hasStrings for web (flutter/engine#43360)

Support hasStrings on web, so that developers can check for clipboard content before showing the paste button, for example, if desired.

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

@@ -65,6 +65,31 @@ class ClipboardMessageHandler {
     });
   }
 
+  /// Handles the platform message which asks if the clipboard contains
+  /// pasteable strings.
+  void hasStringsMethodCall(ui.PlatformMessageResponseCallback? callback) {
+    const MethodCodec codec = JSONMethodCodec();
+    _pasteFromClipboardStrategy.getData().then((String data) {
+      final Map<String, dynamic> map = <String, dynamic>{'value': data.isNotEmpty};
+      callback!(codec.encodeSuccessEnvelope(map));
+    }).catchError((dynamic error) {
+      if (error is UnimplementedError) {
+        // Clipboard.hasStrings not supported.
+        // Passing [null] to [callback] indicates that the platform message isn't
+        // implemented. Look at [MethodChannel.invokeMethod] to see how [null] is
+        // handled.
+        Future<void>.delayed(Duration.zero).then((_) {
+          if (callback != null) {
+            callback(null);
+          }
+        });
+        return;
+      }
+      final Map<String, dynamic> map = <String, dynamic>{'value': false};
+      callback!(codec.encodeSuccessEnvelope(map));
+    });
+  }
+
   void _reportGetDataFailure(ui.PlatformMessageResponseCallback? callback,
       MethodCodec codec, dynamic error) {
     print('Could not get text from clipboard: $error');

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

@@ -580,6 +580,9 @@ class EnginePlatformDispatcher extends ui.PlatformDispatcher {
           case 'Clipboard.getData':
             ClipboardMessageHandler().getDataMethodCall(callback);
             return;
+          case 'Clipboard.hasStrings':
+            ClipboardMessageHandler().hasStringsMethodCall(callback);
+            return;
         }
 
       // Dispatched by the bindings to delay service worker initialization.

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

@@ -87,6 +87,48 @@ Future<void> testMain() async {
       final Map<String, dynamic> result = await completer.future;
       expect(result['text'], testText);
     });
+
+    test('has strings true', () async {
+      clipboardAPIPasteStrategy.testResult = testText;
+      const MethodCodec codec = JSONMethodCodec();
+      final Completer<Map<String, dynamic>> completer = Completer<Map<String, dynamic>>();
+      void callback(ByteData? data) {
+        completer.complete(codec.decodeEnvelope(data!) as Map<String, dynamic>);
+      }
+
+      clipboardMessageHandler.hasStringsMethodCall(callback);
+
+      final Map<String, dynamic> result = await completer.future;
+      expect(result['value'], isTrue);
+    });
+
+    test('has strings false', () async {
+      clipboardAPIPasteStrategy.testResult = '';
+      const MethodCodec codec = JSONMethodCodec();
+      final Completer<Map<String, dynamic>> completer = Completer<Map<String, dynamic>>();
+      void callback(ByteData? data) {
+        completer.complete(codec.decodeEnvelope(data!) as Map<String, dynamic>);
+      }
+
+      clipboardMessageHandler.hasStringsMethodCall(callback);
+
+      final Map<String, dynamic> result = await completer.future;
+      expect(result['value'], isFalse);
+    });
+
+    test('has strings error', () async {
+      clipboardAPIPasteStrategy.errors = true;
+      const MethodCodec codec = JSONMethodCodec();
+      final Completer<Map<String, dynamic>> completer = Completer<Map<String, dynamic>>();
+      void callback(ByteData? data) {
+        completer.complete(codec.decodeEnvelope(data!) as Map<String, dynamic>);
+      }
+
+      clipboardMessageHandler.hasStringsMethodCall(callback);
+
+      final Map<String, dynamic> result = await completer.future;
+      expect(result['value'], isFalse);
+    });
   });
 }
 
@@ -102,8 +144,14 @@ class MockClipboardAPICopyStrategy implements ClipboardAPICopyStrategy {
 class MockClipboardAPIPasteStrategy implements ClipboardAPIPasteStrategy {
   String testResult = '';
 
+  // Whether getData's Future will resolve with an error.
+  bool errors = false;
+
   @override
   Future<String> getData() {
+    if (errors) {
+      return Future<String>.error(Error());
+    }
     return Future<String>.value(testResult);
   }
 }