store [nfc]: Add and use PerAccountStoreAwareStateMixin

Author: chrisbobbe

lib/widgets/message_list.dart

@@ -107,20 +107,13 @@ class MessageList extends StatefulWidget {
   State<StatefulWidget> createState() => _MessageListState();
 }
 
-class _MessageListState extends State<MessageList> {
+class _MessageListState extends State<MessageList> with PerAccountStoreAwareStateMixin<MessageList> {
   MessageListView? model;
 
   @override
-  void didChangeDependencies() {
-    super.didChangeDependencies();
-    final store = PerAccountStoreWidget.of(context);
-    if (model != null && model!.store == store) {
-      // We already have a model, and it's for the right store.
-      return;
-    }
-    // Otherwise, set up the model.  Dispose of any old model.
+  void onNewStore() {
     model?.dispose();
-    _initModel(store);
+    _initModel(PerAccountStoreWidget.of(context));
   }
 
   @override

lib/widgets/store.dart

@@ -140,6 +140,9 @@ class PerAccountStoreWidget extends StatefulWidget {
   ///  * [GlobalStoreWidget.of], for the app's data beyond that of a
   ///    particular account.
   ///  * [InheritedNotifier], which provides the "dependency" mechanism.
+  // TODO(#185): Explain in dartdoc that the returned [PerAccountStore] might
+  //   differ from one call to the next, and to handle that with
+  //   [PerAccountStoreAwareStateMixin].
   static PerAccountStore of(BuildContext context) {
     final widget = context.dependOnInheritedWidgetOfExactType<_PerAccountStoreInheritedWidget>();
     assert(widget != null, 'No PerAccountStoreWidget ancestor');
@@ -246,3 +249,36 @@ class LoadingPage extends StatelessWidget {
     return const Center(child: CircularProgressIndicator());
   }
 }
+
+/// A [State] that uses the ambient [PerAccountStore].
+///
+/// The ambient [PerAccountStore] can be replaced in some circumstances,
+/// such as when an event queue expires. See [PerAccountStoreWidget.of].
+/// When that happens, stateful widgets should
+/// - remove listeners on the old [PerAccountStore], and
+/// - add listeners on the new one.
+/// Use this mixin, overriding [onNewStore], to do that concisely.
+// TODO(#185): Until #185, I think [PerAccountStoreWidget.of] never actually
+//   returns a different [PerAccountStore] from one call to the next.
+//   But it will, and when it does, we want our [StatefulWidgets] to handle it.
+mixin PerAccountStoreAwareStateMixin<T extends StatefulWidget> on State<T> {
+  PerAccountStore? _store;
+
+  /// Called in the first [State.didChangeDependencies] call, and afterward
+  /// when [PerAccountStoreWidget.of] returns a different store than last time.
+  ///
+  /// In this, remove any listeners on the old store
+  /// and add them on the new store.
+  void onNewStore();
+
+  @override
+  void didChangeDependencies() {
+    super.didChangeDependencies();
+
+    final storeNow = PerAccountStoreWidget.of(context);
+    if (_store != storeNow) {
+      onNewStore();
+    }
+    _store = storeNow;
+  }
+}