Android Programming: Pushing the Limits -- Chapter 7:Android IPC -- Messenger

Messenger类实际是对Aidl方式的一层封装。本文只是对如何在Service中使用Messenger类实现与客户端的通信进行讲解，对Messenger的底层不做说明。阅读Android Programming: Pushing the Limits -- Chapter 7:Android IPC -- AIDL了解如何使用Aidl的方式实现服务端与客户端的通信。

在Service中使用Messenger，大部分代码还是跟Android的消息机制打交道，具体一点就是跟Handler，Message打交道。阅读Android -- Looper、Handler、MessageQueue等类之间关系的序列图了解Android的消息机制。

服务端：

Step 1：定义两个变量

private Handler mMessageHandler;
private Messenger mMessenger;

Step 2：对两个变量进行赋值

     @Override
    public void onCreate() {
        super.onCreate();
        HandlerThread handlerThread = new HandlerThread("MessengerService");
        handlerThread.start();
        mMessageHandler = new Handler(handlerThread.getLooper(), new MyHandlerCallback());
        mMessenger = new Messenger(mMessageHandler);
    }

这里我们使用到了HandlerThread进行辅助。而MyHandlerCallback实现了Handler.Callback接口，实现对消息的处理，完成具体操作。

Step 3：实现Handler.Callback接口

 private class MyHandlerCallback implements Handler.Callback{
        @Override
        public boolean handleMessage(Message msg) {
            boolean delivered = false;
            switch (msg.what){
                case MessageApi.SEND_TEXT_MSG:
                    Bundle bundle = msg.getData();
                    delivered = sendTextMessage(bundle.getString(MessageApi.MSG_TEXT_KEY));
                    break;
                case MessageApi.SEND_PHOTO_MSG:
                    delivered = sendPhotoMessage((Bitmap) msg.obj);
                    break;
            }
            Message reply = Message.obtain();
            reply.what = MessageApi.MESSAGE_DELIVERED_MSG;
            Bundle bundle = new Bundle();
            bundle.putBoolean(MessageApi.MSG_DELIVERED_KEY, delivered);
            reply.setData(bundle);
            try {
                // Send message back via Message.replyto
                msg.replyTo.send(reply);
            } catch (RemoteException e) {
                Log.e(TAG, "Error sending message reply!", e);
            }

            return true;
        }
    }

这里与一般的handleMessage没多大区别，主要就是在给客户端回消息时使用到了Message.replyto。因此，可想而知，在客户端发送消息时，如果要接收服务端的消息就必须对消息指定replyto。而replyto实际也是一个Messenger实例。而服务端与客户端使用的消息代码要保持一致，因此这里我们单独用了一个类MessageApi进行存放：

public class MessageApi {
    public static final int SEND_TEXT_MSG = 10;
    public static final int SEND_PHOTO_MSG = 20;
    public static final int MESSAGE_DELIVERED_MSG = 30;
    public static final String MSG_TEXT_KEY = "text_key";
    public static final String MSG_DELIVERED_KEY = "delivered_key";
}

Step 4：返回Binder实例对象到客户端。

    @Override
    public IBinder onBind(Intent intent) {
        Log.d(TAG, "onBind");
        return mMessenger.getBinder();
    }

以上就是服务端的基本步骤了。

服务端示例完整代码：

package com.ldb.android.example.messengerservice.service;

import android.app.Service;
import android.content.Intent;
import android.graphics.Bitmap;
import android.os.Bundle;
import android.os.Handler;
import android.os.HandlerThread;
import android.os.IBinder;
import android.os.Message;
import android.os.Messenger;
import android.os.RemoteException;
import android.support.annotation.Nullable;
import android.util.Log;

import com.ldb.android.example.messengerservice.api.MessageApi;

/**
 * Created by lsp on 2016/9/2.
 */
public class MessengerService extends Service {
    private static final String TAG = "MessengerService";

    private Handler mMessageHandler;
    private Messenger mMessenger;

    @Override
    public void onCreate() {
        super.onCreate();
        HandlerThread handlerThread = new HandlerThread("MessengerService");
        handlerThread.start();
        mMessageHandler = new Handler(handlerThread.getLooper(), new MyHandlerCallback());
        mMessenger = new Messenger(mMessageHandler);
    }

    @Nullable
    @Override
    public IBinder onBind(Intent intent) {
        Log.d(TAG, "onBind");
        return mMessenger.getBinder();
    }

    @Override
    public void onDestroy() {
        super.onDestroy();
        mMessageHandler.getLooper().quit();
    }

    private class MyHandlerCallback implements Handler.Callback{
        @Override
        public boolean handleMessage(Message msg) {
            boolean delivered = false;
            switch (msg.what){
                case MessageApi.SEND_TEXT_MSG:
                    Bundle bundle = msg.getData();
                    delivered = sendTextMessage(bundle.getString(MessageApi.MSG_TEXT_KEY));
                    break;
                case MessageApi.SEND_PHOTO_MSG:
                    delivered = sendPhotoMessage((Bitmap) msg.obj);
                    break;
            }
            Message reply = Message.obtain();
            reply.what = MessageApi.MESSAGE_DELIVERED_MSG;
            Bundle bundle = new Bundle();
            bundle.putBoolean(MessageApi.MSG_DELIVERED_KEY, delivered);
            reply.setData(bundle);
            try {
                // Send message back via Message.replyto
                msg.replyTo.send(reply);
            } catch (RemoteException e) {
                Log.e(TAG, "Error sending message reply!", e);
            }

            return true;
        }
    }

    // Return true when delivered
    private boolean sendPhotoMessage(Bitmap photo) {
        // Implementation left out for brevity
        Log.d(TAG, "sendPhotoMessage");
        return true;
    }
    // Return true when delivered
    private boolean sendTextMessage(String textMessage) {
        // Implementation left out for brevity
        Log.d(TAG, "sendTextMessage: " + textMessage);
        return true;
    }

}

客户端：

Step 1：将服务端的MessageApi类拷贝到客户端。

Step 2：定义三个变量

    private Messenger mRemoteMessenger;
    private Messenger mReplyMessenger;
    private Handler mReplyHandler;

Step 3：为mReplyHandler与mReplyMessenger赋值

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

        mSendButton = (Button) findViewById(R.id.send_button);
        mSendButton.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                sendText();
            }
        });

        HandlerThread handlerThread = new HandlerThread("MessageClient");
        handlerThread.start();
            mReplyHandler = new Handler(handlerThread.getLooper(), new ReplyHandlerCallback());
        mReplyMessenger = new Messenger(mReplyHandler);
    }

与服务端类似。ReplyHandlerCallback实现了Handler.Callback接口.

Step 4：实行Handler.Callback，处理服务端返回的消息。

    private class ReplyHandlerCallback implements Handler.Callback{
        @Override
        public boolean handleMessage(Message msg) {
            switch (msg.what){
                case MessageApi.MESSAGE_DELIVERED_MSG:
                    Bundle bundle = msg.getData();
                    boolean delivered = (boolean) bundle.getBoolean(MessageApi.MSG_DELIVERED_KEY);
                    Log.d(TAG, "delivered: " + delivered);
                    break;
            }
            return true;
        }
    }

Step 5：实现ServiceConnection接口，在onServiceConnected()方法中，我们获取到了服务端的代理Messenger -- mRemoteMessenger， 客户端与服务端的通信都是通过此代理来完成的。

    @Override
    public void onServiceConnected(ComponentName name, IBinder service) {
        mRemoteMessenger = new Messenger(service);
    }

    @Override
    public void onServiceDisconnected(ComponentName name) {
        mRemoteMessenger = null;
    }

Step 6：bindService，从Android 5.0开始，bindService需要通过Explicit Intent。

    @Override
    protected void onResume() {
        super.onResume();
        // Since Android 5.0(Lollipop), bindService should use explicit intent.
        Intent intent = new Intent("com.ldb.android.example.messengerservice.MessengerService");
        bindService(
                createExplicitFromImplicitIntent(this, intent),
                this, BIND_AUTO_CREATE);
    }

Step 7：记得unbindService。

Step 8：发送消息，使用Bundle作为消息内容的载体，不要使用Message.obj，书中是直接使用Message.obj，但是实际操作却报错了，网上建议使用Bundle。如果希望服务端返回消息，则需要指定replyto，实际就是我们在onCreate中实例化的mReplyMessenger，因为它与mReplyHandler相关联，因此服务端通过它返回的消息最终都由mReplyHandler进行处理。

    public void sendText(){
        String textMessage = ((EditText) findViewById(R.id.message_edit_text)).getText().toString();
        Message message = Message.obtain();
        message.what = MessageApi.SEND_TEXT_MSG;
        // Can't use Message.obj
//        message.obj = textMessage;
        // Use Bundle to load the message content.
        Bundle bundle = new Bundle();
        bundle.putString(MessageApi.MSG_TEXT_KEY, textMessage);
        message.setData(bundle);
        // Service would use replyto to send message back.
        message.replyTo = mReplyMessenger;
        try {
            mRemoteMessenger.send(message);
        } catch (RemoteException e) {
            Log.e(TAG, "Error sendText: " + message, e);
        }
    }

以上就是客户端的基本步骤了。

客户端完整代码：

package com.ldb.android.example.messengerclient;

import android.content.ComponentName;
import android.content.Context;
import android.content.Intent;
import android.content.ServiceConnection;
import android.content.pm.PackageManager;
import android.content.pm.ResolveInfo;
import android.os.Handler;
import android.os.HandlerThread;
import android.os.IBinder;
import android.os.Message;
import android.os.Messenger;
import android.os.RemoteException;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.util.Log;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;

import com.ldb.android.example.messengerclient.api.MessageApi;

import java.util.List;

public class MainActivity extends AppCompatActivity implements ServiceConnection{
    private static final String TAG = "MainActivity";

    private Messenger mRemoteMessenger;
    private Messenger mReplyMessenger;
    private Handler mReplyHandler;
    private Button mSendButton;

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

        mSendButton = (Button) findViewById(R.id.send_button);
        mSendButton.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                sendText();
            }
        });

        HandlerThread handlerThread = new HandlerThread("MessageClient");
        handlerThread.start();
            mReplyHandler = new Handler(handlerThread.getLooper(), new ReplyHandlerCallback());
        mReplyMessenger = new Messenger(mReplyHandler);
    }

    @Override
    protected void onResume() {
        super.onResume();
        // Since Android 5.0(Lollipop), bindService should use explicit intent.
        Intent intent = new Intent("com.ldb.android.example.messengerservice.MessengerService");
        bindService(
                createExplicitFromImplicitIntent(this, intent),
                this, BIND_AUTO_CREATE);
    }

    @Override
    protected void onPause() {
        super.onPause();
        unbindService(this);
    }

    @Override
    protected void onDestroy() {
        super.onDestroy();
        mReplyHandler.getLooper().quit();
    }

    @Override
    public void onServiceConnected(ComponentName name, IBinder service) {
        mRemoteMessenger = new Messenger(service);
    }

    @Override
    public void onServiceDisconnected(ComponentName name) {
        mRemoteMessenger = null;
    }

    public void sendText(){
        String textMessage = ((EditText) findViewById(R.id.message_edit_text)).getText().toString();
        Message message = Message.obtain();
        message.what = MessageApi.SEND_TEXT_MSG;
        // Can't use Message.obj
//        message.obj = textMessage;
        // Use Bundle to load the message content.
        Bundle bundle = new Bundle();
        bundle.putString(MessageApi.MSG_TEXT_KEY, textMessage);
        message.setData(bundle);
        // Service would use replyto to send message back.
        message.replyTo = mReplyMessenger;
        try {
            mRemoteMessenger.send(message);
        } catch (RemoteException e) {
            Log.e(TAG, "Error sendText: " + message, e);
        }
    }

    public static Intent createExplicitFromImplicitIntent(Context context, Intent implicitIntent) {
        // Retrieve all services that can match the given intent
        PackageManager pm = context.getPackageManager();
        List<ResolveInfo> resolveInfo = pm.queryIntentServices(implicitIntent, 0);

        // Make sure only one match was found
        if (resolveInfo == null || resolveInfo.size() != 1) {
            return null;
        }

        // Get component info and create ComponentName
        ResolveInfo serviceInfo = resolveInfo.get(0);
        String packageName = serviceInfo.serviceInfo.packageName;
        String className = serviceInfo.serviceInfo.name;
        ComponentName component = new ComponentName(packageName, className);

        // Create a new intent. Use the old one for extras and such reuse
        Intent explicitIntent = new Intent(implicitIntent);

        // Set the component to be explicit
        explicitIntent.setComponent(component);

        return explicitIntent;
    }

    private class ReplyHandlerCallback implements Handler.Callback{
        @Override
        public boolean handleMessage(Message msg) {
            switch (msg.what){
                case MessageApi.MESSAGE_DELIVERED_MSG:
                    Bundle bundle = msg.getData();
                    boolean delivered = (boolean) bundle.getBoolean(MessageApi.MSG_DELIVERED_KEY);
                    Log.d(TAG, "delivered: " + delivered);
                    break;
            }
            return true;
        }
    }
}

布局文件：

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical">

    <EditText
        android:id="@+id/message_edit_text"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"/>

    <Button
        android:id="@+id/send_button"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:text="Send"/>
</LinearLayout>