Add custom system-wide text selection toolbar buttons on Android (#139738)

## Description

This PR adds custom system-wide text selection toolbar buttons on Android.
~~This is a WIP until https://github.com/flutter/flutter/pull/139479 is merged (potential conflicts).~~

## Related Issue

Fixes https://github.com/flutter/flutter/issues/139361

## Tests

Adds 5 tests.

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

@@ -2199,6 +2199,12 @@ class EditableTextState extends State<EditableText> with AutomaticKeepAliveClien
 
   bool get _spellCheckResultsReceived => spellCheckEnabled && spellCheckResults != null && spellCheckResults!.suggestionSpans.isNotEmpty;
 
+  /// The text processing service used to retrieve the native text processing actions.
+  final ProcessTextService _processTextService = DefaultProcessTextService();
+
+  /// The list of native text processing actions provided by the engine.
+  final List<ProcessTextAction> _processTextActions = <ProcessTextAction>[];
+
   /// Whether to create an input connection with the platform for text editing
   /// or not.
   ///
@@ -2410,14 +2416,14 @@ class EditableTextState extends State<EditableText> with AutomaticKeepAliveClien
     clipboardStatus.update();
   }
 
+  bool get _allowPaste {
+    return !widget.readOnly && textEditingValue.selection.isValid;
+  }
+
   /// Paste text from [Clipboard].
   @override
   Future<void> pasteText(SelectionChangedCause cause) async {
-    if (widget.readOnly) {
-      return;
-    }
-    final TextSelection selection = textEditingValue.selection;
-    if (!selection.isValid) {
+    if (!_allowPaste) {
       return;
     }
     // Snapshot the input before using `await`.
@@ -2426,16 +2432,24 @@ class EditableTextState extends State<EditableText> with AutomaticKeepAliveClien
     if (data == null) {
       return;
     }
+    _pasteText(cause, data.text!);
+  }
+
+  void _pasteText(SelectionChangedCause cause, String text) {
+    if (!_allowPaste) {
+      return;
+    }
 
     // After the paste, the cursor should be collapsed and located after the
     // pasted content.
+    final TextSelection selection = textEditingValue.selection;
     final int lastSelectionIndex = math.max(selection.baseOffset, selection.extentOffset);
     final TextEditingValue collapsedTextEditingValue = textEditingValue.copyWith(
       selection: TextSelection.collapsed(offset: lastSelectionIndex),
     );
 
     userUpdateTextEditingValue(
-      collapsedTextEditingValue.replaced(selection, data.text!),
+      collapsedTextEditingValue.replaced(selection, text),
       cause,
     );
     if (cause == SelectionChangedCause.toolbar) {
@@ -2789,7 +2803,35 @@ class EditableTextState extends State<EditableText> with AutomaticKeepAliveClien
       onLiveTextInput: liveTextInputEnabled
           ? () => _startLiveTextInput(SelectionChangedCause.toolbar)
           : null,
-    );
+    )..addAll(_textProcessingActionButtonItems);
+  }
+
+  List<ContextMenuButtonItem> get _textProcessingActionButtonItems {
+    final List<ContextMenuButtonItem> buttonItems = <ContextMenuButtonItem>[];
+    final TextSelection selection = textEditingValue.selection;
+    if (widget.obscureText || !selection.isValid || selection.isCollapsed) {
+      return buttonItems;
+    }
+
+    for (final ProcessTextAction action in _processTextActions) {
+      buttonItems.add(ContextMenuButtonItem(
+        label: action.label,
+        onPressed: () async {
+          final String selectedText = selection.textInside(textEditingValue.text);
+          if (selectedText.isNotEmpty) {
+            final String? processedText = await _processTextService.processTextAction(action.id, selectedText, widget.readOnly);
+            // If an activity does not return a modified version, just hide the toolbar.
+            // Otherwise use the result to replace the selected text.
+            if (processedText != null && _allowPaste) {
+              _pasteText(SelectionChangedCause.toolbar, processedText);
+            } else {
+              hideToolbar();
+            }
+          }
+        },
+      ));
+    }
+    return buttonItems;
   }
 
   // State lifecycle:
@@ -2804,6 +2846,14 @@ class EditableTextState extends State<EditableText> with AutomaticKeepAliveClien
     _scrollController.addListener(_onEditableScroll);
     _cursorVisibilityNotifier.value = widget.showCursor;
     _spellCheckConfiguration = _inferSpellCheckConfiguration(widget.spellCheckConfiguration);
+    _initProcessTextActions();
+  }
+
+  /// Query the engine to initialize the list of text processing actions to show
+  /// in the text selection toolbar.
+  Future<void> _initProcessTextActions() async {
+    _processTextActions.clear();
+    _processTextActions.addAll(await _processTextService.queryTextActions());
   }
 
   // Whether `TickerMode.of(context)` is true and animations (like blinking the

packages/flutter/test/widgets/process_text_utils.dart

+// 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 'package:flutter/foundation.dart';
+import 'package:flutter/services.dart';
+
+const String fakeAction1Id = 'fakeActivity.fakeAction1';
+const String fakeAction2Id = 'fakeActivity.fakeAction2';
+const String fakeAction1Label = 'Action1';
+const String fakeAction2Label = 'Action2';
+
+class MockProcessTextHandler {
+  String? lastCalledActionId;
+  String? lastTextToProcess;
+
+  Future<Object?> handleMethodCall(MethodCall call) async {
+    if (call.method == 'ProcessText.queryTextActions') {
+      // Simulate that only the Android engine will return a non-null result.
+      if (defaultTargetPlatform == TargetPlatform.android) {
+        return <String, String>{
+          fakeAction1Id: fakeAction1Label,
+          fakeAction2Id: fakeAction2Label,
+        };
+      }
+    }
+    if (call.method == 'ProcessText.processTextAction') {
+      final List<dynamic> args = call.arguments as List<dynamic>;
+      final String actionId = args[0] as String;
+      final String textToProcess = args[1] as String;
+      lastCalledActionId = actionId;
+      lastTextToProcess = textToProcess;
+
+      if (actionId == fakeAction1Id) {
+        // Simulates an action that returns a transformed text.
+        return '$textToProcess!!!';
+      }
+      // Simulates an action that failed or does not transform text.
+      return null;
+    }
+    return null;
+  }
+}