Load the database asynchronously

Now the backend has multiple states including loading and the toplevel
app widget was adapted accordingly.

lib/app.dart

@@ -6,12 +6,48 @@ import 'backend.dart';
 import 'screens/home.dart';
 import 'widgets/player_bar.dart';
 
-class App extends StatefulWidget {
+class App extends StatelessWidget {
   @override
-  _AppState createState() => _AppState();
+  Widget build(BuildContext context) {
+    final backend = Backend.of(context);
+
+    return MaterialApp(
+      title: 'Musicus',
+      theme: ThemeData(
+        brightness: Brightness.dark,
+        accentColor: Colors.amber,
+        textSelectionColor: Colors.grey[600],
+        cursorColor: Colors.amber,
+        textSelectionHandleColor: Colors.amber,
+        toggleableActiveColor: Colors.amber,
+        // Added for sliders and FABs. Not everything seems to obey this.
+        colorScheme: ColorScheme.dark(
+          primary: Colors.amber,
+          secondary: Colors.amber,
+        ),
+        fontFamily: 'Libertinus Sans',
+      ),
+      home: Builder(
+        builder: (context) {
+          if (backend.status == BackendStatus.loading) {
+            return Container(
+              color: Theme.of(context).scaffoldBackgroundColor,
+            );
+          } else {
+            return Content();
+          }
+        },
+      ),
+    );
+  }
 }
 
-class _AppState extends State<App> with SingleTickerProviderStateMixin {
+class Content extends StatefulWidget {
+  @override
+  _ContentState createState() => _ContentState();
+}
+
+class _ContentState extends State<Content> with SingleTickerProviderStateMixin {
   final nestedNavigator = GlobalKey<NavigatorState>();
 
   AnimationController playerBarAnimation;
@@ -45,27 +81,11 @@ class _AppState extends State<App> with SingleTickerProviderStateMixin {
 
   @override
   Widget build(BuildContext context) {
-    return MaterialApp(
-      title: 'Musicus',
-      theme: ThemeData(
-        brightness: Brightness.dark,
-        accentColor: Colors.amber,
-        textSelectionColor: Colors.grey[600],
-        cursorColor: Colors.amber,
-        textSelectionHandleColor: Colors.amber,
-        toggleableActiveColor: Colors.amber,
-        // Added for sliders and FABs. Not everything seems to obey this.
-        colorScheme: ColorScheme.dark(
-          primary: Colors.amber,
-          secondary: Colors.amber,
-        ),
-        fontFamily: 'Libertinus Sans',
-      ),
     // The nested Navigator is for every screen from which the player bar at
     // the bottom should be accessible. The WillPopScope widget intercepts
     // taps on the system back button and redirects them to the nested
     // navigator.
-      home: WillPopScope(
+    return WillPopScope(
       onWillPop: () async => !(await nestedNavigator.currentState.maybePop()),
       child: Scaffold(
         body: Navigator(
@@ -86,7 +106,6 @@ class _AppState extends State<App> with SingleTickerProviderStateMixin {
           child: PlayerBar(),
         ),
       ),
-      ),
     );
   }
 

lib/backend.dart

@@ -3,6 +3,11 @@ import 'package:rxdart/rxdart.dart';
 
 import 'database.dart';
 
+enum BackendStatus {
+  loading,
+  ready,
+}
+
 class Backend extends StatefulWidget {
   final Widget child;
 
@@ -18,12 +23,19 @@ class Backend extends StatefulWidget {
 }
 
 class BackendState extends State<Backend> {
-  final db = Database('musicus.sqlite');
-
   final playerActive = BehaviorSubject.seeded(false);
   final playing = BehaviorSubject.seeded(false);
   final position = BehaviorSubject.seeded(0.0);
 
+  Database db;
+  BackendStatus status = BackendStatus.loading;
+
+  @override
+  void initState() {
+    super.initState();
+    _load();
+  }
+
   @override
   Widget build(BuildContext context) {
     return _InheritedBackend(
@@ -32,6 +44,14 @@ class BackendState extends State<Backend> {
     );
   }
 
+  Future<void> _load() async {
+    db = Database('musicus.sqlite');
+
+    setState(() {
+      status = BackendStatus.ready;
+    });
+  }
+
   void startPlayer() {
     playerActive.add(true);
   }