }
-  new_next_tracks_.reset(
-      new database::FutureFetcher<std::vector<std::optional<database::Track>>>(
-          db->GetBulkTracks(current)));
-}
-
 auto Playing::Tick() -> void {
  if (new_track_ && new_track_->Finished()) {
    auto res = new_track_->Result();
    new_track_.reset();
-    if (res && *res) {
-      BindTrack(**res);
+    if (res) {
+      current_track_(*res);
    }
  }
  if (new_next_tracks_ && new_next_tracks_->Finished()) {
    auto res = new_next_tracks_->Result();
    new_next_tracks_.reset();
    if (res) {
-      std::vector<database::Track> filtered;
+      std::vector<std::shared_ptr<database::Track>> filtered;
      for (const auto& t : *res) {
        if (t) {
-          filtered.push_back(*t);
+          filtered.push_back(t);
        }
      }
-      ApplyNextUp(filtered);
+      next_tracks_.set(filtered);
    }
  }
 }

-auto Playing::BindTrack(const database::Track& t) -> void {
-  track_ = t;
-
-  lv_label_set_text(artist_label_,
-                    t.tags().at(database::Tag::kArtist).value_or("").c_str());
-  lv_label_set_text(album_label_,
-                    t.tags().at(database::Tag::kAlbum).value_or("").c_str());
-  lv_label_set_text(title_label_, t.TitleOrFilename().c_str());
-
-  std::optional<int> duration = t.tags().duration;
-  lv_slider_set_range(scrubber_, 0, duration.value_or(1));
-  lv_slider_set_value(scrubber_, 0, LV_ANIM_OFF);
-}
-
-auto Playing::ApplyNextUp(const std::vector<database::Track>& tracks) -> void {
-  // TODO(jacqueline): Do a proper diff to maintain selection.
-  int children = lv_obj_get_child_cnt(next_up_container_);
-  while (children > 0) {
-    lv_obj_del(lv_obj_get_child(next_up_container_, 0));
-    children--;
-  }
-
-  next_tracks_ = tracks;
-
-  if (next_tracks_.empty()) {
-    lv_label_set_text(next_up_label_, "Nothing queued");
-    lv_label_set_text(next_up_hint_, "");
-    return;
-  } else {
-    lv_label_set_text(next_up_label_, "Next up");
-    lv_label_set_text(next_up_hint_, "");
-  }
-
-  for (const auto& track : next_tracks_) {
-    lv_group_add_obj(
-        group_, next_up_label(next_up_container_, track.TitleOrFilename()));
-  }
-}
-
 auto Playing::OnFocusAboveFold() -> void {
  lv_obj_scroll_to_y(content_, 0, LV_ANIM_ON);
 }
@@ -170,8 +170,8 @@ auto TrackBrowser::AddResults(
    initial_page_ = results;
  }

-  auto fn = [&](const database::IndexRecord& record) {
-    auto text = record.text();
+  auto fn = [&](const std::shared_ptr<database::IndexRecord>& record) {
+    auto text = record->text();
    if (!text) {
      // TODO(jacqueline): Display category-specific text.
      text = "[ no data ]";
@@ -19,6 +19,7 @@
 #include "gpios.hpp"
 #include "lvgl_task.hpp"
 #include "modal_confirm.hpp"
+#include "model_playback.hpp"
 #include "nvs.hpp"
 #include "relative_wheel.hpp"
 #include "screen.hpp"
@@ -52,6 +53,10 @@ std::stack<std::shared_ptr<Screen>> UiState::sScreens;
 std::shared_ptr<Screen> UiState::sCurrentScreen;
 std::shared_ptr<Modal> UiState::sCurrentModal;

+models::Playback UiState::sPlaybackModel;
+
+bindey::property<battery::Battery::BatteryState> UiState::sPropBatteryState;
+
 auto UiState::InitBootSplash(drivers::IGpios& gpios) -> bool {
  // Init LVGL first, since the display driver registers itself with LVGL.
  lv_init();
@@ -89,15 +94,33 @@ void UiState::react(const system_fsm::KeyLockChanged& ev) {
 }

 void UiState::react(const system_fsm::BatteryStateChanged&) {
-  UpdateTopBar();
+  if (!sServices) {
+    return;
+  }
+  auto state = sServices->battery().State();
+  if (state) {
+    sPropBatteryState.set(*state);
+  }
 }

 void UiState::react(const audio::PlaybackStarted&) {
-  UpdateTopBar();
+  sPlaybackModel.is_playing.set(true);
 }

 void UiState::react(const audio::PlaybackFinished&) {
-  UpdateTopBar();
+  sPlaybackModel.is_playing.set(false);
+}
+
+void UiState::react(const audio::PlaybackUpdate& ev) {
+  sPlaybackModel.current_track_duration.set(ev.seconds_total);
+  sPlaybackModel.current_track_position.set(ev.seconds_elapsed);
+}
+
+void UiState::react(const audio::QueueUpdate&) {
+  ESP_LOGI(kTag, "current changed!");
+  auto& queue = sServices->track_queue();
+  sPlaybackModel.current_track.set(queue.GetCurrent());
+  sPlaybackModel.upcoming_tracks.set(queue.GetUpcoming(10));
 }

 void UiState::UpdateTopBar() {
@@ -283,21 +306,22 @@ void Browse::react(const internal::RecordSelected& ev) {
  }

  auto record = ev.page->values().at(ev.record);
-  if (record.track()) {
-    ESP_LOGI(kTag, "selected track '%s'", record.text()->c_str());
+  if (record->track()) {
+    ESP_LOGI(kTag, "selected track '%s'", record->text()->c_str());
    auto& queue = sServices->track_queue();
    queue.Clear();
    queue.IncludeLast(std::make_shared<playlist::IndexRecordSource>(
        sServices->database(), ev.initial_page, 0, ev.page, ev.record));
+    ESP_LOGI(kTag, "transit to playing");
    transit<Playing>();
  } else {
-    ESP_LOGI(kTag, "selected record '%s'", record.text()->c_str());
-    auto cont = record.Expand(kRecordsPerPage);
+    ESP_LOGI(kTag, "selected record '%s'", record->text()->c_str());
+    auto cont = record->Expand(kRecordsPerPage);
    if (!cont) {
      return;
    }
    auto query = db->GetPage(&cont.value());
-    std::pmr::string title = record.text().value_or("TODO");
+    std::pmr::string title = record->text().value_or("TODO");
    PushScreen(std::make_shared<screens::TrackBrowser>(
        sServices->database(), title, std::move(query)));
  }
@@ -329,8 +353,9 @@ void Browse::react(const system_fsm::BluetoothDevicesChanged&) {
 static std::shared_ptr<screens::Playing> sPlayingScreen;

 void Playing::entry() {
-  sPlayingScreen.reset(
-      new screens::Playing(sServices->database(), sServices->track_queue()));
+  ESP_LOGI(kTag, "push playing screen");
+  sPlayingScreen.reset(new screens::Playing(
+      sPlaybackModel, sServices->database(), sServices->track_queue()));
  PushScreen(sPlayingScreen);
 }

@@ -339,24 +364,6 @@ void Playing::exit() {
  PopScreen();
 }

-void Playing::react(const audio::PlaybackStarted& ev) {
-  UpdateTopBar();
-  sPlayingScreen->OnTrackUpdate();
-}
-
-void Playing::react(const audio::PlaybackFinished& ev) {
-  UpdateTopBar();
-  sPlayingScreen->OnTrackUpdate();
-}
-
-void Playing::react(const audio::PlaybackUpdate& ev) {
-  sPlayingScreen->OnPlaybackUpdate(ev.seconds_elapsed, ev.seconds_total);
-}
-
-void Playing::react(const audio::QueueUpdate& ev) {
-  sPlayingScreen->OnQueueUpdate();
-}
-
 void Playing::react(const internal::BackPressed& ev) {
  transit<Browse>();
 }
@@ -25,6 +25,7 @@ list(APPEND EXTRA_COMPONENT_DIRS "$ENV{PROJ_PATH}/lib/result")
 list(APPEND EXTRA_COMPONENT_DIRS "$ENV{PROJ_PATH}/lib/shared_string")
 list(APPEND EXTRA_COMPONENT_DIRS "$ENV{PROJ_PATH}/lib/span")
 list(APPEND EXTRA_COMPONENT_DIRS "$ENV{PROJ_PATH}/lib/tinyfsm")
+list(APPEND EXTRA_COMPONENT_DIRS "$ENV{PROJ_PATH}/lib/bindey")

 include($ENV{IDF_PATH}/tools/cmake/project.cmake)