Revert "Revert "Scribble mixin (#104128)" (#114647)" (#114698)

Relands the "Scribble mixin" PR, which was reverted due to breaking a Google test in the last roll.
Breaks the Scribble feature out of TextInputClient in order to avoid breaking changes.

packages/flutter/lib/services.dart

@@ -37,6 +37,7 @@ export 'src/services/raw_keyboard_macos.dart';
 export 'src/services/raw_keyboard_web.dart';
 export 'src/services/raw_keyboard_windows.dart';
 export 'src/services/restoration.dart';
+export 'src/services/scribble.dart';
 export 'src/services/service_extensions.dart';
 export 'src/services/spell_check.dart';
 export 'src/services/system_channels.dart';

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

@@ -15,9 +15,9 @@ import 'binary_messenger.dart';
 import 'hardware_keyboard.dart';
 import 'message_codec.dart';
 import 'restoration.dart';
+import 'scribble.dart';
 import 'service_extensions.dart';
 import 'system_channels.dart';
-import 'text_input.dart';
 
 export 'dart:ui' show ChannelBuffers, RootIsolateToken;
 
@@ -43,7 +43,7 @@ mixin ServicesBinding on BindingBase, SchedulerBinding {
     SystemChannels.system.setMessageHandler((dynamic message) => handleSystemMessage(message as Object));
     SystemChannels.lifecycle.setMessageHandler(_handleLifecycleMessage);
     SystemChannels.platform.setMethodCallHandler(_handlePlatformMessage);
-    TextInput.ensureInitialized();
+    Scribble.ensureInitialized();
     readInitialLifecycleStateFromNativeWindow();
   }
 
@@ -326,7 +326,6 @@ mixin ServicesBinding on BindingBase, SchedulerBinding {
   void setSystemUiChangeCallback(SystemUiChangeCallback? callback) {
     _systemUiChangeCallback = callback;
   }
-
 }
 
 /// Signature for listening to changes in the [SystemUiMode].

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

@@ -0,0 +1,243 @@
+// Copyright 2014 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'dart:ui';
+
+import 'package:flutter/foundation.dart';
+
+import 'message_codec.dart';
+import 'platform_channel.dart';
+import 'system_channels.dart';
+
+/// An interface into system-level handwriting text input.
+///
+/// This is typically used by implemeting the methods in [ScribbleClient] in a
+/// class, usually a [State], and setting an instance of it to [client]. The
+/// relevant methods on [ScribbleClient] will be called in response to method
+/// channel calls on [SystemChannels.scribble].
+///
+/// Currently, handwriting input is supported in the iOS embedder with the Apple
+/// Pencil.
+///
+/// [EditableText] uses this class via [ScribbleClient] to automatically support
+/// handwriting input when [EditableText.scribbleEnabled] is set to true.
+///
+/// See also:
+///
+///  * [SystemChannels.scribble], which is the [MethodChannel] used by this
+///    class, and which has a list of the methods that this class handles.
+class Scribble {
+  Scribble._() {
+    _channel.setMethodCallHandler(_handleScribbleInvocation);
+  }
+
+  /// Ensure that a [Scribble] instance has been set up so that the platform
+  /// can handle messages on the scribble method channel.
+  static void ensureInitialized() {
+    _instance; // ignore: unnecessary_statements
+  }
+
+  /// Set the [MethodChannel] used to communicate with the system's text input
+  /// control.
+  ///
+  /// This is only meant for testing within the Flutter SDK. Changing this
+  /// will break the ability to do handwriting input. This has no effect if
+  /// asserts are disabled.
+  @visibleForTesting
+  static void setChannel(MethodChannel newChannel) {
+    assert(() {
+      _instance._channel = newChannel..setMethodCallHandler(_instance._handleScribbleInvocation);
+      return true;
+    }());
+  }
+
+  static final Scribble _instance = Scribble._();
+
+  /// Set the given [ScribbleClient] as the single active client.
+  ///
+  /// This is usually based on the [ScribbleClient] receiving focus.
+  static set client(ScribbleClient? client) {
+    _instance._client = client;
+  }
+
+  /// Return the current active [ScribbleClient], or null if none.
+  static ScribbleClient? get client => _instance._client;
+
+  ScribbleClient? _client;
+
+  MethodChannel _channel = SystemChannels.scribble;
+
+  final Map<String, ScribbleClient> _scribbleClients = <String, ScribbleClient>{};
+  bool _scribbleInProgress = false;
+
+  /// Used for testing within the Flutter SDK to get the currently registered [ScribbleClient] list.
+  @visibleForTesting
+  static Map<String, ScribbleClient> get scribbleClients => Scribble._instance._scribbleClients;
+
+  /// Returns true if a scribble interaction is currently happening.
+  static bool get scribbleInProgress => _instance._scribbleInProgress;
+
+  Future<dynamic> _handleScribbleInvocation(MethodCall methodCall) async {
+    final String method = methodCall.method;
+    if (method == 'Scribble.focusElement') {
+      final List<dynamic> args = methodCall.arguments as List<dynamic>;
+      _scribbleClients[args[0]]?.onScribbleFocus(Offset((args[1] as num).toDouble(), (args[2] as num).toDouble()));
+      return;
+    } else if (method == 'Scribble.requestElementsInRect') {
+      final List<double> args = (methodCall.arguments as List<dynamic>).cast<num>().map<double>((num value) => value.toDouble()).toList();
+      return _scribbleClients.keys.where((String elementIdentifier) {
+        final Rect rect = Rect.fromLTWH(args[0], args[1], args[2], args[3]);
+        if (!(_scribbleClients[elementIdentifier]?.isInScribbleRect(rect) ?? false)) {
+          return false;
+        }
+        final Rect bounds = _scribbleClients[elementIdentifier]?.bounds ?? Rect.zero;
+        return !(bounds == Rect.zero || bounds.hasNaN || bounds.isInfinite);
+      }).map((String elementIdentifier) {
+        final Rect bounds = _scribbleClients[elementIdentifier]!.bounds;
+        return <dynamic>[elementIdentifier, ...<dynamic>[bounds.left, bounds.top, bounds.width, bounds.height]];
+      }).toList();
+    } else if (method == 'Scribble.scribbleInteractionBegan') {
+      _scribbleInProgress = true;
+      return;
+    } else if (method == 'Scribble.scribbleInteractionFinished') {
+      _scribbleInProgress = false;
+      return;
+    }
+
+    // The methods below are only valid when a client exists, i.e. when a field
+    // is focused.
+    final ScribbleClient? client = _client;
+    if (client == null) {
+      return;
+    }
+
+    final List<dynamic> args = methodCall.arguments as List<dynamic>;
+    switch (method) {
+      case 'Scribble.showToolbar':
+        client.showToolbar();
+        break;
+      case 'Scribble.insertTextPlaceholder':
+        client.insertTextPlaceholder(Size((args[1] as num).toDouble(), (args[2] as num).toDouble()));
+        break;
+      case 'Scribble.removeTextPlaceholder':
+        client.removeTextPlaceholder();
+        break;
+      default:
+        throw MissingPluginException();
+    }
+  }
+
+  /// Registers a [ScribbleClient] with [elementIdentifier] that can be focused
+  /// by the engine.
+  ///
+  /// For example, the registered [ScribbleClient] list is used to respond to
+  /// UIIndirectScribbleInteraction on an iPad.
+  static void registerScribbleElement(String elementIdentifier, ScribbleClient scribbleClient) {
+    _instance._scribbleClients[elementIdentifier] = scribbleClient;
+  }
+
+  /// Unregisters a [ScribbleClient] with [elementIdentifier].
+  static void unregisterScribbleElement(String elementIdentifier) {
+    _instance._scribbleClients.remove(elementIdentifier);
+  }
+
+  List<SelectionRect> _cachedSelectionRects = <SelectionRect>[];
+
+  /// Send the bounding boxes of the current selected glyphs in the client to
+  /// the platform's text input plugin.
+  ///
+  /// These are used by the engine during a UIDirectScribbleInteraction.
+  static void setSelectionRects(List<SelectionRect> selectionRects) {
+    if (!listEquals(_instance._cachedSelectionRects, selectionRects)) {
+      _instance._cachedSelectionRects = selectionRects;
+      _instance._channel.invokeMethod<void>(
+        'Scribble.setSelectionRects',
+        selectionRects.map((SelectionRect rect) {
+          return <num>[rect.bounds.left, rect.bounds.top, rect.bounds.width, rect.bounds.height, rect.position];
+        }).toList(),
+      );
+    }
+  }
+}
+
+/// An interface to interact with the engine for handwriting text input.
+///
+/// This is currently only used to handle
+/// [UIIndirectScribbleInteraction](https://developer.apple.com/documentation/uikit/uiindirectscribbleinteraction),
+/// which is responsible for manually receiving handwritten text input in UIKit.
+/// The Flutter engine uses this to receive handwriting input on Flutter text
+/// input fields.
+mixin ScribbleClient {
+  /// A unique identifier for this element.
+  String get elementIdentifier;
+
+  /// Called by the engine when the [ScribbleClient] should receive focus.
+  ///
+  /// For example, this method is called during a UIIndirectScribbleInteraction.
+  ///
+  /// The [Offset] indicates the location where the focus event happened, which
+  /// is typically where the cursor should be placed.
+  void onScribbleFocus(Offset offset);
+
+  /// Tests whether the [ScribbleClient] overlaps the given rectangle bounds,
+  /// where the rectangle bounds are in global coordinates.
+  bool isInScribbleRect(Rect rect);
+
+  /// The current bounds of the [ScribbleClient].
+  Rect get bounds;
+
+  /// Requests that the client show the editing toolbar.
+  ///
+  /// This is used when the platform changes the selection during scribble
+  /// input.
+  void showToolbar();
+
+  /// Requests that the client add a text placeholder to reserve visual space
+  /// in the text.
+  ///
+  /// For example, this is called when responding to UIKit requesting
+  /// a text placeholder be added at the current selection, such as when
+  /// requesting additional writing space with iPadOS14 Scribble.
+  void insertTextPlaceholder(Size size);
+
+  /// Requests that the client remove the text placeholder.
+  void removeTextPlaceholder();
+}
+
+/// Represents a selection rect for a character and it's position in the text.
+///
+/// This is used to report the current text selection rect and position data
+/// to the engine for Scribble support on iPadOS 14.
+@immutable
+class SelectionRect {
+  /// Constructor for creating a [SelectionRect] from a text [position] and
+  /// [bounds].
+  const SelectionRect({required this.position, required this.bounds});
+
+  /// The position of this selection rect within the text String.
+  final int position;
+
+  /// The rectangle representing the bounds of this selection rect within the
+  /// currently focused [RenderEditable]'s coordinate space.
+  final Rect bounds;
+
+  @override
+  bool operator ==(Object other) {
+    if (identical(this, other)) {
+      return true;
+    }
+    if (runtimeType != other.runtimeType) {
+      return false;
+    }
+    return other is SelectionRect
+        && other.position == position
+        && other.bounds == bounds;
+  }
+
+  @override
+  int get hashCode => Object.hash(position, bounds);
+
+  @override
+  String toString() => 'SelectionRect($position, $bounds)';
+}

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

@@ -222,6 +222,38 @@ class SystemChannels {
       JSONMethodCodec(),
   );
 
+  /// A JSON [MethodChannel] for handling handwriting input.
+  ///
+  /// This method channel is used by iPadOS 14's Scribble feature where writing
+  /// with an Apple Pencil on top of a text field inserts text into the field.
+  ///
+  /// The following methods are defined for this channel:
+  ///
+  ///  * `Scribble.focusElement`: Indicates that focus is requested at the given
+  ///    [Offset].
+  ///
+  ///  * `Scribble.requestElementsInRect`: Returns a List of identifiers and
+  ///    bounds for the [ScribbleClient]s that lie within the given Rect.
+  ///
+  ///  * `Scribble.scribbleInteractionBegan`: Indicates that handwriting input
+  ///    has started.
+  ///
+  ///  * `Scribble.scribbleInteractionFinished`: Indicates that handwriting input
+  ///    has ended.
+  ///
+  ///  * `Scribble.showToolbar`: Requests that the toolbar be shown, such as
+  ///    when selection is changed by handwriting.
+  ///
+  ///  * `Scribble.insertTextPlaceholder`: Requests that visual writing space is
+  ///    reserved.
+  ///
+  ///  * `Scribble.removeTextPlaceholder`: Requests that any placeholder writing
+  ///    space is removed.
+  static const MethodChannel scribble = OptionalMethodChannel(
+      'flutter/scribble',
+      JSONMethodCodec(),
+  );
+
   /// A [MethodChannel] for handling spell check for text input.
   ///
   /// This channel exposes the spell check framework for supported platforms.