Added v4 wrapper for JavaScriptChannelParams.

This commit adds a Android specific wrapper for the
JavaScriptChannelParams class from the webview_platform_interface.
The wrapper should call the native Android JavaScript channel API.

Unit tests are also included.

Author: mvanbeusekom

packages/webview_flutter/webview_flutter_android/lib/src/v4/android_webview_javascript_channel.dart

@@ -0,0 +1,29 @@
+// 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.
+
+import 'package:webview_flutter_platform_interface/v4/webview_flutter_platform_interface.dart';
+
+import '../android_webview.dart';
+
+/// Exposes a channel to receive calls from javaScript.
+///
+/// See [AndroidWebViewController.addJavaScriptChannel].
+class AndroidWebViewJavaScriptChannel extends JavaScriptChannel {
+  /// Creates a new [AndroidWebViewJavaScriptChannel] based the supplied [JavaScriptChannelParams].
+  AndroidWebViewJavaScriptChannel.fromJavaScriptChannelParams({
+    required JavaScriptChannelParams params,
+  })  : _javaScriptChannelParams = params,
+        super(params.name);
+
+  final JavaScriptChannelParams _javaScriptChannelParams;
+
+  @override
+  void postMessage(String message) {
+    _javaScriptChannelParams.onMessageReceived(
+      JavaScriptMessage(
+        message: message,
+      ),
+    );
+  }
+}

packages/webview_flutter/webview_flutter_android/test/v4/android_webview_javascript_channel_test.dart

@@ -0,0 +1,33 @@
+// 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.
+
+import 'package:flutter_test/flutter_test.dart';
+import 'package:mockito/annotations.dart';
+import 'package:mockito/mockito.dart';
+import 'package:webview_flutter_android/src/v4/android_webview_javascript_channel.dart';
+import 'package:webview_flutter_platform_interface/v4/webview_flutter_platform_interface.dart';
+
+import 'android_webview_javascript_channel_test.mocks.dart';
+
+@GenerateMocks(<Type>[JavaScriptChannelParams])
+void main() {
+  TestWidgetsFlutterBinding.ensureInitialized();
+
+  test('postMessage should call onMessageReceived', () {
+    final JavaScriptChannelParams mockChannelParams =
+        MockJavaScriptChannelParams();
+
+    const JavaScriptMessage message =
+        JavaScriptMessage(message: 'test message');
+
+    when(mockChannelParams.name).thenReturn('test_name');
+    when(mockChannelParams.onMessageReceived).thenReturn((_) => message);
+
+    AndroidWebViewJavaScriptChannel.fromJavaScriptChannelParams(
+            params: mockChannelParams)
+        .postMessage('test message');
+
+    verify(mockChannelParams.onMessageReceived(message)).called(1);
+  });
+}

packages/webview_flutter/webview_flutter_android/test/v4/android_webview_javascript_channel_test.mocks.dart

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+// Mocks generated by Mockito 5.1.0 from annotations
+// in webview_flutter_android/test/v4/android_webview_javascript_channel_test.dart.
+// Do not manually edit this file.
+
+import 'package:mockito/mockito.dart' as _i1;
+import 'package:webview_flutter_platform_interface/v4/src/platform_webview_controller.dart'
+    as _i2;
+import 'package:webview_flutter_platform_interface/v4/src/webview_platform.dart'
+    as _i3;
+
+// ignore_for_file: type=lint
+// ignore_for_file: avoid_redundant_argument_values
+// ignore_for_file: avoid_setters_without_getters
+// ignore_for_file: comment_references
+// ignore_for_file: implementation_imports
+// ignore_for_file: invalid_use_of_visible_for_testing_member
+// ignore_for_file: prefer_const_constructors
+// ignore_for_file: unnecessary_parenthesis
+// ignore_for_file: camel_case_types
+
+/// A class which mocks [JavaScriptChannelParams].
+///
+/// See the documentation for Mockito's code generation for more information.
+class MockJavaScriptChannelParams extends _i1.Mock
+    implements _i2.JavaScriptChannelParams {
+  MockJavaScriptChannelParams() {
+    _i1.throwOnMissingStub(this);
+  }
+
+  @override
+  String get name =>
+      (super.noSuchMethod(Invocation.getter(#name), returnValue: '') as String);
+  @override
+  void Function(_i3.JavaScriptMessage) get onMessageReceived =>
+      (super.noSuchMethod(Invocation.getter(#onMessageReceived),
+              returnValue: (_i3.JavaScriptMessage __p0) {})
+          as void Function(_i3.JavaScriptMessage));
+}