Android接口描述语言（AIDL）

AIDL定义客户端和服务器端都遵守的接口，进行进程间通信。

Note:AIDL使用场景：来自多个应用的客户端通过IPC访问服务，并且服务需要处理多线程的场景；

如果你没有来自不同客户端的并发场景，建议使用Binder接口；如果你需要处理IPC，但是没有多线程的场景，建议使用Messenger。（这两种用法在http://www.cnblogs.com/konger/p/3923518.html有介绍）

AIDL接口调用是直接的函数调用方式，同进程和其他进程调用之间有所区别：

• 同进程调用AIDL接口，AIDL接口代码在调用该接口的线程中完成。（如果仅仅是同进程调用AIDL，就建议用Binder接口）
• 其他进程调用AIDL，那们会在AIDL所属进程的线程池中分派一个线程来执行AIDL代码，所以AIDL可能被多线程同时访问，需要保证线程安全。
• 可以通过oneway关键字来修改远程调用的行为属性，如果使用这个关键字，那么远程调用会直接返回而不等结果，不会阻塞调用者线程的运行。关键字oneway对于本地调用没有任何影响。（疑问：直接返回的话，存在两个问题：1、结果怎么获取？2、后台运行可能会被中断？）

定义AIDL接口

在src目录下用Java语法定义.aidl后缀的接口文件，客户端和服务端都需要保存一份接口文件的拷贝。编译时，SDK会为aild文件在gen目录下生成IBinder接口。服务端实现这个接口，客户端绑定该服务，调用接口中的方法实现进程间通信。

创建AIDL实现的服务端，步骤如下：

1. 创建*.aidl文件：定义接口
2. 实现接口：SDK根据aidl文件生成AIDL接口，它包含一个名为Stub的抽象内部类，该类声明了所有aidl描述的方法，你必须在代码里继承该Stub类并实现里面定义的方法。
3. 向客户端公开接口：继承Service，复写onBind方法，返回Stub的实现类。

注意：aidl文件一旦发布，注意客户端和服务端的接口兼容性，要改必须一起改。

1. 创建aidl文件

AIDL接口可以定义多个带入参和返回值的函数，参数和返回值可以是任意类型，甚至是其他AIDL接口

默认地，AIDL支持如下数据类型：

• Java语言的基本类型（int,long,char,boolean等）
• String
• CharSequence
• List
  列表中的元素必须如下三种类型：AIDL支持的数据类型、其他AIDL生成的接口、自定义的可序列化类型，接收到的实际类型是ArrayList（？）
• Map
  类型要求与List相同，不支持Map，接收到的实际类型是HashMap

你需要import以上类型之外的其他类型，即使在同一个包中。

当定义服务接口时，需要注意：

• 方法可以由0或者多个参数，可以返回具体的值或者无返回值
• 所有非基础类型的参数必须指定参数方向：入参in 出参out inout， 默认是in
• 所有在aidl文件中的注释都会包含在自动生成的IBinder接口文件（除非注释在import和package之前）
• AIDL只支持方法，不支持变量

aidl例子代码：

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

// 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);
}

保存你的aidl文件在工程的src目录下，当你编译时，SDK工具会自动在gen目录下生成IBinder接口文件，生成的文件文件名与aidl文件一致，后缀是.java（IRemoteService.aidl对应IRemoteService.java）。

如果使用Eclipse，增量编译会立即自动生成java接口文件。如果你使用Ant工具，你需要使用ant debug或者ant release编译。

2. 实现接口

自动生成的java接口文件包含名为Stub的抽象子类（YourInterface.Stub）。

继承Stub类，实现里面的接口。

例子代码：

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
    }
};

mBinder是Stub的实例，为服务定义了远程调用接口。下一步，客户端通过mBinder调用服务。

实现AIDL接口，有如下规则：

• 线程安全，调用可能来自多个线程。
• 默认的，RPC是同步调用，如果服务需要花费较长时间才能完成处理，不建议在应用主线程中调用，否则会导致ANR。
• 服务不会抛任何一场给客户端

3. 暴露接口给客户端

继承Service，实现onBind，返回mBinder（Stub类），例子如下：

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
        }
    };
}

当客户端调用bindService连接服务，客户端的onServiceConnected回调函数会受到mBinder实例，通过服务的onBind函数。

客户端和服务端在不同的应用时，客户端需要一份aidl文件的拷贝。

客户端接到IBinder的onServiceConnected回调，需要调用YourServiceInterface.Stub.asInterface(service)转换为YourServiceInterface类型。例子如下：

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;
    }
};

通过IPC传递对象

自定义的类需要实现Parcelable接口，步骤如下：

1. 实现Parcelable接口
2. 实现writeToParcel，将当前类的状态写入Parcel
3. 添加静态字段CREATOR，实现Parcelable.Creator
4. 最后，创建AIDL文件声明这个可打包的类（见下文的Rect.aidl），如果使用的是自定义的编译过程，那么不要编译此AIDL文件，它像C语言的头文件一样不需要编译。

AIDL会使用这些方法的成员序列化和反序列化对象。下面的代码演示如何使Rect类支持序列化（parcelable）

package android.graphics;

// Declare Rect so AIDL can find it and knows that it implements
// the parcelable protocol.
parcelable Rect;

下面的例子演示了如何让Rect类实现Parcelable协议。

import android.os.Parcel;
import android.os.Parcelable;

public final class Rect implements Parcelable {
    public int left;
    public int top;
    public int right;
    public int bottom;

    public static final Parcelable.Creator<Rect> CREATOR = new
Parcelable.Creator<Rect>() {
        public Rect createFromParcel(Parcel in) {
            return new Rect(in);
        }

        public Rect[] newArray(int size) {
            return new Rect[size];
        }
    };

    public Rect() {
    }

    private Rect(Parcel in) {
        readFromParcel(in);
    }

    public void writeToParcel(Parcel out) {
        out.writeInt(left);
        out.writeInt(top);
        out.writeInt(right);
        out.writeInt(bottom);
    }

    public void readFromParcel(Parcel in) {
        left = in.readInt();
        top = in.readInt();
        right = in.readInt();
        bottom = in.readInt();
    }
}

调用IPC方法

这里给出调用远端AIDL接口的步骤：

    1. 在 src/ 目录下包含.adil文件。

    2. 声明一个IBinder接口（通过.aidl文件生成的）的实例。

    3. 实现ServiceConnection.

    4. 调用Context.bindService()绑定你的ServiceConnection实现类的对象（也就是远程服务端）。

    5. 在onServiceConnected()方法中会接收到IBinder对象（也就是服务端），调用YourInterfaceName.Stub.asInterface((IBinder)service)将返回值转换为YourInterface类型。

    6. 调用接口中定义的方法，并且应该总是捕获连接被打断时抛出的DeadObjectException异常，这是远端方法可能会抛出唯一异常。

    7. 调用Context.unbindService()方法断开连接。

这里有几个关于调用IPC服务的提示：

• 对象是在进程间会进行引用计数
• 可以发送匿名对象作为方法的参数

更多关于服务绑定的内容请看Bound Services相关文档。

下面是AIDL-created服务的演示代码，该代码是从ApiDemos工程中的Remote Service中提取的。

public static class Binding extends Activity {
    /** The primary interface we will be calling on the service. */
    IRemoteService mService = null;
    /** Another interface we use on the service. */
    ISecondary mSecondaryService = null;

    Button mKillButton;
    TextView mCallbackText;

    private boolean mIsBound;

    /**
     * Standard initialization of this activity.  Set up the UI, then wait
     * for the user to poke it before doing anything.
     */
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        setContentView(R.layout.remote_service_binding);

        // Watch for button clicks.
        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.");
    }

    /**
     * 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(Binding.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(Binding.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);
        }
    };

    private OnClickListener mBindListener = new OnClickListener() {
        public void onClick(View v) {
            // Establish a couple connections with the service, binding
            // by interface names.  This allows other applications to be
            // installed that replace the remote service by implementing
            // the same interface.
            bindService(new Intent(IRemoteService.class.getName()),
                    mConnection, Context.BIND_AUTO_CREATE);
            bindService(new Intent(ISecondary.class.getName()),
                    mSecondaryConnection, Context.BIND_AUTO_CREATE);
            mIsBound = true;
            mCallbackText.setText("Binding.");
        }
    };

    private OnClickListener mUnbindListener = new OnClickListener() {
        public void onClick(View v) {
            if (mIsBound) {
                // If we have received the service, and hence registered with
                // it, then now is the time to unregister.
                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 OnClickListener mKillListener = new 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(Binding.this,
                            R.string.remote_call_failed,
                            Toast.LENGTH_SHORT).show();
                }
            }
        }
    };

    // ----------------------------------------------------------------------
    // Code showing how to deal with callbacks.
    // ----------------------------------------------------------------------

    /**
     * This implementation is used to receive callbacks from the remote
     * service.
     */
    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));
        }
    };

    private static final int BUMP_MSG = 1;

    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);
            }
        }

    };
}

翻译自：http://developer.android.com/guide/components/aidl.html

参考自：http://www.cnblogs.com/hibraincol/archive/2011/09/06/2169325.html（翻译的非常好）