Handle the loading state whilst appending many tracks better

1) Update the queue length periodically so that the user can see we're
   working
2) Clear any previous track and display "loading..." instead

lua/playing.lua

@@ -226,7 +226,18 @@ return screen:new {
         end
       end),
       playback.track:bind(function(track)
-        if not track then return end
+        if not track then
+          if queue.loading:get() then
+            title:set { text = "Loading..." }
+          else
+            title:set { text = "" }
+          end
+          artist:set { text = "" }
+          cur_time:set { text = format_time(0) }
+          end_time:set { text = format_time(0) }
+          scrubber:set { value = 0 }
+          return
+        end
         if track.duration then
           end_time:set { text = format_time(track.duration) }
         else

src/tangara/audio/audio_events.hpp

@@ -103,6 +103,7 @@ struct QueueUpdate : tinyfsm::Event {
     kRepeatingLastTrack,
     kTrackFinished,
     kDeserialised,
+    kBulkLoadingUpdate,
   };
   Reason reason;
 };

src/tangara/audio/audio_fsm.cpp

@@ -112,10 +112,13 @@ void AudioState::react(const QueueUpdate& ev) {
         cmd.new_track = std::monostate{};
       }
       break;
+    case QueueUpdate::kBulkLoadingUpdate:
+      // Bulk loading updates are informational only; a separate QueueUpdate
+      // event will be sent when loading is done.
     case QueueUpdate::kDeserialised:
-    default:
       // The current track is deserialised separately in order to retain seek
       // position.
+    default:
       return;
   }
 

src/tangara/audio/track_queue.cpp

@@ -26,6 +26,7 @@
 #include "database/track.hpp"
 #include "events/event_queue.hpp"
 #include "memory_resource.hpp"
+#include "random.hpp"
 #include "tasks.hpp"
 #include "track_queue.hpp"
 #include "ui/ui_fsm.hpp"
@@ -190,6 +191,8 @@ auto TrackQueue::append(Item i) -> void {
     // doesn't block.
     bg_worker_.Dispatch<void>([=, this]() {
       database::TrackIterator it = std::get<database::TrackIterator>(i);
+
+      size_t next_update_at = 10;
       while (true) {
         auto next = *it;
         if (!next) {
@@ -205,7 +208,16 @@ auto TrackQueue::append(Item i) -> void {
           }
         }
         it++;
+
+        // Appending very large iterators can take a while. Send out periodic
+        // queue updates during them so that the user has an idea what's going
+        // on.
+        if (!--next_update_at) {
+          next_update_at = util::sRandom->RangeInclusive(10, 20);
+          notifyChanged(false, Reason::kBulkLoadingUpdate);
+        }
       }
+
       {
         const std::unique_lock<std::shared_mutex> lock(mutex_);
         updateShuffler(was_queue_empty);

src/tangara/ui/ui_fsm.cpp

@@ -225,6 +225,7 @@ lua::Property UiState::sQueueRandom{false, [](const lua::LuaValue& val) {
                                       sServices->track_queue().random(new_val);
                                       return true;
                                     }};
+lua::Property UiState::sQueueLoading{false};
 
 lua::Property UiState::sVolumeCurrentPct{
     0, [](const lua::LuaValue& val) {
@@ -424,7 +425,7 @@ void UiState::react(const system_fsm::BatteryStateChanged& ev) {
   }
 }
 
-void UiState::react(const audio::QueueUpdate&) {
+void UiState::react(const audio::QueueUpdate& update) {
   auto& queue = sServices->track_queue();
   auto queue_size = queue.totalSize();
   sQueueSize.setDirect(static_cast<int>(queue_size));
@@ -439,6 +440,12 @@ void UiState::react(const audio::QueueUpdate&) {
   sQueueRandom.setDirect(queue.random());
   sQueueRepeat.setDirect(queue.repeat());
   sQueueReplay.setDirect(queue.replay());
+
+  if (update.reason == audio::QueueUpdate::Reason::kBulkLoadingUpdate) {
+    sQueueLoading.setDirect(true);
+  } else {
+    sQueueLoading.setDirect(false);
+  }
 }
 
 void UiState::react(const audio::PlaybackUpdate& ev) {
@@ -614,6 +621,7 @@ void Lua::entry() {
             {"replay", &sQueueReplay},
             {"repeat_track", &sQueueRepeat},
             {"random", &sQueueRandom},
+            {"loading", &sQueueLoading},
         });
     registry.AddPropertyModule("volume",
                                {

src/tangara/ui/ui_fsm.hpp

@@ -122,6 +122,7 @@ class UiState : public tinyfsm::Fsm<UiState> {
   static lua::Property sQueueReplay;
   static lua::Property sQueueRepeat;
   static lua::Property sQueueRandom;
+  static lua::Property sQueueLoading;
 
   static lua::Property sVolumeCurrentPct;
   static lua::Property sVolumeCurrentDb;