store [nfc]: Introduce DataBinding

This indirection will be useful as a customization point
for use in tests.

This provides the core of zulip#30: uses of PerAccountStoreWidget are now
testable, in that subsequent commits in this series will start writing
such tests and won't require any further changes in lib/, only test/.

lib/main.dart

@@ -3,6 +3,7 @@ import 'package:flutter/material.dart';
 
 import 'licenses.dart';
 import 'log.dart';
+import 'model/binding.dart';
 import 'widgets/app.dart';
 
 void main() {
@@ -11,5 +12,6 @@ void main() {
     return true;
   }());
   LicenseRegistry.addLicense(additionalLicenses);
+  LiveDataBinding.ensureInitialized();
   runApp(const ZulipApp());
 }

lib/model/binding.dart

@@ -0,0 +1,81 @@
+import 'package:flutter/foundation.dart';
+
+import '../widgets/store.dart';
+import 'store.dart';
+
+/// A singleton service providing the app's data.
+///
+/// Only one instance will be constructed in the lifetime of the app,
+/// by calling the `ensureInitialized` static method on a subclass.
+/// This instance can be accessed as [instance].
+///
+/// Most code should not interact with the bindings directly.
+/// Instead, use the corresponding higher-level APIs that expose the bindings'
+/// functionality in a widget-oriented way.
+///
+/// This piece of architecture is modelled on the "binding" classes in Flutter
+/// itself.  For discussion, see [BindingBase], [WidgetsFlutterBinding], and
+/// [TestWidgetsFlutterBinding].
+/// This version is simplified because we don't (yet?) have enough complexity
+/// to put into these bindings to need to use mixins to split them up.
+abstract class DataBinding {
+  DataBinding() {
+    assert(_instance == null);
+    initInstance();
+  }
+
+  /// The single instance of [DataBinding].
+  static DataBinding get instance => checkInstance(_instance);
+  static DataBinding? _instance;
+
+  static T checkInstance<T extends DataBinding>(T? instance) {
+    assert(() {
+      if (instance == null) {
+        throw FlutterError.fromParts([
+          ErrorSummary('Zulip binding has not yet been initialized.'),
+          ErrorHint(
+            'In the app, this is done by the `LiveDataBinding.ensureInitialized()` call '
+            'in the `void main()` method.',
+          ),
+        ]);
+      }
+      return true;
+    }());
+    return instance!;
+  }
+
+  @protected
+  @mustCallSuper
+  void initInstance() {
+    _instance = this;
+  }
+
+  /// Prepare the app's [GlobalStore], loading the necessary data.
+  ///
+  /// Generally the app should call this function only once.
+  ///
+  /// This is part of the implementation of [GlobalStoreWidget].
+  /// In application code, use [GlobalStoreWidget.of] to get access
+  /// to a [GlobalStore].
+  Future<GlobalStore> loadGlobalStore();
+}
+
+/// A concrete binding for use in the live application.
+///
+/// The global store returned by [loadGlobalStore], and consequently by
+/// [GlobalStoreWidget.of] in application code, will be a [LiveGlobalStore].
+/// It therefore uses a live server and live, persistent local database.
+class LiveDataBinding extends DataBinding {
+  /// Initialize the binding if necessary, and ensure it is a [LiveDataBinding].
+  static LiveDataBinding ensureInitialized() {
+    if (DataBinding._instance == null) {
+      LiveDataBinding();
+    }
+    return DataBinding.instance as LiveDataBinding;
+  }
+
+  @override
+  Future<GlobalStore> loadGlobalStore() {
+    return LiveGlobalStore.load();
+  }
+}

lib/widgets/store.dart

@@ -1,5 +1,6 @@
 import 'package:flutter/material.dart';
 
+import '../model/binding.dart';
 import '../model/store.dart';
 
 /// Provides access to the app's data.
@@ -54,7 +55,7 @@ class _GlobalStoreWidgetState extends State<GlobalStoreWidget> {
   void initState() {
     super.initState();
     (() async {
-      final store = await LiveGlobalStore.load();
+      final store = await DataBinding.instance.loadGlobalStore();
       setState(() {
         this.store = store;
       });