异常信息

Android原生项目引入了一个flutter module，flutter module用到了一个flutter写的sdk，sdk需要使用webview来执行js代码，因此sdk引用了flutter_webview_plugin，然后运行时报错：

E/flutter: [ERROR:flutter/lib/ui/ui_dart_state.cc(186)] Unhandled Exception: MissingPluginException(No implementation found for method launch on channel flutter_webview_plugin)
    #0      MethodChannel._invokeMethod (package:flutter/src/services/platform_channel.dart:156:7)
    <asynchronous suspension>
    #1      FlutterWebviewPlugin.launch (package:flutter_webview_plugin/src/base.dart:225:5)
    <asynchronous suspension>

看下使用的flutter_webview_plugin的版本：

执行flutter pub get：

$ flutter pub get
Running "flutter pub get" in polkawallet...                        738ms
The plugin `flutter_aes_ecb_pkcs5` is built using an older version of the Android plugin API which assumes that it's running in a full-Flutter environment. It may have undefined behaviors when Flutter is integrated into an existing app as a module.
The plugin can be updated to the v2 Android Plugin APIs by following https://flutter.dev/go/android-plugin-migration.
The plugin `flutter_qr_scan` is built using an older version of the Android plugin API which assumes that it's running in a full-Flutter environment. It may have undefined behaviors when Flutter is integrated into an existing app as a module.
The plugin can be updated to the v2 Android Plugin APIs by following https://flutter.dev/go/android-plugin-migration.
The plugin `flutter_webview_plugin` is built using an older version of the Android plugin API which assumes that it's running in a full-Flutter environment. It may have undefined behaviors when Flutter is integrated into an existing app as a module.
The plugin can be updated to the v2 Android Plugin APIs by following https://flutter.dev/go/android-plugin-migration.

注意到上面输出信息中的：

The plugin `flutter_webview_plugin` is built using an older version of the Android plugin API which assumes that it's running in a full-Flutter environment. It may have undefined behaviors when Flutter is integrated into an existing app as a module.
The plugin can be updated to the v2 Android Plugin APIs by following https://flutter.dev/go/android-plugin-migration.

提示我们，这个 flutter_webview_plugin 是使用旧版本的 plugin API 开发的插件，它是假设在全flutter环境的项目中使用的，如果在一个Android app的module中使用，可能会出现未知的异常，要对插件进行升级可以参考：https://flutter.dev/docs/development/packages-and-plugins/plugin-api-migration

关于flutter_webview_plugin

github：https://github.com/fluttercommunity/flutter_webview_plugin/
所有发布的版本：https://github.com/fluttercommunity/flutter_webview_plugin/tags

在flutter中使用webview需要导入一个第三方插件：flutter_webview_plugin
（flutter_webview_plugin的介绍文档：https://pub.dartlang.org/packages/flutter_webview_plugin）

这个插件停更在2020年4月…不更新了

在app开发中经常需要到webview控件，但是flutter本身是没有webview相关控件的，因此需要插件来提供webview控件，flutter中比较常用的webview插件主要有2个，分别是：flutter_webview_plugin和webview_flutter。
flutter_webview_plugin：https://pub.dartlang.org/packages/flutter_webview_plugin
webview_flutter： https://pub.flutter-io.cn/packages/webview_flutter

flutter_webview_plugin插件的使用

使用方式很简单，按照文档 https://pub.dartlang.org/packages/flutter_webview_plugin 的说明一步步来就行了。

一、获得插件

首先，我们先在pubspec.yaml文件中添加依赖

dependencies:
       flutter_webview_plugin: ^0.3.4

然后执行 flutter pub get 就会帮我们下载插件

二、在项目中引入

插件下载完成后就可以在项目中引入了

import 'package:flutter_webview_plugin/flutter_webview_plugin.dart';

三、代码中具体使用webview

解决

网上看了一大堆解决方式，有些参考吧…

阿里巴巴的flutter_boost项目的webview相关异常：flutter_webview_plugin异常 #653 看着比较靠谱，折腾了一通… 尝试使用 git log --grep="webview" 和 git log --grep="plugin"想查看能不能搜索到相关commit记录，git log --grep="plugin"可以搜索到插件相关的修改，但查看代码发现好像最新版本的代码中又被改掉了?..本以为可以看下现成的解决方案的…

只好静下来自己分析异常…
仔细看下FlutterWebviewPlugin的源码：

/**
 * FlutterWebviewPlugin
 */
public class FlutterWebviewPlugin implements MethodCallHandler, PluginRegistry.ActivityResultListener {
    private Activity activity;
    private WebviewManager webViewManager;
    private Context context;
    static MethodChannel channel;
    private static final String CHANNEL_NAME = "flutter_webview_plugin";
    private static final String JS_CHANNEL_NAMES_FIELD = "javascriptChannelNames";

    public static void registerWith(PluginRegistry.Registrar registrar) {
        if (registrar.activity() != null) {
            channel = new MethodChannel(registrar.messenger(), CHANNEL_NAME);
            final FlutterWebviewPlugin instance = new FlutterWebviewPlugin(registrar.activity(), registrar.activeContext());
            registrar.addActivityResultListener(instance);
            channel.setMethodCallHandler(instance);
        }
    }


...

好家伙… 必须要activity不为null时才进行注册插件，而项目中使用Flutter时并没有启动一个Activity，所以registerWith就不能成功注册插件了…
这种插件注册方式其实是flutter v1版本的插件注册方式，flutter v2版本的插件注册方式参看：https://flutter.dev/docs/development/packages-and-plugins/plugin-api-migration，其中说的非常清楚，

The new API has the advantage of providing a cleaner set of accessors for lifecycle dependent components compared to the old APIs. For instance PluginRegistry.Registrar.activity() could return null if Flutter isn’t attached to any activities.

new API在Flutter没有绑定到任何activity的情况下，PluginRegistry.Registrar.activity() 可以返回null。

然而flutter_webview_plugin插件停更在2020年4月…代码不更新了，还是使用旧的apiPluginRegistry.Registrar.activity()。

于是考虑用动态代理解决…

参考：

先stop app，再运行app：
Unhandled Exception: MissingPluginException(No implementation found for method launch on channel)

Flutter 解决 MissingPluginException(No implementation found for method xxx on channel xxx)

Unhandled Exception: MissingPluginException(No implementation found for method ****** on channel plugins.flutter.io/firebase_messaging) on IOS

MissingPluginException(No implementation found for method close on channel flutter_webview_plugin)

flutter_webview_plugin插件的使用
flutter插件推荐之 flutter_webview_plugin

Flutter应用开发之webview_flutter插件
在 Flutter 中使用 WebView

Unhandled Exception: MissingPluginException(No implementation found for method close on channel flutter_webview_plugin)

flutter和Android混编下出现“No implementation found for method xxxx on channel”错误

flutter_webview_plugin异常 #653

Flutter plugin registrar.context() and registrar.activity() always null

Javaweb学习（一）之动态代理（增强方法的返回值）