Support Clipboard.getData and Clipboard.setData for Flutter for web (flutter/engine#15455)

* copy/paste solution for chrome. copy solution for firefox. using clipboardAPI.

* addressing PR comments

* update licences file

engine/src/flutter/ci/licenses_golden/licenses_flutter

@@ -388,6 +388,7 @@ FILE: ../../../flutter/lib/web_ui/lib/src/engine/bitmap_canvas.dart
 FILE: ../../../flutter/lib/web_ui/lib/src/engine/browser_detection.dart
 FILE: ../../../flutter/lib/web_ui/lib/src/engine/browser_location.dart
 FILE: ../../../flutter/lib/web_ui/lib/src/engine/canvas_pool.dart
+FILE: ../../../flutter/lib/web_ui/lib/src/engine/clipboard.dart
 FILE: ../../../flutter/lib/web_ui/lib/src/engine/color_filter.dart
 FILE: ../../../flutter/lib/web_ui/lib/src/engine/compositor/canvas.dart
 FILE: ../../../flutter/lib/web_ui/lib/src/engine/compositor/canvas_kit_canvas.dart

engine/src/flutter/lib/web_ui/lib/src/engine.dart

@@ -24,6 +24,7 @@ part 'engine/bitmap_canvas.dart';
 part 'engine/browser_detection.dart';
 part 'engine/browser_location.dart';
 part 'engine/canvas_pool.dart';
+part 'engine/clipboard.dart';
 part 'engine/color_filter.dart';
 part 'engine/compositor/canvas.dart';
 part 'engine/compositor/canvas_kit_canvas.dart';

engine/src/flutter/lib/web_ui/lib/src/engine/clipboard.dart

@@ -0,0 +1,109 @@
+// Copyright 2013 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.
+
+part of engine;
+
+/// Handles clipboard related platform messages.
+class ClipboardMessageHandler {
+  /// Helper to handle copy to clipboard functionality.
+  final CopyToClipboardStrategy _copyToClipboardStrategy =
+      CopyToClipboardStrategy();
+
+  /// Helper to handle copy to clipboard functionality.
+  final PasteFromClipboardStrategy _pasteFromClipboardStrategy =
+      PasteFromClipboardStrategy();
+
+  /// Handles the platform message which stores the given text to the clipboard.
+  void setDataMethodCall(MethodCall methodCall) {
+    _copyToClipboardStrategy.setData(methodCall.arguments['text']);
+  }
+
+  /// Handles the platform message which retrieves text data from the clipboard.
+  void getDataMethodCall(ui.PlatformMessageResponseCallback callback) {
+    _pasteFromClipboardStrategy.getData().then((String data) {
+      const MethodCodec codec = JSONMethodCodec();
+      final Map<String, dynamic> map = {'text': data};
+      callback(codec.encodeSuccessEnvelope(map));
+    }).catchError(
+        (error) => print('Could not get text from clipboard: $error'));
+  }
+}
+
+/// Provides functionality for writing text to clipboard.
+///
+/// A concrete implementation is picked at runtime based on the available
+/// APIs and the browser.
+abstract class CopyToClipboardStrategy {
+  factory CopyToClipboardStrategy() {
+    return (html.window.navigator.clipboard.writeText != null)
+        ? ClipboardAPICopyStrategy()
+        : ExecCommandCopyStrategy();
+  }
+
+  /// Places the text onto the browser Clipboard.
+  void setData(String text);
+}
+
+/// Provides functionality for reading text from clipboard.
+///
+/// A concrete implementation is picked at runtime based on the available
+/// APIs and the browser.
+abstract class PasteFromClipboardStrategy {
+  factory PasteFromClipboardStrategy() {
+    return (browserEngine == BrowserEngine.firefox ||
+            html.window.navigator.clipboard.readText == null)
+        ? ExecCommandPasteStrategy()
+        : ClipboardAPIPasteStrategy();
+  }
+
+  /// Returns text from the system Clipboard.
+  Future<String> getData();
+}
+
+/// Provides copy functionality for browsers which supports ClipboardAPI.
+///
+/// Works on Chrome and Firefox browsers.
+/// See: https://developer.mozilla.org/en-US/docs/Web/API/Clipboard_API
+class ClipboardAPICopyStrategy implements CopyToClipboardStrategy {
+  @override
+  void setData(String text) {
+    html.window.navigator.clipboard
+        .writeText(text)
+        .catchError((error) => print('Could not copy text: $error'));
+  }
+}
+
+/// Provides paste functionality for browsers which supports `clipboard.readText`.
+///
+/// Works on Chrome. Firefox only supports `readText` if the target element is
+/// in content editable mode.
+/// See: https://developer.mozilla.org/en-US/docs/Web/Guide/HTML/Editable_content
+/// See: https://developer.mozilla.org/en-US/docs/Web/API/Clipboard_API
+class ClipboardAPIPasteStrategy implements PasteFromClipboardStrategy {
+  @override
+  Future<String> getData() async {
+    return html.window.navigator.clipboard.readText();
+  }
+}
+
+/// Provides a fallback strategy for browsers which does not support ClipboardAPI.
+class ExecCommandCopyStrategy implements CopyToClipboardStrategy {
+  @override
+  void setData(String text) {
+    // TODO(nurhan): https://github.com/flutter/flutter/issues/48578
+    print('Clipboard is only implemented for browsers '
+        'supporting Clipboard API. Use context menu for text editing.');
+  }
+}
+
+/// Provides a fallback strategy for browsers which does not support ClipboardAPI.
+class ExecCommandPasteStrategy implements PasteFromClipboardStrategy {
+  @override
+  Future<String> getData() {
+    // TODO(nurhan): https://github.com/flutter/flutter/issues/48581
+    // TODO(nurhan): https://github.com/flutter/flutter/issues/48580
+    print('Paste is not implemented for this browser.');
+    throw UnimplementedError();
+  }
+}

engine/src/flutter/lib/web_ui/lib/src/engine/window.dart

@@ -159,10 +159,10 @@ class EngineWindow extends ui.Window {
             // There are no default system sounds on web.
             return;
           case 'Clipboard.setData':
+            ClipboardMessageHandler().setDataMethodCall(decoded);
+            return;
           case 'Clipboard.getData':
-            // TODO(nurhan): https://github.com/flutter/flutter/issues/46020
-            print('WARNING: Clipboard API unimplemented for Flutter for Web. '
-                'Use context menu for text editing.');
+            ClipboardMessageHandler().getDataMethodCall(callback);
             return;
         }
         break;