[webview_flutter]Allow specifying a navigation delegate(Android and Dart).

packages/webview_flutter/CHANGELOG.md

@@ -1,3 +1,7 @@
+## 0.3.4
+
+* Support specifying navigation delegates that can prevent navigations from being executed.
+
 ## 0.3.3+2
 
 * Exclude LongPress handler from semantics tree since it does nothing.

packages/webview_flutter/android/build.gradle

@@ -44,4 +44,8 @@ android {
     lintOptions {
         disable 'InvalidPackage'
     }
+
+    dependencies {
+        implementation 'androidx.webkit:webkit:1.0.0'
+    }
 }

packages/webview_flutter/android/src/main/java/io/flutter/plugins/webviewflutter/FlutterWebView.java

@@ -4,7 +4,9 @@
 
 package io.flutter.plugins.webviewflutter;
 
+import android.annotation.TargetApi;
 import android.content.Context;
+import android.os.Build;
 import android.view.View;
 import android.webkit.WebStorage;
 import android.webkit.WebView;
@@ -21,6 +23,7 @@ public class FlutterWebView implements PlatformView, MethodCallHandler {
   private static final String JS_CHANNEL_NAMES_FIELD = "javascriptChannelNames";
   private final WebView webView;
   private final MethodChannel methodChannel;
+  private final FlutterWebViewClient flutterWebViewClient;
 
   @SuppressWarnings("unchecked")
   FlutterWebView(Context context, BinaryMessenger messenger, int id, Map<String, Object> params) {
@@ -31,12 +34,15 @@ public class FlutterWebView implements PlatformView, MethodCallHandler {
     methodChannel = new MethodChannel(messenger, "plugins.flutter.io/webview_" + id);
     methodChannel.setMethodCallHandler(this);
 
+    flutterWebViewClient = new FlutterWebViewClient(methodChannel);
     applySettings((Map<String, Object>) params.get("settings"));
 
     if (params.containsKey(JS_CHANNEL_NAMES_FIELD)) {
       registerJavaScriptChannelNames((List<String>) params.get(JS_CHANNEL_NAMES_FIELD));
     }
 
+    webView.setWebViewClient(flutterWebViewClient);
+
     if (params.containsKey("initialUrl")) {
       String url = (String) params.get("initialUrl");
       webView.loadUrl(url);
@@ -135,6 +141,7 @@ private void updateSettings(MethodCall methodCall, Result result) {
     result.success(null);
   }
 
+  @TargetApi(Build.VERSION_CODES.KITKAT)
   private void evaluateJavaScript(MethodCall methodCall, final Result result) {
     String jsString = (String) methodCall.arguments;
     if (jsString == null) {
@@ -178,6 +185,9 @@ private void applySettings(Map<String, Object> settings) {
         case "jsMode":
           updateJsMode((Integer) settings.get(key));
           break;
+        case "hasNavigationDelegate":
+          flutterWebViewClient.setHasNavigationDelegate((boolean) settings.get(key));
+          break;
         default:
           throw new IllegalArgumentException("Unknown WebView setting: " + key);
       }

packages/webview_flutter/android/src/main/java/io/flutter/plugins/webviewflutter/FlutterWebViewClient.java

@@ -0,0 +1,124 @@
+// 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.
+
+package io.flutter.plugins.webviewflutter;
+
+import android.annotation.TargetApi;
+import android.os.Build;
+import android.util.Log;
+import android.webkit.WebResourceRequest;
+import android.webkit.WebView;
+import androidx.webkit.WebViewClientCompat;
+import io.flutter.plugin.common.MethodChannel;
+import java.util.HashMap;
+import java.util.Map;
+
+// We need to use WebViewClientCompat to get
+// shouldOverrideUrlLoading(WebView view, WebResourceRequest request)
+// invoked by the webview on older Android devices, without it pages that use iframes will
+// be broken when a navigationDelegate is set on Android version earlier than N.
+class FlutterWebViewClient extends WebViewClientCompat {
+  private static final String TAG = "FlutterWebViewClient";
+  private final MethodChannel methodChannel;
+  private boolean hasNavigationDelegate;
+
+  FlutterWebViewClient(MethodChannel methodChannel) {
+    this.methodChannel = methodChannel;
+  }
+
+  void setHasNavigationDelegate(boolean hasNavigationDelegate) {
+    this.hasNavigationDelegate = hasNavigationDelegate;
+  }
+
+  @TargetApi(Build.VERSION_CODES.LOLLIPOP)
+  @Override
+  public boolean shouldOverrideUrlLoading(WebView view, WebResourceRequest request) {
+    if (!hasNavigationDelegate) {
+      return false;
+    }
+    notifyOnNavigationRequest(
+        request.getUrl().toString(), request.getRequestHeaders(), view, request.isForMainFrame());
+    // We must make a synchronous decision here whether to allow the navigation or not,
+    // if the Dart code has set a navigation delegate we want that delegate to decide whether
+    // to navigate or not, and as we cannot get a response from the Dart delegate synchronously we
+    // return true here to block the navigation, if the Dart delegate decides to allow the
+    // navigation the plugin will later make an addition loadUrl call for this url.
+    //
+    // Since we cannot call loadUrl for a subframe, we currently only allow the delegate to stop
+    // navigations that target the main frame, if the request is not for the main frame
+    // we just return false to allow the navigation.
+    //
+    // For more details see: https://github.com/flutter/flutter/issues/25329#issuecomment-464863209
+    return request.isForMainFrame();
+  }
+
+  @Override
+  public boolean shouldOverrideUrlLoading(WebView view, String url) {
+    if (!hasNavigationDelegate) {
+      return false;
+    }
+    // This version of shouldOverrideUrlLoading is only invoked by the webview on devices with
+    // webview versions  earlier than 67(it is also invoked when hasNavigationDelegate is false).
+    // On these devices we cannot tell whether the navigation is targeted to the main frame or not.
+    // We proceed assuming that the navigation is targeted to the main frame. If the page had any
+    // frames they will be loaded in the main frame instead.
+    Log.w(
+        TAG,
+        "Using a navigationDelegate with an old webview implementation, pages with frames or iframes will not work");
+    notifyOnNavigationRequest(url, null, view, true);
+    return true;
+  }
+
+  private void notifyOnNavigationRequest(
+      String url, Map<String, String> headers, WebView webview, boolean isMainFrame) {
+    HashMap<String, Object> args = new HashMap<>();
+    args.put("url", url);
+    args.put("isForMainFrame", isMainFrame);
+    if (isMainFrame) {
+      methodChannel.invokeMethod(
+          "navigationRequest", args, new OnNavigationRequestResult(url, headers, webview));
+    } else {
+      methodChannel.invokeMethod("navigationRequest", args);
+    }
+  }
+
+  private static class OnNavigationRequestResult implements MethodChannel.Result {
+    private final String url;
+    private final Map<String, String> headers;
+    private final WebView webView;
+
+    private OnNavigationRequestResult(String url, Map<String, String> headers, WebView webView) {
+      this.url = url;
+      this.headers = headers;
+      this.webView = webView;
+    }
+
+    @Override
+    public void success(Object shouldLoad) {
+      Boolean typedShouldLoad = (Boolean) shouldLoad;
+      if (typedShouldLoad) {
+        loadUrl();
+      }
+    }
+
+    @Override
+    public void error(String errorCode, String s1, Object o) {
+      throw new IllegalStateException("navigationRequest calls must succeed");
+    }
+
+    @Override
+    public void notImplemented() {
+      throw new IllegalStateException(
+          "navigationRequest must be implemented by the webview method channel");
+    }
+
+    private void loadUrl() {
+      if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.LOLLIPOP) {
+        webView.loadUrl(url, headers);
+      } else {
+        webView.loadUrl(url);
+      }
+    }
+  }
+}

packages/webview_flutter/example/android/gradle.properties

@@ -1 +1,2 @@
 org.gradle.jvmargs=-Xmx1536M
+android.useAndroidX=true

packages/webview_flutter/example/lib/main.dart

@@ -3,11 +3,27 @@
 // found in the LICENSE file.
 
 import 'dart:async';
+import 'dart:convert';
 import 'package:flutter/material.dart';
 import 'package:webview_flutter/webview_flutter.dart';
 
 void main() => runApp(MaterialApp(home: WebViewExample()));
 
+const String kNavigationExamplePage = '''
+<!DOCTYPE html><html>
+<head><title>Navigation Delegate Example</title></head>
+<body>
+<p>
+The navigation delegate is set to block navigation to the youtube website.
+</p>
+<ul>
+<ul><a href="https://www.youtube.com/">https://www.youtube.com/</a></ul>
+<ul><a href="https://www.google.com/">https://www.google.com/</a></ul>
+</ul>
+</body>
+</html>
+''';
+
 class WebViewExample extends StatelessWidget {
   final Completer<WebViewController> _controller =
       Completer<WebViewController>();
@@ -37,6 +53,14 @@ class WebViewExample extends StatelessWidget {
           javascriptChannels: <JavascriptChannel>[
             _toasterJavascriptChannel(context),
           ].toSet(),
+          navigationDelegate: (NavigationRequest request) {
+            if (request.url.startsWith('https://www.youtube.com/')) {
+              print('blocking navigation to $request}');
+              return NavigationDecision.prevent;
+            }
+            print('allowing navigation to $request');
+            return NavigationDecision.navigate;
+          },
         );
       }),
       floatingActionButton: favoriteButton(),
@@ -76,12 +100,12 @@ class WebViewExample extends StatelessWidget {
 
 enum MenuOptions {
   showUserAgent,
-  toast,
   listCookies,
   clearCookies,
   addToCache,
   listCache,
   clearCache,
+  navigationDelegate,
 }
 
 class SampleMenu extends StatelessWidget {
@@ -102,13 +126,6 @@ class SampleMenu extends StatelessWidget {
               case MenuOptions.showUserAgent:
                 _onShowUserAgent(controller.data, context);
                 break;
-              case MenuOptions.toast:
-                Scaffold.of(context).showSnackBar(
-                  SnackBar(
-                    content: Text('You selected: $value'),
-                  ),
-                );
-                break;
               case MenuOptions.listCookies:
                 _onListCookies(controller.data, context);
                 break;
@@ -124,6 +141,9 @@ class SampleMenu extends StatelessWidget {
               case MenuOptions.clearCache:
                 _onClearCache(controller.data, context);
                 break;
+              case MenuOptions.navigationDelegate:
+                _onNavigationDelegateExample(controller.data, context);
+                break;
             }
           },
           itemBuilder: (BuildContext context) => <PopupMenuItem<MenuOptions>>[
@@ -132,10 +152,6 @@ class SampleMenu extends StatelessWidget {
                   child: const Text('Show user agent'),
                   enabled: controller.hasData,
                 ),
-                const PopupMenuItem<MenuOptions>(
-                  value: MenuOptions.toast,
-                  child: Text('Make a toast'),
-                ),
                 const PopupMenuItem<MenuOptions>(
                   value: MenuOptions.listCookies,
                   child: Text('List cookies'),
@@ -156,6 +172,10 @@ class SampleMenu extends StatelessWidget {
                   value: MenuOptions.clearCache,
                   child: Text('Clear cache'),
                 ),
+                const PopupMenuItem<MenuOptions>(
+                  value: MenuOptions.navigationDelegate,
+                  child: Text('Navigation Delegate example'),
+                ),
               ],
         );
       },
@@ -218,6 +238,13 @@ class SampleMenu extends StatelessWidget {
     ));
   }
 
+  void _onNavigationDelegateExample(
+      WebViewController controller, BuildContext context) async {
+    final String contentBase64 =
+        base64Encode(const Utf8Encoder().convert(kNavigationExamplePage));
+    controller.loadUrl('data:text/html;base64,$contentBase64');
+  }
+
   Widget _getCookieList(String cookies) {
     if (cookies == null || cookies == '""') {
       return Container();