Respond to TextInputClient.reattach messages. (#43959)

packages/flutter/lib/src/services/system_channels.dart

@@ -159,6 +159,13 @@ class SystemChannels {
   ///    second argument is a [String] consisting of the stringification of one
   ///    of the values of the [TextInputAction] enum.
   ///
+  ///  * `TextInputClient.requestExistingInputState`: The embedding may have
+  ///    lost its internal state about the current editing client, if there is
+  ///    one. The framework should call `TextInput.setClient` and
+  ///    `TextInput.setEditingState` again with its most recent information. If
+  ///    there is no existing state on the framework side, the call should
+  ///    fizzle.
+  ///
   ///  * `TextInputClient.onConnectionClosed`: The text input connection closed
   ///    on the platform side. For example the application is moved to
   ///    background or used closed the virtual keyboard. This method informs

packages/flutter/lib/src/widgets/editable_text.dart

@@ -1395,7 +1395,8 @@ class EditableTextState extends State<EditableText> with AutomaticKeepAliveClien
     if (!_hasInputConnection) {
       final TextEditingValue localValue = _value;
       _lastKnownRemoteTextEditingValue = localValue;
-      _textInputConnection = TextInput.attach(this,
+      _textInputConnection = TextInput.attach(
+        this,
         TextInputConfiguration(
           inputType: widget.keyboardType,
           obscureText: widget.obscureText,

packages/flutter/test/services/text_input_test.dart

@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+import 'dart:convert' show utf8;
+
 import 'package:flutter/services.dart';
 import 'package:flutter_test/flutter_test.dart' show TestWidgetsFlutterBinding;
 import '../flutter_test_alternative.dart';
@@ -9,6 +11,66 @@ import '../flutter_test_alternative.dart';
 void main() {
   TestWidgetsFlutterBinding.ensureInitialized();
 
+  group('TextInput message channels', () {
+    FakeTextChannel fakeTextChannel;
+    FakeTextInputClient client;
+
+    setUp(() {
+      fakeTextChannel = FakeTextChannel((MethodCall call) async {});
+      TextInput.setChannel(fakeTextChannel);
+      client = FakeTextInputClient();
+    });
+
+    tearDown(() {
+      TextInputConnection.debugResetId();
+      TextInput.setChannel(SystemChannels.textInput);
+    });
+
+    test('text input client handler responds to reattach with setClient', () async {
+      TextInput.attach(client, client.configuration);
+      fakeTextChannel.validateOutgoingMethodCalls(<MethodCall>[
+        MethodCall('TextInput.setClient', <dynamic>[1, client.configuration.toJson()]),
+      ]);
+
+      fakeTextChannel.incoming(const MethodCall('TextInputClient.requestExistingInputState', null));
+
+      expect(fakeTextChannel.outgoingCalls.length, 2);
+      fakeTextChannel.validateOutgoingMethodCalls(<MethodCall>[
+        // From original attach
+        MethodCall('TextInput.setClient', <dynamic>[1, client.configuration.toJson()]),
+        // From requestExistingInputState
+        MethodCall('TextInput.setClient', <dynamic>[1, client.configuration.toJson()]),
+      ]);
+    });
+
+    test('text input client handler responds to reattach with setClient and text state', () async {
+      final TextInputConnection connection = TextInput.attach(client, client.configuration);
+      fakeTextChannel.validateOutgoingMethodCalls(<MethodCall>[
+        MethodCall('TextInput.setClient', <dynamic>[1, client.configuration.toJson()]),
+      ]);
+
+      const TextEditingValue editingState = TextEditingValue(text: 'foo');
+      connection.setEditingState(editingState);
+      fakeTextChannel.validateOutgoingMethodCalls(<MethodCall>[
+        MethodCall('TextInput.setClient', <dynamic>[1, client.configuration.toJson()]),
+        MethodCall('TextInput.setEditingState', editingState.toJSON()),
+      ]);
+
+      fakeTextChannel.incoming(const MethodCall('TextInputClient.requestExistingInputState', null));
+
+      expect(fakeTextChannel.outgoingCalls.length, 4);
+      fakeTextChannel.validateOutgoingMethodCalls(<MethodCall>[
+        // attach
+        MethodCall('TextInput.setClient', <dynamic>[1, client.configuration.toJson()]),
+        // set editing state 1
+        MethodCall('TextInput.setEditingState', editingState.toJSON()),
+        // both from requestExistingInputState
+        MethodCall('TextInput.setClient', <dynamic>[1, client.configuration.toJson()]),
+        MethodCall('TextInput.setEditingState', editingState.toJSON()),
+      ]);
+    });
+  });
+
   group('TextInputConfiguration', () {
     test('sets expected defaults', () {
       const TextInputConfiguration configuration = TextInputConfiguration();
@@ -113,7 +175,7 @@ void main() {
   });
 }
 
-class FakeTextInputClient extends TextInputClient {
+class FakeTextInputClient implements TextInputClient {
   String latestMethodCall = '';
 
   @override
@@ -135,4 +197,68 @@ class FakeTextInputClient extends TextInputClient {
   void connectionClosed() {
     latestMethodCall = 'connectionClosed';
   }
+
+  TextInputConfiguration get configuration => const TextInputConfiguration();
+}
+
+class FakeTextChannel implements MethodChannel {
+  FakeTextChannel(this.outgoing) : assert(outgoing != null);
+
+  Future<void> Function(MethodCall) outgoing;
+  Future<void> Function(MethodCall) incoming;
+
+  List<MethodCall> outgoingCalls = <MethodCall>[];
+
+  @override
+  BinaryMessenger get binaryMessenger => throw UnimplementedError();
+
+  @override
+  MethodCodec get codec => const JSONMethodCodec();
+
+  @override
+  Future<List<T>> invokeListMethod<T>(String method, [dynamic arguments]) => throw UnimplementedError();
+
+  @override
+  Future<Map<K, V>> invokeMapMethod<K, V>(String method, [dynamic arguments]) => throw UnimplementedError();
+
+  @override
+  Future<T> invokeMethod<T>(String method, [dynamic arguments]) {
+    final MethodCall call = MethodCall(method, arguments);
+    outgoingCalls.add(call);
+    return outgoing(call);
+  }
+
+  @override
+  String get name => 'flutter/textinput';
+
+
+  @override
+  void setMethodCallHandler(Future<void> Function(MethodCall call) handler) {
+    incoming = handler;
+  }
+
+  @override
+  void setMockMethodCallHandler(Future<void> Function(MethodCall call) handler)  => throw UnimplementedError();
+
+  void validateOutgoingMethodCalls(List<MethodCall> calls) {
+    expect(outgoingCalls.length, calls.length);
+    bool hasError = false;
+    for (int i = 0; i < calls.length; i++) {
+      final ByteData outgoingData = codec.encodeMethodCall(outgoingCalls[i]);
+      final ByteData expectedData = codec.encodeMethodCall(calls[i]);
+      final String outgoingString = utf8.decode(outgoingData.buffer.asUint8List());
+      final String expectedString = utf8.decode(expectedData.buffer.asUint8List());
+
+      if (outgoingString != expectedString) {
+        print(
+          'Index $i did not match:\n'
+          '  actual: ${outgoingCalls[i]}'
+          '  expected: ${calls[i]}');
+        hasError = true;
+      }
+    }
+    if (hasError) {
+      fail('Calls did not match.');
+    }
+  }
 }