log: Support reportErrorToUserBriefly.

The imports in `lib/log.dart` are for the code references in
reportErrorToUserBriefly's dart doc.

Author: gnprice

lib/log.dart

@@ -1,3 +1,6 @@
+import 'package:flutter/material.dart';
+
+import 'widgets/app.dart';
 
 /// Whether [debugLog] should do anything.
 ///
@@ -30,3 +33,19 @@ bool debugLog(String message) {
   }());
   return true;
 }
+
+/// Display an error message.
+///
+/// This shows a [SnackBar] containing the message after app startup,
+/// otherwise logs it to the console.
+///
+/// See also: [ZulipApp._reportErrorToUserBriefly]
+void Function(String message) reportErrorToUserBriefly = _defaultReportErrorToUserBriefly;
+
+void _defaultReportErrorToUserBriefly(String message) {
+  // If this callback is still in place, then the app's widget tree
+  // hasn't mounted yet even as far as the [Navigator].
+  // So there's not much we can do to tell the user;
+  // just log, in case the user is actually a developer watching the console.
+  assert(debugLog(message));
+}

lib/widgets/app.dart

@@ -5,6 +5,7 @@ import 'package:flutter/material.dart';
 import 'package:flutter/scheduler.dart';
 import 'package:flutter_gen/gen_l10n/zulip_localizations.dart';
 
+import '../log.dart';
 import '../model/localizations.dart';
 import '../model/narrow.dart';
 import 'about_zulip.dart';
@@ -92,9 +93,14 @@ class ZulipApp extends StatefulWidget {
   /// Useful in tests.
   final List<NavigatorObserver>? navigatorObservers;
 
+  static void _reportErrorToUserBriefly(String message) {
+    scaffoldMessenger?.showSnackBar(SnackBar(content: Text(message)));
+  }
+
   void _declareReady() {
     assert(navigatorKey.currentContext != null);
     _ready.value = true;
+    reportErrorToUserBriefly = _reportErrorToUserBriefly;
   }
 
   @override

test/widgets/app_test.dart

@@ -1,6 +1,7 @@
 import 'package:checks/checks.dart';
 import 'package:flutter/material.dart';
 import 'package:flutter_test/flutter_test.dart';
+import 'package:zulip/log.dart';
 import 'package:zulip/model/database.dart';
 import 'package:zulip/widgets/app.dart';
 import 'package:zulip/widgets/inbox.dart';
@@ -164,5 +165,21 @@ void main() {
       check(ZulipApp.scaffoldMessenger).isNotNull();
       check(ZulipApp.ready).value.isTrue();
     });
+
+    testWidgets('reportErrorToUserBriefly shows snack bar', (tester) async {
+      await tester.pumpWidget(const ZulipApp());
+
+      // Prior to app startup, reportErrorToUserBriefly only logs.
+      reportErrorToUserBriefly('test reportErrorToUserBriefly');
+      check(ZulipApp.ready).value.isFalse();
+      await tester.pump();
+      check(find.text('test reportErrorToUserBriefly').evaluate()).isEmpty();
+      check(ZulipApp.ready).value.isTrue();
+
+      // After app startup, reportErrorToUserBriefly displays a snack bar.
+      reportErrorToUserBriefly('test reportErrorToUserBriefly');
+      await tester.pump();
+      check(find.text('test reportErrorToUserBriefly').evaluate()).single;
+    });
   });
 }