app: Add ZulipApp.scaffoldMessenger

Also generalize ValueNotifierChecks to ValueListenableChecks.

We can't write checks-extensions for ZulipApp.scaffoldMessenger
or ZulipApp.ready, though, because Dart doesn't have extensions
for static members.  Apparently they're currently working on them:
  dart-lang/language#723

Co-authored-by: Zixuan James Li <[email protected]>

lib/widgets/app.dart

@@ -63,6 +63,22 @@ class ZulipApp extends StatefulWidget {
   /// to be mounted.
   static final navigatorKey = GlobalKey<NavigatorState>();
 
+  /// The [ScaffoldMessengerState] for the app.
+  ///
+  /// This is null during the app's early startup, while [ready] is still false.
+  ///
+  /// For code that exists entirely outside the widget tree and has no natural
+  /// [BuildContext] of its own, this enables controlling snack bars.
+  /// Where a relevant [BuildContext] does exist, prefer using that instead,
+  /// with [ScaffoldMessenger.of].
+  static ScaffoldMessengerState? get scaffoldMessenger {
+    final context = navigatorKey.currentContext;
+    if (context == null) return null;
+    // Not maybeOf; we use MaterialApp, which provides ScaffoldMessenger,
+    // so it's a bug if navigatorKey is mounted somewhere lacking that.
+    return ScaffoldMessenger.of(context);
+  }
+
   /// Reset the state of [ZulipApp] statics, for testing.
   ///
   /// TODO refactor this better, perhaps unify with ZulipBinding

test/flutter_checks.dart

@@ -2,6 +2,7 @@
 library;
 
 import 'package:checks/checks.dart';
+import 'package:flutter/foundation.dart';
 import 'package:flutter/material.dart';
 import 'package:flutter/services.dart';
 
@@ -51,7 +52,7 @@ extension RouteSettingsChecks<T> on Subject<RouteSettings> {
   Subject<Object?> get arguments => has((s) => s.arguments, 'arguments');
 }
 
-extension ValueNotifierChecks<T> on Subject<ValueNotifier<T>> {
+extension ValueListenableChecks<T> on Subject<ValueListenable<T>> {
   Subject<T> get value => has((c) => c.value, 'value');
 }
 

test/widgets/app_test.dart

@@ -157,4 +157,17 @@ void main() {
         ..bottom.isLessThan(2 / 3 * screenHeight);
     });
   });
+
+  group('scaffoldMessenger', () {
+    testWidgets('scaffoldMessenger becomes non-null after startup', (tester) async {
+      addTearDown(testBinding.reset);
+      await tester.pumpWidget(const ZulipApp());
+
+      check(ZulipApp.scaffoldMessenger).isNull();
+      check(ZulipApp.ready).value.isFalse();
+      await tester.pump();
+      check(ZulipApp.scaffoldMessenger).isNotNull();
+      check(ZulipApp.ready).value.isTrue();
+    });
+  });
 }