compose_box: Send "stopped typing" notices on app lifecycle updates

This is a bonus feature that covers some cases that FocusNode doesn't
cover.  We send a "stopped typing" notice when the app loses focus or
become invisible.

An example of WidgetsBindingObserver can be found here:
  https://api.flutter.dev/flutter/widgets/WidgetsBindingObserver-class.html

Signed-off-by: Zixuan James Li <[email protected]>

Author: PIG208

lib/widgets/compose_box.dart

@@ -288,14 +288,34 @@ class _ContentInput extends StatefulWidget {
   State<_ContentInput> createState() => _ContentInputState();
 }
 
-class _ContentInputState extends State<_ContentInput> {
+class _ContentInputState extends State<_ContentInput> with WidgetsBindingObserver {
   String? _prevText;
 
   @override
   void initState() {
     super.initState();
     widget.controller.addListener(_contentChanged);
     widget.focusNode.addListener(_focusChanged);
+    // The focus node does not notify [AppLifecycleState] changes,
+    // so we also listen if the app is visible and in focus.
+    WidgetsBinding.instance.addObserver(this);
+  }
+
+  @override
+  void didChangeAppLifecycleState(AppLifecycleState state) {
+    super.didChangeAppLifecycleState(state);
+    switch (state) {
+      case AppLifecycleState.detached:
+      case AppLifecycleState.inactive:
+      case AppLifecycleState.hidden:
+      case AppLifecycleState.paused:
+        final store = PerAccountStoreWidget.of(context);
+        store.typingNotifier.stoppedComposing();
+      case AppLifecycleState.resumed:
+        // The app becoming visible and getting input focus doesn't
+        // necessarily mean that the user started typing, so do nothing.
+        break;
+    }
   }
 
   @override
@@ -313,6 +333,7 @@ class _ContentInputState extends State<_ContentInput> {
   void dispose() {
     widget.controller.removeListener(_contentChanged);
     widget.focusNode.removeListener(_focusChanged);
+    WidgetsBinding.instance.removeObserver(this);
     super.dispose();
   }
 

test/widgets/compose_box_test.dart

@@ -4,6 +4,7 @@ import 'package:checks/checks.dart';
 import 'package:file_picker/file_picker.dart';
 import 'package:flutter_checks/flutter_checks.dart';
 import 'package:http/http.dart' as http;
+import 'package:flutter/services.dart';
 import 'package:flutter/material.dart';
 import 'package:flutter_test/flutter_test.dart';
 import 'package:image_picker/image_picker.dart';
@@ -330,6 +331,26 @@ void main() {
       // [TextEditingController.selection].  Still, we expect no
       // "typing started" request if the text remains the same.
     });
+
+    testWidgets('unfocusing app stops typing notification', (tester) async {
+      Future<void> setAppLifeCycleState(AppLifecycleState state) async {
+        // While this state lives on [ServicesBinding], testWidgets resets it
+        // for us when the test ends:
+        //   https://github.com/flutter/flutter/blob/c78c166e3ecf963ca29ed503e710fd3c71eda5c9/packages/flutter_test/lib/src/binding.dart#L1189
+        final ByteData? message = const StringCodec().encodeMessage(state.toString());
+        await tester.binding.defaultBinaryMessenger
+          .handlePlatformMessage('flutter/lifecycle', message, (_) {});
+      }
+
+      await prepareComposeBox(tester, narrow: narrow, account: account);
+
+      await checkStartTyping(tester, narrow);
+
+      connection.prepare(json: {});
+      setAppLifeCycleState(AppLifecycleState.paused);
+      await tester.pump(Duration.zero);
+      checkTypingRequest(TypingOp.stop, narrow);
+    });
   });
 
   group('message-send request response', () {