使用AIDL实现IPC通信之——实现服务端主动发送数据给客户端_android aidl服务端主动向客户端发消息

作者：2023面试高手 | 2024-03-06 13:31:26

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android aidl服务端主动向客户端发消息

前一篇文章讲了怎么在客户端使用AIDL实现IPC通信，调用远程服务端的方法。但是，远程服务端并不能主动给客户端返回信息。在很多情况下是需要远程服务端主动给客户端返回数据，客户端只需要进行监听即可，这是典型的观察者模式。这篇文章主要来解决一下这个问题。

代码主要来自ApiDemos/App/Service/Remote Service Binding，下面对代码进行说明。

1、首先是AIDL接口定义

这里定义了三个接口，首先是IRemoteService，这个接口主要是用于客户端注册和解注册回调接口，这样服务端就可以往客户端回传数据。


package com.easyliu.demo.aidl;
 
import com.easyliu.demo.aidl.IRemoteServiceCallback;
/**
 * Example of defining an interface for calling on to a remote service
 * (running in another process).
 */
interface IRemoteService {
    /**
     * Often you want to allow a service to call back to its clients.
     * This shows how to do so, by registering a callback interface with
     * the service.
     */
    void registerCallback(IRemoteServiceCallback cb);
    
    /**
     * Remove a previously registered callback interface.
     */
    void unregisterCallback(IRemoteServiceCallback cb);
}

然后是IRemoteServiceCallback,这个是回调接口，用于往客户端回传信息。由于AIDL接口中不支持一般的interface，所以接口也得是aidl接口类型，如下所示：


package com.easyliu.demo.aidl;
 
/**
 * Example of a callback interface used by IRemoteService to send
 * synchronous notifications back to its clients.  Note that this is a
 * one-way interface so the server does not block waiting for the client.
 */
oneway interface IRemoteServiceCallback {
    /**
     * Called when the service has a new value for you.
     */
    void valueChanged(int value);
}

最后是另一个aidl接口ISecondary，接口中定义了两个方法，如下所示。有一个方法是获取服务端所在进程的PID，这样我们可以看到当客户端根据这个PID杀死服务端进程的时候会出现什么反应，这个后面再说。


interface ISecondary {
    /**
     * Request the PID of this service, to do evil things with it.
     */
    int getPid();
    
    /**
     * This demonstrates the basic types that you can use as parameters
     * and return values in AIDL.
     */
    void basicTypes(int anInt, long aLong, boolean aBoolean, float aFloat,
            double aDouble, String aString);
}

2、AIDL接口在服务端的实现

service端的代码如下所示。有几点需要讲解：

1、使用RemoteCallbackList来保存客户端传过来的回调接口，使用它可以保证服务端接收到的对象和客户端的是同一个对象。

2、IRemoteSevice的两个方法分别是注册接口和解注册接口，使用RemoteCallbackList的register和unregister方法。

3、ISecondary的getPid方法当中返回当前服务端进程的PID。

4、在service的oncreate方法中发送了一个消息给消息队列，Handler接收到这个消息之后给服务端发送一个值，发送完成之后每隔一秒发送一个消息，这样客户端每隔一秒就会收到服务端发来的值，这个值就是一个累加的数字。

5、调用RemoteCallbackList当中保存的回调接口发送数据有固定的写法，如下所示。首先得开始广播，然后得到list当中的每一项，然后调用此接口的方法。当所有注册的接口都回调完成之后，需要结束广播。


final int N = mCallbacks.beginBroadcast();
                    for (int i=0; i<N; i++) {
                        try {
                            mCallbacks.getBroadcastItem(i).valueChanged(value);
                        } catch (RemoteException e) {
                            // The RemoteCallbackList will take care of removing
                            // the dead object for us.
                        }
                    }
                    mCallbacks.finishBroadcast();

远程Service代码：


package com.easyliu.demo.aidldemo;
 
import android.app.Activity;
import android.app.Notification;
import android.app.NotificationManager;
import android.app.PendingIntent;
import android.app.Service;
import android.content.ComponentName;
import android.content.Context;
import android.content.Intent;
import android.content.ServiceConnection;
import android.os.Bundle;
import android.os.Handler;
import android.os.IBinder;
import android.os.Message;
import android.os.Process;
import android.os.RemoteCallbackList;
import android.os.RemoteException;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.Button;
import android.widget.TextView;
import android.widget.Toast;
 
import com.easyliu.demo.aidl.IRemoteService;
import com.easyliu.demo.aidl.IRemoteServiceCallback;
import com.easyliu.demo.aidl.ISecondary;
 
public class RemoteService extends Service {
    /**
     * This is a list of callbacks that have been registered with the
     * service.  Note that this is package scoped (instead of private) so
     * that it can be accessed more efficiently from inner classes.
     */
    final RemoteCallbackList<IRemoteServiceCallback> mCallbacks
            = new RemoteCallbackList<IRemoteServiceCallback>();
    private int mValue = 0;
    private static final int REPORT_MSG = 1;
    @Override
    public void onCreate() {
        mHandler.sendEmptyMessage(REPORT_MSG);
    }
 
    @Override
    public void onDestroy() {
        // Tell the user we stopped.
        Toast.makeText(this, R.string.remote_service_stopped, Toast.LENGTH_SHORT).show();
        // Unregister all callbacks.
        mCallbacks.kill();
        // Remove the next pending message to increment the counter, stopping
        // the increment loop.
        mHandler.removeMessages(REPORT_MSG);
    }
    
    @Override
    public IBinder onBind(Intent intent) {
        // Select the interface to return.  If your service only implements
        // a single interface, you can just return it here without checking
        // the Intent.
        if (IRemoteService.class.getName().equals(intent.getAction())) {
            return mBinder;
        }
        if (ISecondary.class.getName().equals(intent.getAction())) {
            return mSecondaryBinder;
        }
        return null;
    }
 
    /**
     * The IRemoteInterface is defined through IDL
     */
    private final IRemoteService.Stub mBinder = new IRemoteService.Stub() {
        public void registerCallback(IRemoteServiceCallback cb) {
            if (cb != null) mCallbacks.register(cb);
        }
        public void unregisterCallback(IRemoteServiceCallback cb) {
            if (cb != null) mCallbacks.unregister(cb);
        }
    };
 
    /**
     * A secondary interface to the service.
     */
    private final ISecondary.Stub mSecondaryBinder = new ISecondary.Stub() {
        public int getPid() {
            return Process.myPid();
        }
        public void basicTypes(int anInt, long aLong, boolean aBoolean,
                float aFloat, double aDouble, String aString) {
        }
    };
 
    @Override
    public void onTaskRemoved(Intent rootIntent) {
        Toast.makeText(this, "Task removed: " + rootIntent, Toast.LENGTH_LONG).show();
    }
    
    /**
     * Our Handler used to execute operations on the main thread.  This is used
     * to schedule increments of our value.
     */
    private final Handler mHandler = new Handler() {
        @Override public void handleMessage(Message msg) {
            switch (msg.what) {
                // It is time to bump the value!
                case REPORT_MSG: {
                    // Up it goes.
                    int value = ++mValue;
                    // Broadcast to all clients the new value.
                    final int N = mCallbacks.beginBroadcast();
                    for (int i=0; i<N; i++) {
                        try {
                            mCallbacks.getBroadcastItem(i).valueChanged(value);
                        } catch (RemoteException e) {
                            // The RemoteCallbackList will take care of removing
                            // the dead object for us.
                        }
                    }
                    mCallbacks.finishBroadcast();
                    // Repeat every 1 second.
                    sendMessageDelayed(obtainMessage(REPORT_MSG), 1*1000);
                } break;
                default:
                    super.handleMessage(msg);
            }
        }
    };
}

3、在Manifest文件里面注册Service

在Service当中加了几个action，用于别的组件通过Intent隐式启动此Service。


<service
            android:name=".RemoteService"
            android:process=":remote">
            <intent-filter>
                <action android:name="com.easyliu.demo.aidl.IRemoteService" />
                <action android:name="com.easyliu.demo.aidl.ISecondary" />
                <action android:name="com.easyliu.demo.aidldemo.RemoteService" />
            </intent-filter>
        </service>

4、客户端的实现
客户端界面主要是由三个按钮：绑定、解除绑定、杀死服务端进程，然后还有一个显示状态的文本控件。

布局文件如下所示：


<?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:gravity="center"
    android:orientation="vertical">
 
    <Button
        android:id="@+id/bind"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="@string/bind_service">
        <requestFocus />
    </Button>
 
    <Button
        android:id="@+id/unbind"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="@string/unbind_service"></Button>
 
    <Button
        android:id="@+id/kill"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="@string/kill_process"></Button>
 
    <TextView
        android:id="@+id/callback"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_weight="0"
        android:gravity="center_horizontal"
        android:paddingTop="4dip"
        android:textAppearance="?android:attr/textAppearanceMedium" />
</LinearLayout>

主Activity代码如下所示。有几点需要说明：

1、点击BIND SERVICE按钮的时候，同时绑定ISecondary和IRemoteService，返回相应的接口。同时，给返回的IRemoteService接口注册一个回调接口，用于接收服务端发来的信息。IRemoteServiceCallback回调接口如下所示，在注释中已经有了说明，由于valuedChanged方法是运行客户端的Binder线程当中，是不能直接访问主UI当中的控件的，所以需要通过Handler切换到主UI线程中去执行。

注意：如果valuedChanged比较耗时的话，必须确保RemoteService当中的valueChanged方法不是运行在主UI当中，不然会导致服务端无法响应。

同理：在客户端调用服务端的方法的时候，如果服务端的方法比较耗时，我们就得避免在客户端的UI线程当中去访问远程方法，不然会导致客户端无响应。


 private IRemoteServiceCallback mCallback = new IRemoteServiceCallback.Stub() {
        /**
         * This is called by the remote service regularly to tell us about
         * new values.  Note that IPC calls are dispatched through a thread
         * pool running in each process, so the code executing here will
         * NOT be running in our main thread like most other things -- so,
         * to update the UI, we need to use a Handler to hop over there.
         */
        public void valueChanged(int value) {
            mHandler.sendMessage(mHandler.obtainMessage(BUMP_MSG, value, 0));
        }
    };

mHandler的实现如下所示：


  private Handler mHandler = new Handler() {
        @Override
        public void handleMessage(Message msg) {
            switch (msg.what) {
                case BUMP_MSG:
                    mCallbackText.setText("Received from service: " + msg.arg1);
                    break;
                default:
                    super.handleMessage(msg);
            }
        }
    };

2、点击UNBIND SERVICE按钮的时候，需要先解注册之前注册的I RemoteServiceCallback回调接口，然后再unbindService。

3、在执行bindService的时候，代码如下所示，第三个参数有几个可选项，一般选Context.BIND_AUTO_CREATE，意思是如果在绑定过程中，Service进程被意外杀死了，系统还会自动重新启动被绑定的Service。所以当我们点击KILL PROCESS按钮的时候会杀死Service进程，但是马上又会自动重启，重新调用onServiceConnected方法重新绑定。当然，这个参数还有别的一些选择。


bindService(intent,
        mConnection, Context.BIND_AUTO_CREATE);

主Activity代码：


package com.easyliu.demo.aidldemo;
 
import android.app.Activity;
import android.content.ComponentName;
import android.content.Context;
import android.content.Intent;
import android.content.ServiceConnection;
import android.os.Handler;
import android.os.IBinder;
import android.os.Message;
import android.os.Process;
import android.os.RemoteException;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.TextView;
import android.widget.Toast;
 
import com.easyliu.demo.aidl.IRemoteService;
import com.easyliu.demo.aidl.IRemoteServiceCallback;
import com.easyliu.demo.aidl.ISecondary;
 
public class BindActivity extends AppCompatActivity {
    IRemoteService mService = null;
    ISecondary mSecondaryService = null;
    Button mKillButton;
    TextView mCallbackText;
    private boolean mIsBound;
    private static final int BUMP_MSG = 1;
 
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.remote_service_binding);
        Button button = (Button) findViewById(R.id.bind);
        button.setOnClickListener(mBindListener);
        button = (Button) findViewById(R.id.unbind);
        button.setOnClickListener(mUnbindListener);
        mKillButton = (Button) findViewById(R.id.kill);
        mKillButton.setOnClickListener(mKillListener);
        mKillButton.setEnabled(false);
        mCallbackText = (TextView) findViewById(R.id.callback);
        mCallbackText.setText("Not attached.");
    }
    private Handler mHandler = new Handler() {
        @Override
        public void handleMessage(Message msg) {
            switch (msg.what) {
                case BUMP_MSG:
                    mCallbackText.setText("Received from service: " + msg.arg1);
                    break;
                default:
                    super.handleMessage(msg);
            }
        }
    };
    /**
     * Class for interacting with the main interface of the service.
     */
    private ServiceConnection mConnection = new ServiceConnection() {
        public void onServiceConnected(ComponentName className,
                                       IBinder service) {
            // This is called when the connection with the service has been
            // established, giving us the service object we can use to
            // interact with the service.  We are communicating with our
            // service through an IDL interface, so get a client-side
            // representation of that from the raw service object.
            mService = IRemoteService.Stub.asInterface(service);
            mKillButton.setEnabled(true);
            mCallbackText.setText("Attached.");
 
            // We want to monitor the service for as long as we are
            // connected to it.
            try {
                mService.registerCallback(mCallback);
            } catch (RemoteException e) {
                // In this case the service has crashed before we could even
                // do anything with it; we can count on soon being
                // disconnected (and then reconnected if it can be restarted)
                // so there is no need to do anything here.
            }
 
            // As part of the sample, tell the user what happened.
            Toast.makeText(BindActivity.this, R.string.remote_service_connected,
                    Toast.LENGTH_SHORT).show();
        }
 
        public void onServiceDisconnected(ComponentName className) {
            // This is called when the connection with the service has been
            // unexpectedly disconnected -- that is, its process crashed.
            mService = null;
            mKillButton.setEnabled(false);
            mCallbackText.setText("Disconnected.");
 
            // As part of the sample, tell the user what happened.
            Toast.makeText(BindActivity.this, R.string.remote_service_disconnected,
                    Toast.LENGTH_SHORT).show();
        }
    };
 
    /**
     * Class for interacting with the secondary interface of the service.
     */
    private ServiceConnection mSecondaryConnection = new ServiceConnection() {
        public void onServiceConnected(ComponentName className,
                                       IBinder service) {
            // Connecting to a secondary interface is the same as any
            // other interface.
            mSecondaryService = ISecondary.Stub.asInterface(service);
            mKillButton.setEnabled(true);
        }
 
        public void onServiceDisconnected(ComponentName className) {
            mSecondaryService = null;
            mKillButton.setEnabled(false);
        }
    };
    /**
     * 绑定
     */
    private View.OnClickListener mBindListener = new View.OnClickListener() {
        public void onClick(View v) {
            Intent intent=new Intent(IRemoteService.class.getName());
            intent.setPackage("com.easyliu.demo.aidldemo");
            //注意这里的Context.BIND_AUTO_CREATE，这意味这如果在绑定的过程中，
            //如果Service由于某种原因被Destroy了，Android还会自动重新启动被绑定的Service。
            // 你可以点击Kill Process 杀死Service看看结果
            bindService(intent,
                    mConnection, Context.BIND_AUTO_CREATE);
            intent=new Intent(ISecondary.class.getName());
            intent.setPackage("com.easyliu.demo.aidldemo");
            bindService(intent,
                    mSecondaryConnection, Context.BIND_AUTO_CREATE);
            mIsBound = true;
            mCallbackText.setText("Binding.");
        }
    };
    /**
     * 解除绑定
     */
    private View.OnClickListener mUnbindListener = new View.OnClickListener() {
        public void onClick(View v) {
            if (mIsBound) {
                if (mService != null) {
                    try {
                        mService.unregisterCallback(mCallback);
                    } catch (RemoteException e) {
                        // There is nothing special we need to do if the service
                        // has crashed.
                    }
                }
                // Detach our existing connection.
                unbindService(mConnection);
                unbindService(mSecondaryConnection);
                mKillButton.setEnabled(false);
                mIsBound = false;
                mCallbackText.setText("Unbinding.");
            }
        }
    };
 
    private View.OnClickListener mKillListener = new View.OnClickListener() {
        public void onClick(View v) {
            // To kill the process hosting our service, we need to know its
            // PID.  Conveniently our service has a call that will return
            // to us that information.
            if (mSecondaryService != null) {
                try {
                    int pid = mSecondaryService.getPid();
                    // Note that, though this API allows us to request to
                    // kill any process based on its PID, the kernel will
                    // still impose standard restrictions on which PIDs you
                    // are actually able to kill.  Typically this means only
                    // the process running your application and any additional
                    // processes created by that app as shown here; packages
                    // sharing a common UID will also be able to kill each
                    // other's processes.
                    Process.killProcess(pid);
                    mCallbackText.setText("Killed service process.");
                } catch (RemoteException ex) {
                    // Recover gracefully from the process hosting the
                    // server dying.
                    // Just for purposes of the sample, put up a notification.
                    Toast.makeText(BindActivity.this,
                            R.string.remote_call_failed,
                            Toast.LENGTH_SHORT).show();
                }
            }
        }
    };
    /**
     * 远程回调接口实现
     */
    private IRemoteServiceCallback mCallback = new IRemoteServiceCallback.Stub() {
        /**
         * This is called by the remote service regularly to tell us about
         * new values.  Note that IPC calls are dispatched through a thread
         * pool running in each process, so the code executing here will
         * NOT be running in our main thread like most other things -- so,
         * to update the UI, we need to use a Handler to hop over there.
         */
        public void valueChanged(int value) {
            mHandler.sendMessage(mHandler.obtainMessage(BUMP_MSG, value, 0));
        }
    };
}

5、执行效果

当点击BIND SERVICE按钮，就会每隔一秒收到服务端发来的消息：

当点击KILL PROCESS按钮的时候，首先会杀死Service进程，然后马上就会重新启动Service进程，重新绑定。这是因为在bindService的时候，第三个参数设置为了Context.BIND_AUTO_CREATE，所以就会出现这样的效果。

此外：

1、在进行IPC通信的时候，还可以验证权限，只有具有某个权限的APP才能绑定此服务然后返回Binder，然后使用此Binder进行通信。关于权限验证这里不讲。

2、同时需要给返回的IBinder对象设置了一个死亡代理，当远端Service由于某种原因死亡的时候，就会调用此回调方法，我们就可以在此方法当中进行一些操作，比如，重新bindService等。这个在前一节有讲。

3、以上例程显示了两个AIDL接口均在同一个Service里面进行实现的情况。由于Service是四大组件之一，是一种系统资源，不适合无限制的增加Service，最好是把所有的AIDL放在同一个Service当中去管理。可以自己写一个工具专门用来管理所有的AIDL连接，把工具设置成单例，然后暴露给客户端一些接口即可。

代码下载地址：https://github.com/EasyLiu-Ly/AIDLDemo

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