Allow changing the webview's platform specific implementation

packages/webview_flutter/lib/platform_interface.dart

@@ -0,0 +1,74 @@
+// Copyright 2019 The Chromium 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:async';
+
+import 'package:flutter/foundation.dart';
+import 'package:flutter/gestures.dart';
+import 'package:flutter/widgets.dart';
+
+/// Interface for talking to the webview's platform implementation.
+///
+/// An instance implementing this interface is passed to the `onWebViewPlatformCreated` callback that is
+/// passed to [WebViewPlatformBuilder#onWebViewPlatformCreated].
+abstract class WebViewPlatform {
+  /// Loads the specified URL.
+  ///
+  /// If `headers` is not null and the URL is an HTTP URL, the key value paris in `headers` will
+  /// be added as key value pairs of HTTP headers for the request.
+  ///
+  /// `url` must not be null.
+  ///
+  /// Throws an ArgumentError if `url` is not a valid URL string.
+  Future<void> loadUrl(
+    String url,
+    Map<String, String> headers,
+  ) {
+    throw UnimplementedError(
+        "WebView loadUrl is not implemented on the current platform");
+  }
+
+  // As the PR currently focus about the wiring I've only moved loadUrl to the new way, so
+  // the discussion is more focused.
+  // In this temporary state WebViewController still uses a method channel directly for all other
+  // method calls so we need to expose the webview ID.
+  // TODO(amirh): remove this before publishing this package.
+  int get id;
+}
+
+typedef WebViewPlatformCreatedCallback = void Function(
+    WebViewPlatform webViewPlatform);
+
+/// Interface building a platform WebView implementation.
+///
+/// [WebView#platformBuilder] controls the builder that is used by [WebView].
+/// [AndroidWebViewPlatform] and [CupertinoWebViewPlatform] are the default implementations
+/// for Android and iOS respectively.
+abstract class WebViewBuilder {
+  /// Builds a new WebView.
+  ///
+  /// Returns a Widget tree that embeds the created webview.
+  ///
+  /// `creationParams` are the initial parameters used to setup the webview.
+  ///
+  /// `onWebViewPlatformCreated` will be invoked after the platform specific [WebViewPlatform]
+  /// implementation is created with the [WebViewPlatform] instance as a parameter.
+  ///
+  /// `gestureRecognizers` specifies which gestures should be consumed by the web view.
+  /// It is possible for other gesture recognizers to be competing with the web view on pointer
+  /// events, e.g if the web view is inside a [ListView] the [ListView] will want to handle
+  /// vertical drags. The web view will claim gestures that are recognized by any of the
+  /// recognizers on this list.
+  /// When `gestureRecognizers` is empty or null, the web view will only handle pointer events for gestures that
+  /// were not claimed by any other gesture recognizer.
+  Widget build({
+    BuildContext context,
+    // TODO(amirh): convert this to be the actual parameters.
+    // I'm starting without it as the PR is starting to become pretty big.
+    // I'll followup with the conversion PR.
+    Map<String, dynamic> creationParams,
+    WebViewPlatformCreatedCallback onWebViewPlatformCreated,
+    Set<Factory<OneSequenceGestureRecognizer>> gestureRecognizers,
+  });
+}

packages/webview_flutter/lib/src/webview_android.dart

@@ -0,0 +1,54 @@
+// Copyright 2019 The Chromium 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/gestures.dart';
+import 'package:flutter/services.dart';
+import 'package:flutter/widgets.dart';
+
+import '../platform_interface.dart';
+import 'webview_method_channel.dart';
+
+/// Builds an Android webview.
+///
+/// This is used as the default implementation for [WebView.platformBuilder] on Android. It uses
+/// an [AndroidView] to embed the webview in the widget hierarchy, and uses a method channel to
+/// communicate with the platform code.
+class AndroidWebViewBuilder implements WebViewBuilder {
+  @override
+  Widget build({
+    BuildContext context,
+    Map<String, dynamic> creationParams,
+    WebViewPlatformCreatedCallback onWebViewPlatformCreated,
+    Set<Factory<OneSequenceGestureRecognizer>> gestureRecognizers,
+  }) {
+    return GestureDetector(
+      // We prevent text selection by intercepting the long press event.
+      // This is a temporary stop gap due to issues with text selection on Android:
+      // https://github.com/flutter/flutter/issues/24585 - the text selection
+      // dialog is not responding to touch events.
+      // https://github.com/flutter/flutter/issues/24584 - the text selection
+      // handles are not showing.
+      // TODO(amirh): remove this when the issues above are fixed.
+      onLongPress: () {},
+      excludeFromSemantics: true,
+      child: AndroidView(
+        viewType: 'plugins.flutter.io/webview',
+        onPlatformViewCreated: (int id) {
+          if (onWebViewPlatformCreated == null) {
+            return;
+          }
+          onWebViewPlatformCreated(MethodChannelWebViewPlatform(id));
+        },
+        gestureRecognizers: gestureRecognizers,
+        // WebView content is not affected by the Android view's layout direction,
+        // we explicitly set it here so that the widget doesn't require an ambient
+        // directionality.
+        layoutDirection: TextDirection.rtl,
+        creationParams: creationParams,
+        creationParamsCodec: const StandardMessageCodec(),
+      ),
+    );
+  }
+}

packages/webview_flutter/lib/src/webview_cupertino.dart

@@ -0,0 +1,39 @@
+// Copyright 2019 The Chromium 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/gestures.dart';
+import 'package:flutter/services.dart';
+import 'package:flutter/widgets.dart';
+
+import '../platform_interface.dart';
+import 'webview_method_channel.dart';
+
+/// Builds an iOS webview.
+///
+/// This is used as the default implementation for [WebView.platformBuilder] on iOS. It uses
+/// a [UiKitView] to embed the webview in the widget hierarchy, and uses a method channel to
+/// communicate with the platform code.
+class CupertinoWebViewBuilder implements WebViewBuilder {
+  @override
+  Widget build({
+    BuildContext context,
+    Map<String, dynamic> creationParams,
+    WebViewPlatformCreatedCallback onWebViewPlatformCreated,
+    Set<Factory<OneSequenceGestureRecognizer>> gestureRecognizers,
+  }) {
+    return UiKitView(
+      viewType: 'plugins.flutter.io/webview',
+      onPlatformViewCreated: (int id) {
+        if (onWebViewPlatformCreated == null) {
+          return;
+        }
+        onWebViewPlatformCreated(MethodChannelWebViewPlatform(id));
+      },
+      gestureRecognizers: gestureRecognizers,
+      creationParams: creationParams,
+      creationParamsCodec: const StandardMessageCodec(),
+    );
+  }
+}

packages/webview_flutter/lib/src/webview_method_channel.dart

@@ -0,0 +1,37 @@
+// Copyright 2019 The Chromium 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:async';
+
+import 'package:flutter/services.dart';
+
+import '../platform_interface.dart';
+
+/// A [WebViewPlatform] that uses a method channel to control the webview.
+class MethodChannelWebViewPlatform implements WebViewPlatform {
+  MethodChannelWebViewPlatform(this._id)
+      : _channel = MethodChannel('plugins.flutter.io/webview_$_id');
+
+  final int _id;
+
+  final MethodChannel _channel;
+
+  @override
+  Future<void> loadUrl(
+    String url,
+    Map<String, String> headers,
+  ) async {
+    assert(url != null);
+    // TODO(amirh): remove this on when the invokeMethod update makes it to stable Flutter.
+    // https://github.com/flutter/flutter/issues/26431
+    // ignore: strong_mode_implicit_dynamic_method
+    return _channel.invokeMethod('loadUrl', <String, dynamic>{
+      'url': url,
+      'headers': headers,
+    });
+  }
+
+  @override
+  int get id => _id;
+}

packages/webview_flutter/lib/webview_flutter.dart

@@ -9,6 +9,10 @@ import 'package:flutter/gestures.dart';
 import 'package:flutter/services.dart';
 import 'package:flutter/widgets.dart';
 
+import 'platform_interface.dart';
+import 'src/webview_android.dart';
+import 'src/webview_cupertino.dart';
+
 typedef void WebViewCreatedCallback(WebViewController controller);
 
 enum JavascriptMode {
@@ -121,6 +125,39 @@ class WebView extends StatefulWidget {
   })  : assert(javascriptMode != null),
         super(key: key);
 
+  static WebViewBuilder _platformBuilder;
+
+  /// Sets a custom [WebViewBuilder].
+  ///
+  /// This property can be set to use a custom platform implementation for WebViews.
+  ///
+  /// Setting `platformBuilder` doesn't affect [WebView]s that were already created.
+  ///
+  /// The default value is [AndroidWebViewBuilder] on Android and [CupertinoWebViewBuilder] on iOs.
+  static set platformBuilder(WebViewBuilder platformBuilder) {
+    _platformBuilder = platformBuilder;
+  }
+
+  /// The [WebViewBuilder] that's used to create new [WebView]s.
+  ///
+  /// The default value is [AndroidWebViewBuilder] on Android and [CupertinoWebViewBuilder] on iOs.
+  static WebViewBuilder get platformBuilder {
+    if (_platformBuilder == null) {
+      switch (defaultTargetPlatform) {
+        case TargetPlatform.android:
+          _platformBuilder = AndroidWebViewBuilder();
+          break;
+        case TargetPlatform.iOS:
+          _platformBuilder = CupertinoWebViewBuilder();
+          break;
+        default:
+          throw UnsupportedError(
+              "Trying to use the default webview implementation for $defaultTargetPlatform but there isn't a default one");
+      }
+    }
+    return _platformBuilder;
+  }
+
   /// If not null invoked once the web view is created.
   final WebViewCreatedCallback onWebViewCreated;
 
@@ -229,40 +266,12 @@ class _WebViewState extends State<WebView> {
 
   @override
   Widget build(BuildContext context) {
-    if (defaultTargetPlatform == TargetPlatform.android) {
-      return GestureDetector(
-        // We prevent text selection by intercepting the long press event.
-        // This is a temporary stop gap due to issues with text selection on Android:
-        // https://github.com/flutter/flutter/issues/24585 - the text selection
-        // dialog is not responding to touch events.
-        // https://github.com/flutter/flutter/issues/24584 - the text selection
-        // handles are not showing.
-        // TODO(amirh): remove this when the issues above are fixed.
-        onLongPress: () {},
-        excludeFromSemantics: true,
-        child: AndroidView(
-          viewType: 'plugins.flutter.io/webview',
-          onPlatformViewCreated: _onPlatformViewCreated,
-          gestureRecognizers: widget.gestureRecognizers,
-          // WebView content is not affected by the Android view's layout direction,
-          // we explicitly set it here so that the widget doesn't require an ambient
-          // directionality.
-          layoutDirection: TextDirection.rtl,
-          creationParams: _CreationParams.fromWidget(widget).toMap(),
-          creationParamsCodec: const StandardMessageCodec(),
-        ),
-      );
-    } else if (defaultTargetPlatform == TargetPlatform.iOS) {
-      return UiKitView(
-        viewType: 'plugins.flutter.io/webview',
-        onPlatformViewCreated: _onPlatformViewCreated,
-        gestureRecognizers: widget.gestureRecognizers,
-        creationParams: _CreationParams.fromWidget(widget).toMap(),
-        creationParamsCodec: const StandardMessageCodec(),
-      );
-    }
-    return Text(
-        '$defaultTargetPlatform is not yet supported by the webview_flutter plugin');
+    return WebView.platformBuilder.build(
+      context: context,
+      creationParams: _CreationParams.fromWidget(widget).toMap(),
+      onWebViewPlatformCreated: _onWebViewPlatformCreated,
+      gestureRecognizers: widget.gestureRecognizers,
+    );
   }
 
   @override
@@ -279,8 +288,9 @@ class _WebViewState extends State<WebView> {
         (WebViewController controller) => controller._updateWidget(widget));
   }
 
-  void _onPlatformViewCreated(int id) {
-    final WebViewController controller = WebViewController._(id, widget);
+  void _onWebViewPlatformCreated(WebViewPlatform platformController) {
+    final WebViewController controller =
+        WebViewController._(platformController.id, platformController, widget);
     _controller.complete(controller);
     if (widget.onWebViewCreated != null) {
       widget.onWebViewCreated(controller);
@@ -385,6 +395,7 @@ class _WebSettings {
 class WebViewController {
   WebViewController._(
     int id,
+    this._platformInterface,
     this._widget,
   ) : _channel = MethodChannel('plugins.flutter.io/webview_$id') {
     _settings = _WebSettings.fromWidget(_widget);
@@ -394,6 +405,8 @@ class WebViewController {
 
   final MethodChannel _channel;
 
+  final WebViewPlatform _platformInterface;
+
   _WebSettings _settings;
 
   WebView _widget;
@@ -434,6 +447,9 @@ class WebViewController {
 
   /// Loads the specified URL.
   ///
+  /// If `headers` is not null and the URL is an HTTP URL, the key value paris in `headers` will
+  /// be added as key value pairs of HTTP headers for the request.
+  ///
   /// `url` must not be null.
   ///
   /// Throws an ArgumentError if `url` is not a valid URL string.
@@ -443,13 +459,7 @@ class WebViewController {
   }) async {
     assert(url != null);
     _validateUrlString(url);
-    // TODO(amirh): remove this on when the invokeMethod update makes it to stable Flutter.
-    // https://github.com/flutter/flutter/issues/26431
-    // ignore: strong_mode_implicit_dynamic_method
-    return _channel.invokeMethod('loadUrl', <String, dynamic>{
-      'url': url,
-      'headers': headers,
-    });
+    return _platformInterface.loadUrl(url, headers);
   }
 
   /// Accessor to the current URL that the WebView is displaying.