store: Move PerAccountStoreWidget down into individual routes

Fixes: #21

Author: gnprice

lib/widgets/app.dart

@@ -3,6 +3,7 @@ import 'package:flutter/material.dart';
 import '../model/store.dart';
 import 'compose_box.dart';
 import 'message_list.dart';
+import 'page.dart';
 import 'store.dart';
 
 class ZulipApp extends StatelessWidget {
@@ -21,13 +22,13 @@ class ZulipApp extends StatelessWidget {
       //   https://m3.material.io/theme-builder#/custom
       colorScheme: ColorScheme.fromSeed(seedColor: kZulipBrandColor));
     return GlobalStoreWidget(
-      child: PerAccountStoreWidget(
-        // Just one account for now.
-        accountId: LiveGlobalStore.fixtureAccountId,
-        child: MaterialApp(
-          title: 'Zulip',
-          theme: theme,
-          home: const HomePage())));
+      child: MaterialApp(
+        title: 'Zulip',
+        theme: theme,
+        home: const PerAccountStoreWidget(
+          // Just one account for now.
+          accountId: LiveGlobalStore.fixtureAccountId,
+          child: HomePage())));
   }
 }
 
@@ -70,7 +71,7 @@ class HomePage extends StatelessWidget {
           const SizedBox(height: 16),
           ElevatedButton(
             onPressed: () => Navigator.push(context,
-              MaterialPageRoute(builder: (context) =>
+              MaterialAccountPageRoute(context: context, builder: (context) =>
                 const MessageListPage())),
             child: const Text("All messages")),
         ])));

lib/widgets/page.dart

@@ -0,0 +1,50 @@
+
+import 'package:flutter/material.dart';
+
+import 'store.dart';
+
+mixin AccountPageRouteMixin<T> on PageRoute<T> {
+  int get accountId;
+
+  @override
+  Widget buildPage(BuildContext context, Animation<double> animation, Animation<double> secondaryAnimation) {
+    return PerAccountStoreWidget(
+      accountId: accountId,
+      child: super.buildPage(context, animation, secondaryAnimation));
+  }
+}
+
+class MaterialAccountPageRoute<T> extends MaterialPageRoute<T> with AccountPageRouteMixin<T> {
+  MaterialAccountPageRoute({
+    required BuildContext context,
+    required super.builder,
+    super.settings,
+    super.maintainState,
+    super.fullscreenDialog,
+    super.allowSnapshotting,
+  }) : accountId = PerAccountStoreWidget.accountIdOf(context);
+
+  @override
+  final int accountId;
+}
+
+class AccountPageRouteBuilder<T> extends PageRouteBuilder<T> with AccountPageRouteMixin<T> {
+  AccountPageRouteBuilder({
+    required BuildContext context,
+    super.settings,
+    required super.pageBuilder,
+    super.transitionsBuilder,
+    super.transitionDuration,
+    super.reverseTransitionDuration,
+    super.opaque,
+    super.barrierDismissible,
+    super.barrierColor,
+    super.barrierLabel,
+    super.maintainState,
+    super.fullscreenDialog,
+    super.allowSnapshotting,
+  }) : accountId = PerAccountStoreWidget.accountIdOf(context);
+
+  @override
+  final int accountId;
+}

lib/widgets/store.dart

@@ -132,16 +132,38 @@ class PerAccountStoreWidget extends StatefulWidget {
   /// [BuildContext.dependOnInheritedWidgetOfExactType].
   ///
   /// See also:
-  ///  * [InheritedNotifier], which provides the "dependency" mechanism.
+  ///  * [accountIdOf], for the account ID corresponding to the same data.
   ///  * [GlobalStoreWidget.of], for the app's data beyond that of a
   ///    particular account.
+  ///  * [InheritedNotifier], which provides the "dependency" mechanism.
   static PerAccountStore of(BuildContext context) {
     final widget = context
         .dependOnInheritedWidgetOfExactType<_PerAccountStoreInheritedWidget>();
     assert(widget != null, 'No PerAccountStoreWidget ancestor');
     return widget!.store;
   }
 
+  /// Our account ID for the relevant account for this widget.
+  ///
+  /// As with [of], the data is taken from the closest [PerAccountStoreWidget]
+  /// that encloses the given context.  Throws an error if there is no enclosing
+  /// [PerAccountStoreWidget].
+  ///
+  /// Unlike [of], this method does not create a dependency relationship, and
+  /// updates to the [PerAccountStoreWidget] will not cause the calling widget
+  /// to be rebuilt.  As a result, this should not be called from build methods,
+  /// but is appropriate to use in interaction event handlers.  For more, see
+  /// [BuildContext.findAncestorWidgetOfExactType].
+  ///
+  /// Like [of], the cost of this method is O(1) with a small constant factor.
+  static int accountIdOf(BuildContext context) {
+    final element = context.getElementForInheritedWidgetOfExactType<_PerAccountStoreInheritedWidget>();
+    assert(element != null, 'No PerAccountStoreWidget ancestor');
+    final widget = element!.findAncestorWidgetOfExactType<PerAccountStoreWidget>();
+    assert(widget != null);
+    return widget!.accountId;
+  }
+
   @override
   State<PerAccountStoreWidget> createState() => _PerAccountStoreWidgetState();
 }