RemoteView – Android系统内部通讯机制

一、Remote

Remote（远程的）是Android系统提供的一种跨进程通信（IPC）机制，允许一个应用程序（或进程）调用另一个进程中的服务或UI元素。通过Remote机制，应用程序可以像访问本地服务或UI元素一样访问远程服务或UI元素，这为Android应用程序开发提供了更多的灵活性。

二、Remotely

Android提供了两种方式实现对Remote的远程访问，一种是通过AIDL（Android接口定义语言）生成Java接口的方式，另一种则是通过RemoteView实现。

三、Remote Control

RemoteView（远程视图）是一种特殊的View，使用它可以在一个进程中创建一个View，然后将该View在另一个进程中显示。RemoteView在设计上基本和普通View一致，能够支持大部分的View功能和属性，但是有一定的限制，例如：

- 不支持自定义的绘制代码
- 只支持一部分的View属性，如：layout_width、layout_height、background、padding等属性
- 只能访问远程进程中已经注册的布局文件或View ID
- 只能更新布局中已存在的View（即不能动态增加Views） 

四、Remote软件下载

在实际应用中，如果需要在一个应用程序中访问另一个应用程序中的RemoteView，我们需要在另一个应用程序中注册一个RemoteView布局文件或者View ID，并在需要显示该View的时候，通过RemoteViews类构造远程视图，然后通过AppWidgetManager来更新该远程视图。下面是一个注册RemoteView布局文件并展示的示例代码：

// 在另一个进程中的RemoteService中注册RemoteView布局
public class RemoteService extends Service {
 
    private static final int LAYOUT_ID = R.layout.remote_view_layout;

    @Override
    public IBinder onBind(Intent intent) {
        return new RemoteBinder();
    }

    private class RemoteBinder extends IRemoteBinder.Stub() {
 
        @Override
        public RemoteViews getRemoteView() throws RemoteException {
            RemoteViews remoteViews = new RemoteViews(getApplicationContext().getPackageName(), LAYOUT_ID);
            // 设置点击事件
            Intent intent = new Intent(getApplicationContext(), RemoteActivity.class);
            PendingIntent pendingIntent = PendingIntent.getActivity(getApplicationContext(), 0, intent, 0);
            remoteViews.setOnClickPendingIntent(R.id.button, pendingIntent);
            return remoteViews;
        }
    }
}

// 在本地应用程序中通过RemoteViews获取远程视图并展示
public class LocalActivity extends Activity {
 
    private static final String PACKAGE_NAME = "com.example.remote";

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
 
        AppWidgetManager manager = AppWidgetManager.getInstance(this);
        RemoteViews remoteViews = new RemoteViews(PACKAGE_NAME, R.layout.remote_view_layout);
        ComponentName componentName = new ComponentName(PACKAGE_NAME, RemoteService.class.getName());
        // 获取另一个进程中RemoteService返回的RemoteViews
        remoteViews = manager.getAppWidgetOptions(componentName).getBoolean(AppWidgetManager.OPTION_APPWIDGET_HOST_CATEGORY, false)
                ? manager.createWidget(this, R.xml.widget_gadget) : remoteViews;
        // 更新本地View
        ViewGroup viewGroup = findViewById(R.id.container);
        viewGroup.addView(remoteViews.apply(this, viewGroup));
    }
}

五、Remove

除了RemoteView，在使用AIDL调用Remote服务时，我们也可以在Binder中返回RemoteView，这样就能在服务端创建一个View，然后在客户端中展示。

// 服务端实现
public class RemoteService extends Service {

    @Override
    public IBinder onBind(Intent intent) {
        return new RemoteBinder();
    }

    private class RemoteBinder extends IRemoteBinder.Stub() {

        @Override
        public RemoteViews getRemoteView() throws RemoteException {
            // 创建一个RemoteView并返回
            RemoteViews remoteViews = new RemoteViews(getApplicationContext().getPackageName(), R.layout.remote_view_layout);
            return remoteViews;
        }

        @Override
        public IBinder getRemoteBinder() throws RemoteException {
            // 返回带有远程View的Binder
            return new RemoteViewBinder(new RemoteViews(getApplicationContext().getPackageName(), R.layout.remote_view_layout));
        }
    }

    // 返回带有远程View的Binder
    private class RemoteViewBinder extends IRemoteBinder.Stub() {
        RemoteViews remoteViews;

        public RemoteViewBinder(RemoteViews remoteViews) {
            this.remoteViews = remoteViews;
        }

        @Override
        public RemoteViews getRemoteView() throws RemoteException {
            return remoteViews;
        }
    }
}

// 客户端实现
public class LocalActivity extends Activity {

    IRemoteBinder remoteService;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        // 绑定RemoteService服务
        Intent intent = new Intent(this, RemoteService.class);
        bindService(intent, conn, Context.BIND_AUTO_CREATE);
    }

    private ServiceConnection conn = new ServiceConnection() {
        @Override
        public void onServiceConnected(ComponentName name, IBinder service) {
            remoteService = IRemoteBinder.Stub.asInterface(service);
            try {
                // 获取带有远程View的Binder
                IRemoteBinder remoteBinder = remoteService.getRemoteBinder();
                // 展示远程View
                RemoteViews remoteViews = remoteBinder.getRemoteView();
                ViewGroup viewGroup = findViewById(R.id.container);
                viewGroup.addView(remoteViews.apply(getApplicationContext(), viewGroup));
            } catch (RemoteException e) {
                e.printStackTrace();
            }
        }

        @Override
        public void onServiceDisconnected(ComponentName name) {

        }
    };
}