Load fonts asynchronously on a bg task

This saves a second or two from bootup; AFAICT this *mostly* reclaims
the dynamic fonts boot time regression.

lua/font.lua

@@ -1,6 +1,36 @@
 local lvgl = require("lvgl")
 
-return {
-  fusion_12 = lvgl.Font("//lua/fonts/fusion12"),
-  fusion_10 = lvgl.Font("//lua/fonts/fusion10"),
+local fonts = {}
+local fonts_priv = {
+  has_invoked_cb = false,
+  cb = nil,
 }
+
+function fonts_priv.has_loaded_all()
+  return fonts.fusion_12 and fonts.fusion_10
+end
+
+function fonts_priv.invoke_cb()
+  if fonts_priv.has_invoked_cb or not fonts_priv.cb then return end
+  if not fonts_priv.has_loaded_all() then return end
+  fonts_priv.has_invoked_cb = true
+  fonts_priv.cb()
+end
+
+lvgl.Font("//lua/fonts/fusion12", function(font)
+  fonts.fusion_12 = font
+  fonts_priv.invoke_cb()
+end)
+
+lvgl.Font("//lua/fonts/fusion10", function(font)
+  fonts.fusion_10 = font
+  fonts_priv.invoke_cb()
+end)
+
+function fonts.on_loaded(cb)
+  fonts_priv.cb = cb
+  fonts_priv.has_invoked_cb = false
+  fonts_priv.invoke_cb()
+end
+
+return fonts

lua/main.lua

@@ -1,4 +1,10 @@
+-- Load fonts first, since they're parsed asynchronously and we can do much of
+-- the other UI setup in parallel.
 local font = require("font")
+
+-- require() everything else needed for the main menu + global bindings. Do
+-- this now instead of in init_ui because loading and parsing the scripts can
+-- take a while.
 local vol = require("volume")
 local theme = require("theme")
 local controls = require("controls")
@@ -7,55 +13,66 @@ local sd_card = require("sd_card")
 local backstack = require("backstack")
 local main_menu = require("main_menu")
 
-local theme_dark = require("theme_dark")
-theme.set(theme_dark)
+local function init_ui()
+  -- Load the theme within init_ui because the theme needs fonts to be ready.
+  local theme_dark = require("theme_dark")
+  theme.set(theme_dark)
 
-local lock_time = time.ticks()
+  local lock_time = time.ticks()
 
--- Set up property bindings that are used across every screen.
-GLOBAL_BINDINGS = {
-  -- Show an alert with the current volume whenever the volume changes
-  vol.current_pct:bind(function(pct)
-    require("alerts").show(function()
-      local container = lvgl.Object(nil, {
-        w = lvgl.PCT(80),
-        h = lvgl.SIZE_CONTENT,
-        flex = {
-          flex_direction = "column",
-          justify_content = "center",
-          align_items = "center",
-          align_content = "center",
-        },
-        radius = 8,
-        pad_all = 2,
-      })
-      theme.set_style(container, "pop_up")
-      container:Label {
-        text = string.format("Volume %i%%", pct),
-        text_font = font.fusion_10
-      }
-      container:Bar {
-        w = lvgl.PCT(100),
-        h = 8,
-        range = { min = 0, max = 100 },
-        value = pct,
-      }
-      container:center()
-    end)
-  end),
-  -- When the device has been locked for a while, default to showing the now
-  -- playing screen after unlocking.
-  controls.lock_switch:bind(function(locked)
-    if locked then
-      lock_time = time.ticks()
-    elseif time.ticks() - lock_time > 8000 then
-      local queue = require("queue")
-      if queue.size:get() > 0 then
-        require("playing"):pushIfNotShown()
+  -- Set up property bindings that are used across every screen.
+  GLOBAL_BINDINGS = {
+    -- Show an alert with the current volume whenever the volume changes
+    vol.current_pct:bind(function(pct)
+      require("alerts").show(function()
+        local container = lvgl.Object(nil, {
+          w = lvgl.PCT(80),
+          h = lvgl.SIZE_CONTENT,
+          flex = {
+            flex_direction = "column",
+            justify_content = "center",
+            align_items = "center",
+            align_content = "center",
+          },
+          radius = 8,
+          pad_all = 2,
+        })
+        theme.set_style(container, "pop_up")
+        container:Label {
+          text = string.format("Volume %i%%", pct),
+          text_font = font.fusion_10
+        }
+        container:Bar {
+          w = lvgl.PCT(100),
+          h = 8,
+          range = { min = 0, max = 100 },
+          value = pct,
+        }
+        container:center()
+      end)
+    end),
+    -- When the device has been locked for a while, default to showing the now
+    -- playing screen after unlocking.
+    controls.lock_switch:bind(function(locked)
+      if locked then
+        lock_time = time.ticks()
+      elseif time.ticks() - lock_time > 8000 then
+        local queue = require("queue")
+        if queue.size:get() > 0 then
+          require("playing"):pushIfNotShown()
+        end
       end
-    end
-  end),
-  sd_card.mounted:bind(function(mounted)
-    backstack.reset(main_menu:new())
-  end),
-}
+    end),
+    sd_card.mounted:bind(function(mounted)
+      backstack.reset(main_menu:new())
+    end),
+  }
+end
+
+-- Wait for fonts to finish, then show the main menu.
+-- We could show an intermediary Lua-controlled splash/loading UI whilst we
+-- wait, but in practice loading the fonts takes only a few hundred ms longer
+-- than all the other UI init.
+font.on_loaded(function()
+  init_ui()
+end)