HelloFlutter--FlutterActivity

自谷歌发布Flutter release版本几天后才开始学习Flutter，实在惭愧。在了解完一些基础知识之后开始尝试将编写的简单Flutter module打包进Android项目中。本文章将尝试过程中遇到的一些问题和笔记记录下来。
本篇文章只是闭门造车的结果，如有任何错误很抱歉！请帮忙指出，多谢了

Android项目依赖Flutter项目

对于已有的Android项目来说，将所有页面都换成flutter页面不太现实，只能从一些简单的页面入手逐个替换。

Flutter项目跟Android工程根文件夹是同级的，它不同于普通的Android module存在于Android工程根目录下。在AndroidStudio中创建Flutter module，也并不会将该项目放到Android项目目录中，而是默认选择Android项目根目录的同级目录下。
在依赖Flutter module的时候，首先需要在项目的setting.gradle加入如下依赖

include ':app'
//加入下面配置
setBinding(new Binding([gradle: this]))
evaluate(new File(
        settingsDir.parentFile,
        'flutter_module/.android/include_flutter.groovy'
))

以上配置的Flutter module的位置是出于Android根目录同级目录下，如果Flutter module的路径不同需要另外设置File函数的参数。编译项目，会在Android项目下生成名为flutter的module，正常来说该module不需要去修改代码，只需要在app的build.gradle中依赖该fluttermodule即可。

dependencies {
    ...
    // 加入下面配置
    implementation project(':flutter')
}

自此，完成Android项目对Flutter项目的依赖

FlutterActivity

在创建的Flutter项目的.andorid module中，只有一个类，那就是MainActivity类。
其继承自FlutterActivity，运行该Flutter工程时，Android项目的入口就是该MainActivity类。

该FlutterActivity类是Flutter项目的页面入口，Flutter为Android项目提供了FlutterView和FlutterFragment作为展示页面，附着在Activity上面。而FlutterActivity使用的便是FlutterView。
那么，从开发的角度，接下来引出几个问题？

继承FlutterActivity只能默认进入Flutter设定的首页？
Flutter页面的生命周期如何管理？
Flutter页面与Android原生页面之间如何通讯？
Flutter页面是如何绘制的？

查看源码

查看FlutterActivity的类声明，该类实现了三个接口

public class FlutterActivity extends Activity implements 
Provider, PluginRegistry, ViewFactory {
  ...
}

这三个接口作用如下

Provider：只有一个简单的方法，那就是getFlutterView()返回当前Activity中的Flutter页面
PluginRegistry：插件注册相关的类，以后的文章再详细讲述
ViewFactory：该接口有三个方法，分别是
```
public interface ViewFactory {
      FlutterView createFlutterView(Context var1);

      FlutterNativeView createFlutterNativeView();

      boolean retainFlutterNativeView();
  }   
```
1. FlutterView createFlutterView(Context context)：该方法比较直观，就是生成一个Flutter的页面，供Activity展示。但是并没有在源码中找到引用它的地方。FlutterActivity的实现返回值是null。
2. FlutterNativeView createFlutterNativeView()：从字面意思是生成一个Flutter的原生View，但是并没有在源码中找到引用它的地方。FlutterActivity的实现返回值也是null。
3. boolean retainFlutterNativeView()：字面意思，保留Flutter原生页面。是一个boolean类型的值，但是并没有在源码中找到引用它的地方。FlutterActivity的实现返回值是false。
  通过查看FlutterActivity所继承的三个接口，我们并没有找到FlutterActivity中直接生成FlutterView的线索，只能从实例变量中查找。

FlutterActivityDelegate

在进行一番阅读之后，发现该委派类。在Android源码中有很多使用委派模式的地方，该处也算是一个。并且，在FlutterActivity中，FlutterActivityDelegate对象会跟随Activity的生命周期方法被调用同名方法。查看FlutterActivityDelegate的源码

构造方法

public FlutterActivityDelegate(Activity activity, FlutterActivityDelegate.ViewFactory viewFactory) {
    this.activity = (Activity)Preconditions.checkNotNull(activity);
    this.viewFactory = (FlutterActivityDelegate.ViewFactory)Preconditions.checkNotNull(viewFactory);
}

其构造方法需要传入一个Activity对象，还有FlutterActivityDelegate.ViewFactory对象。但在上文已经发现FlutterActivityDelegate.ViewFactory的方法并无引用的地方，这里只需要着重关注Activity对象就好了。

同名生命周期方法：查看FlutterActivityDelegate类源码，该类定义了一些列对象Activity生命周期函数的同名函数。并分别运行在FlutterActivity类的对应生命周期中，由此可见Flutter页面的生命周期是由该委托类处理的。接下来逐个分析生命周期函数

onCreate：该方法中实现了flutterView的生成。查看代码，由于代码量大，这里只显示关键代码

    public void onCreate(Bundle savedInstanceState) {
        ...
        this.flutterView = this.viewFactory.createFlutterView(this.activity);
        if (this.flutterView == null) {
            FlutterNativeView nativeView = this.viewFactory.createFlutterNativeView();
            this.flutterView = new FlutterView(this.activity, (AttributeSet)null, nativeView);
            this.flutterView.setLayoutParams(matchParent);
            this.activity.setContentView(this.flutterView);
            this.launchView = this.createLaunchView();
            if (this.launchView != null) {
                this.addLaunchView();
            }
        }
        ...
    }

从FlutterActivity实现的ViewFactory方法我们已经得知，传递给委托类FlutterActivityDelegate实例的ViewFactory并没有生成FlutterView可供FlutterActivityDelegate使用。所以只能继续查看this.flutterView = new FlutterView(this.activity, (AttributeSet)null, nativeView)之后的代码。

public class FlutterView extends SurfaceView implements BinaryMessenger, TextureRegistry, AccessibilityStateChangeListener {
    public FlutterView(Context context, AttributeSet attrs, FlutterNativeView nativeView) {
        super(context, attrs);
        ...
        Activity activity = (Activity)this.getContext();
        if (nativeView == null) {
            this.mNativeView = new FlutterNativeView(activity.getApplicationContext());
        } else {
            this.mNativeView = nativeView;
        }

        this.mNativeView.attachViewAndActivity(this, activity);
        ...
    }
}

我们可以看到FlutterView继承自SurfaceView，在其构造方法中。如果传递的FlutterNativeView如果为空，那将会重新创建一个默认的FlutterNativeView。接着看

public class FlutterNativeView implements BinaryMessenger {
    public FlutterNativeView(Context context) {
        this(context, false);
    }

    public FlutterNativeView(Context context, boolean isBackgroundView) {
        this.mNextReplyId = 1;
        this.mPendingReplies = new HashMap();
        this.mContext = context;
        this.mPluginRegistry = new FlutterPluginRegistry(this, context);
        this.attach(this, isBackgroundView);
        this.assertAttached();
        this.mMessageHandlers = new HashMap();
    }
}

在这里我们可以看到FlutterNativeView实现了BinaryMessenger接口，而BinaryMessenger是一个数据信息交流对象，其接口声明如下

public interface BinaryMessenger {
    /**
    *Sends a binary message to the Flutter application.
    *Parameters:
    *channel - the name String of the logical channel used for the message.
    *message - the message payload, a direct-allocated ByteBuffer with the message bytes between position zero and current position, or null.
    */
    void send(String var1, ByteBuffer var2);
    
     /**
     * Sends a binary message to the Flutter application, optionally expecting a reply.
     * Any uncaught exception thrown by the reply callback will be caught and logged.
     * <p>
     * Parameters:
     * channel - the name String of the logical channel used for the message.
     * message - the message payload, a direct-allocated ByteBuffer with the message bytes between position zero and current position, or null.
     * callback - a BinaryMessenger.BinaryReply callback invoked when the Flutter application responds to the message, possibly null.
     */
    void send(String var1, ByteBuffer var2, BinaryMessenger.BinaryReply var3);

    /**
     * Registers a handler to be invoked when the Flutter application sends a message to its host platform.
     * Registration overwrites any previous registration for the same channel name. Use a null handler to deregister.
     * <p>
     * If no handler has been registered for a particular channel, any incoming message on that channel will be handled silently by sending a null reply.
     * <p>
     * Parameters:
     * channel - the name String of the channel.
     * handler - a BinaryMessenger.BinaryMessageHandler to be invoked on incoming messages, or null.
     */
    void setMessageHandler(String var1, BinaryMessenger.BinaryMessageHandler var2);

    /**
     * Binary message reply callback. Used to submit a reply to an incoming message from Flutter. 
     * Also used in the dual capacity to handle a reply received from Flutter after sending a message.
     */
    public interface BinaryReply {
        /**
         * Handles the specified reply.
         * Parameters:
         * reply - the reply payload, a direct-allocated ByteBuffer or null. 
         * Senders of outgoing replies must place the reply bytes between position zero and current position. 
         * Reply receivers can read from the buffer directly.
         */
        void reply(ByteBuffer var1);
    }
    
    /**
     * Handler for incoming binary messages from Flutter.
     */
    public interface BinaryMessageHandler {
        /**
         * Handles the specified message.
         * Handler implementations must reply to all incoming messages, 
         * by submitting a single reply message to the given BinaryMessenger.BinaryReply. 
         * Failure to do so will result in lingering Flutter reply handlers. The reply may be submitted asynchronously.
         * <p>
         * Any uncaught exception thrown by this method will be caught by the messenger implementation and logged, 
         * and a null reply message will be sent back to Flutter.
         * <p>
         * Parameters:
         * message - the message ByteBuffer payload, possibly null.
         * reply - A BinaryMessenger.BinaryReply used for submitting a reply back to Flutter.
         */
        void onMessage(ByteBuffer var1, BinaryMessenger.BinaryReply var2);
    }
}

要命的是Flutter框架在Android中还没有注释可以看，只能从官网查看文档。
这是一个用于在Flutter和Native之间交换数据的接口类，已知FlutterView已经实现了SurfaceView，flutterNativeView负责FlutterView和Flutter之间的通讯，再使用Skia绘制页面。

总结

在阅读完FlutterActivity的部分源码以后，得出了以上几个问题的答案。

继承FlutterActivity之后，重写ViewFactory中的方法可以进入不同的FlutterView页面。
在FlutterActivityDelegate委托类里，实现了对FlutterActivity和Flutter页面生命周期的管理
HelloFlutter——MethodChannel(Native&Flutter数据交互)
FlutterView继承了SurfaceView，使用FlutterNativeView在Android和Flutter之间作为通讯的桥梁，之后调用Skia框架绘制页面。这也是其与RN和其他依赖于WebView的混合开发的框架不同的根源。
本篇文章只是闭门造车的结果，如有任何错误很抱歉！请帮忙指出，多谢了