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文章目录
  1. 1. 什么是Bound Services？
  2. 2. 构造一个Bound Services
    1. 2.1. 直接实现IBinder接口。
    2. 2.2. 使用Messenger（Messenger的内部也是使用AIDL实现的）。
    3. 2.3. 使用AIDL
      1. 2.3.1. 创建.aidl文件。
      2. 2.3.2. service端实现接口。
      3. 2.3.3. service返回接口对象给client。
      4. 2.3.4. client获取service端接口。
  3. 3. 绑定Service。

最近工作上的开发，许多与framework有关：跨进程通信、service和AIDL。这里结合官方文档总结一下。

什么是Bound Services？

Bound Services是一种能接受其他组件绑定的services。组件通过与它绑定，可以向services发起调用，services应答并可以反馈消息。组件也称client端，这里的组件指三种：activity、service、content provider。

构造一个Bound Services

Bound Services需要提供一个实现了IBinder接口的对象，用于client与services通信。该对象在public IBinder onBind(Intent intent)返回给client。services提供三种方式获取IBinder对象。

直接实现IBinder接口。

这种方法使用的场景是，services是当前应用私有的，只为当前应用服务。代码如下（注：本文所有代码来自谷歌官网文档）

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public class LocalService extends Service {
    // Binder given to clients
    private final IBinder mBinder = new LocalBinder();
    // Random number generator
    private final Random mGenerator = new Random();

    /**
     * Class used for the client Binder.  Because we know this service always
     * runs in the same process as its clients, we don't need to deal with IPC.
     */
    public class LocalBinder extends Binder {
        LocalService getService() {
            // Return this instance of LocalService so clients can call public methods
            return LocalService.this;
        }
    }

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

    /** method for clients */
    public int getRandomNumber() {
      return mGenerator.nextInt(100);
    }
}


public class BindingActivity extends Activity {
    LocalService mService;
    boolean mBound = false;

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

    @Override
    protected void onStart() {
        super.onStart();
        // Bind to LocalService
        Intent intent = new Intent(this, LocalService.class);
        bindService(intent, mConnection, Context.BIND_AUTO_CREATE);
    }

    @Override
    protected void onStop() {
        super.onStop();
        // Unbind from the service
        if (mBound) {
            unbindService(mConnection);
            mBound = false;
        }
    }

    /** Called when a button is clicked (the button in the layout file attaches to
      * this method with the android:onClick attribute) */
    public void onButtonClick(View v) {
        if (mBound) {
            // Call a method from the LocalService.
            // However, if this call were something that might hang, then this request should
            // occur in a separate thread to avoid slowing down the activity performance.
            int num = mService.getRandomNumber();
            Toast.makeText(this, "number: " + num, Toast.LENGTH_SHORT).show();
        }
    }

    /** Defines callbacks for service binding, passed to bindService() */
    private ServiceConnection mConnection = new ServiceConnection() {

        @Override
        public void onServiceConnected(ComponentName className,
                IBinder service) {
            // We've bound to LocalService, cast the IBinder and get LocalService instance
            LocalBinder binder = (LocalBinder) service;
            mService = binder.getService();
            mBound = true;
        }

        @Override
        public void onServiceDisconnected(ComponentName arg0) {
            mBound = false;
        }
    };
}

使用Messenger（Messenger的内部也是使用AIDL实现的）。

（1）services通过构造一个handler H接受client的消息 （2）使用H构造一个Messenger M（3）在onBind()返回M.getBinder() (4)client在onServiceConnected(ComponentName className,IBinder binder)中，使用binder构造一个Messenger，使用这个Messenger就可以向services发送消息，services在H接收消息。（5）client如果想接受services的消息，则client发送的消息体Message的replyTo参数带上自己Messenger即可。代码如下

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public class MessengerService extends Service {
    /** Command to the service to display a message */
    static final int MSG_SAY_HELLO = 1;

    /**
     * Handler of incoming messages from clients.
     */
    class IncomingHandler extends Handler {
        @Override
        public void handleMessage(Message msg) {
            switch (msg.what) {
                case MSG_SAY_HELLO:
                    Toast.makeText(getApplicationContext(), "hello!", Toast.LENGTH_SHORT).show();
                    break;
                default:
                    super.handleMessage(msg);
            }
        }
    }

    /**
     * Target we publish for clients to send messages to IncomingHandler.
     */
    final Messenger mMessenger = new Messenger(new IncomingHandler());

    /**
     * When binding to the service, we return an interface to our messenger
     * for sending messages to the service.
     */
    @Override
    public IBinder onBind(Intent intent) {
        Toast.makeText(getApplicationContext(), "binding", Toast.LENGTH_SHORT).show();
        return mMessenger.getBinder();
    }
}
public class ActivityMessenger extends Activity {
    /** Messenger for communicating with the service. */
    Messenger mService = null;

    /** Flag indicating whether we have called bind on the service. */
    boolean mBound;

    /**
     * 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 object we can use to
            // interact with the service.  We are communicating with the
            // service using a Messenger, so here we get a client-side
            // representation of that from the raw IBinder object.
            mService = new Messenger(service);
            mBound = true;
        }

        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;
            mBound = false;
        }
    };

    public void sayHello(View v) {
        if (!mBound) return;
        // Create and send a message to the service, using a supported 'what' value
        Message msg = Message.obtain(null, MessengerService.MSG_SAY_HELLO, 0, 0);
        try {
            mService.send(msg);
        } catch (RemoteException e) {
            e.printStackTrace();
        }
    }

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

    @Override
    protected void onStart() {
        super.onStart();
        // Bind to the service
        bindService(new Intent(this, MessengerService.class), mConnection,
            Context.BIND_AUTO_CREATE);
    }

    @Override
    protected void onStop() {
        super.onStop();
        // Unbind from the service
        if (mBound) {
            unbindService(mConnection);
            mBound = false;
        }
    }
}

使用AIDL

android接口定义语言。定义了程序接口，这些接口client和service端都能使用，能进行跨进程调用。

创建.aidl文件。

使用java语法创建后缀为.aidl的接口文件。放在service端的src目录，并且所有需要bind service的app也需要将aidl文件放入src。Android SDK Tool会根据aidl文件生成java接口。aidl文件及生成的java文件如下：

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// IRemoteService.aidl
package com.example.myapp;

// Declare any non-default types here with import statements

/** Example service interface */
interface IRemoteService {
/** Request the process ID of this service, to do evil things with it. */
int getPid();

/** Demonstrates some 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);
}

/*___Generated_by_IDEA___*/

/*
 * This file is auto-generated.  DO NOT MODIFY.
 * Original file: /Users/kongyouji/github/test/src/com/example/myapp/IRemoteService.aidl
 */
package com.example.myapp;
// Declare any non-default types here with import statements

/**
 * Example service interface
 */
public interface IRemoteService extends android.os.IInterface {
    /**
     * Local-side IPC implementation stub class.
     */
    public static abstract class Stub extends android.os.Binder implements com.example.myapp.IRemoteService {
        private static final java.lang.String DESCRIPTOR = "com.example.myapp.IRemoteService";

        /**
         * Construct the stub at attach it to the interface.
         */
        public Stub() {
            this.attachInterface(this, DESCRIPTOR);
        }

        /**
         * Cast an IBinder object into an com.example.myapp.IRemoteService interface,
         * generating a proxy if needed.
         */
        public static com.example.myapp.IRemoteService asInterface(android.os.IBinder obj) {
            if ((obj == null)) {
                return null;
            }
            android.os.IInterface iin = obj.queryLocalInterface(DESCRIPTOR);
            if (((iin != null) && (iin instanceof com.example.myapp.IRemoteService))) {
                return ((com.example.myapp.IRemoteService) iin);
            }
            return new com.example.myapp.IRemoteService.Stub.Proxy(obj);
        }

        @Override
        public android.os.IBinder asBinder() {
            return this;
        }

        @Override
        public boolean onTransact(int code, android.os.Parcel data, android.os.Parcel reply, int flags) throws android.os.RemoteException {
            switch (code) {
                case INTERFACE_TRANSACTION: {
                    reply.writeString(DESCRIPTOR);
                    return true;
                }
                case TRANSACTION_getPid: {
                    data.enforceInterface(DESCRIPTOR);
                    int _result = this.getPid();
                    reply.writeNoException();
                    reply.writeInt(_result);
                    return true;
                }
                case TRANSACTION_basicTypes: {
                    data.enforceInterface(DESCRIPTOR);
                    int _arg0;
                    _arg0 = data.readInt();
                    long _arg1;
                    _arg1 = data.readLong();
                    boolean _arg2;
                    _arg2 = (0 != data.readInt());
                    float _arg3;
                    _arg3 = data.readFloat();
                    double _arg4;
                    _arg4 = data.readDouble();
                    java.lang.String _arg5;
                    _arg5 = data.readString();
                    this.basicTypes(_arg0, _arg1, _arg2, _arg3, _arg4, _arg5);
                    reply.writeNoException();
                    return true;
                }
            }
            return super.onTransact(code, data, reply, flags);
        }

        private static class Proxy implements com.example.myapp.IRemoteService {
            private android.os.IBinder mRemote;

            Proxy(android.os.IBinder remote) {
                mRemote = remote;
            }

            @Override
            public android.os.IBinder asBinder() {
                return mRemote;
            }

            public java.lang.String getInterfaceDescriptor() {
                return DESCRIPTOR;
            }

            /**
             * Request the process ID of this service, to do evil things with it.
             */
            @Override
            public int getPid() throws android.os.RemoteException {
                android.os.Parcel _data = android.os.Parcel.obtain();
                android.os.Parcel _reply = android.os.Parcel.obtain();
                int _result;
                try {
                    _data.writeInterfaceToken(DESCRIPTOR);
                    mRemote.transact(Stub.TRANSACTION_getPid, _data, _reply, 0);
                    _reply.readException();
                    _result = _reply.readInt();
                } finally {
                    _reply.recycle();
                    _data.recycle();
                }
                return _result;
            }

            /**
             * Demonstrates some basic types that you can use as parameters
             * and return values in AIDL.
             */
            @Override
            public void basicTypes(int anInt, long aLong, boolean aBoolean, float aFloat, double aDouble, java.lang.String aString) throws android.os.RemoteException {
                android.os.Parcel _data = android.os.Parcel.obtain();
                android.os.Parcel _reply = android.os.Parcel.obtain();
                try {
                    _data.writeInterfaceToken(DESCRIPTOR);
                    _data.writeInt(anInt);
                    _data.writeLong(aLong);
                    _data.writeInt(((aBoolean) ? (1) : (0)));
                    _data.writeFloat(aFloat);
                    _data.writeDouble(aDouble);
                    _data.writeString(aString);
                    mRemote.transact(Stub.TRANSACTION_basicTypes, _data, _reply, 0);
                    _reply.readException();
                } finally {
                    _reply.recycle();
                    _data.recycle();
                }
            }
        }

        static final int TRANSACTION_getPid = (android.os.IBinder.FIRST_CALL_TRANSACTION + 0);
        static final int TRANSACTION_basicTypes = (android.os.IBinder.FIRST_CALL_TRANSACTION + 1);
    }

    /**
     * Request the process ID of this service, to do evil things with it.
     */
    public int getPid() throws android.os.RemoteException;

    /**
     * Demonstrates some basic types that you can use as parameters
     * and return values in AIDL.
     */
    public void basicTypes(int anInt, long aLong, boolean aBoolean, float aFloat, double aDouble, java.lang.String aString) throws android.os.RemoteException;
}

service端实现接口。

上述生成的java有内部类Stub，它继承自android.os.Binder对象并抽象实现了aidl接口。service需要继承Stub对象，并实现抽象接口。代码如下，mBinder对象将在client绑定service时返回。

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private final IRemoteService.Stub mBinder = new IRemoteService.Stub() {
    public int getPid(){
        return Process.myPid();
    }
    public void basicTypes(int anInt, long aLong, boolean aBoolean,
        float aFloat, double aDouble, String aString) {
        // Does nothing
    }
};

service返回接口对象给client。

service将实现Stub的对象返回给client端，代码如下：

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public class RemoteService extends Service {
    @Override
    public void onCreate() {
        super.onCreate();
    }

    @Override
    public IBinder onBind(Intent intent) {
        // Return the interface
        return mBinder;
    }

    private final IRemoteService.Stub mBinder = new IRemoteService.Stub() {
        public int getPid(){
            return Process.myPid();
        }
        public void basicTypes(int anInt, long aLong, boolean aBoolean,
            float aFloat, double aDouble, String aString) {
            // Does nothing
        }
    };
}

client获取service端接口。

在onServiceConnected中，通过调用Stub.asInterface获得service接口对象。通过这个对象，client就可以直接调用service操作了。代码如下：

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Intent intent = new Intent(this, RemoteService.class);
bindService(intent, mConnection, Context.BIND_AUTO_CREATE);

IRemoteService mIRemoteService;
private ServiceConnection mConnection = new ServiceConnection() {
    // Called when the connection with the service is established
    public void onServiceConnected(ComponentName className, IBinder service) {
        // Following the example above for an AIDL interface,
        // this gets an instance of the IRemoteInterface, which we can use to call on the service
        mIRemoteService = IRemoteService.Stub.asInterface(service);
    }

    // Called when the connection with the service disconnects unexpectedly
    public void onServiceDisconnected(ComponentName className) {
        Log.e(TAG, "Service has unexpectedly disconnected");
        mIRemoteService = null;
    }
};

绑定Service。

client实现ServiceConnection接口，并且调用bindService()绑定，调用unBindService()解除绑定。以上例子都有实现，这里不再重复代码。下面是需要注意的点.
（1）如果service仅仅通过bindService()启动（onStartCommand()未被调用过），则在bindService()的第三个参数应该写为BIND_AUTO_CREATE，在Service未启动时，它会create一个Service并且与其绑定。
（2）如果Service仅仅通过bindService()启动，则Android系统会自动管理Service的生命周期，在它所有client都解绑后，自动destory。如果onStartCommand()被调用过，则需要我们自己维护它的生命周期，调用stopSelf()或stopService()销毁Service。

文章目录
  1. 1. 什么是Bound Services？
  2. 2. 构造一个Bound Services
    1. 2.1. 直接实现IBinder接口。
    2. 2.2. 使用Messenger（Messenger的内部也是使用AIDL实现的）。
    3. 2.3. 使用AIDL
      1. 2.3.1. 创建.aidl文件。
      2. 2.3.2. service端实现接口。
      3. 2.3.3. service返回接口对象给client。
      4. 2.3.4. client获取service端接口。
  3. 3. 绑定Service。