Android学习--Service之AIDL

概述

AIDL:Android Interface Definition Language.

通常情况下，在Android中，一个进程是无法访问另一个进程的内存空间的；如果要实现此功能，就需要把对象解码成操作系统能够理解的原始字节，然后才能跨进程传送，AIDL就是用来完成这项工作的。

注意，只有service需要与其他应用进行IPC、并且需要处理多线程任务时，才需要使用AIDL。(如果service不需要进行IPC，可以通过继承Binder实现对外接口；如果需要进行IPC 、但是不需要处理多线程任务，可以使用Messenger实现对外接口。)

定义AIDL接口

AIDL的接口要定义在后缀名为.aidl的文件中，并把该文件保存在持有该service、及打算绑定该service的应用程序的src/文件夹下。在应用程序中创建了.aidl文件后，Android SDK会在此.aidl文件的基础上自动生成一个IBinder接口(此接口的代码存放在工程gen/文件夹下)。在service端，必须在适当的时候实现此IBinder接口；在客户端，可以通过此IBinder与service进行IPC(在客户端不用具体实现此IBinder接口)。

创建包含AIDL功能的service，步骤如下：

1、创建.aidl文件：在此文件中定义包含函数签名的编程接口；

2、实现接口：第一步完成后，Android SDK会基于.aidl文件生成一个相应的接口，此接口有一个名字为Stub的抽象内部类(该类继承了Binder并且抽象实现了AIDL接口);必须在service中继承此Stub内部类并实现其内部的函数。

3、将此接口暴露给客户：实现Service并在其onBind()方法中返回Stub的实现。

现将各步骤详细介绍如下：

1、创建.aidl文件

必须用Java语言创建.aidl文件；每个.aidl文件中只能定义一个接口，并且里面只包含接口的声明和方法的签名。

.aidl文件中可以使用任何数据类型(主要用于方法的参数和返回值)，甚至其他AIDL生成的接口；但是，AIDL默认支持的只有以下数据类型(对于其他类型必须import引入)：

*Java中所有的基本数据类型，及String、ChaSequence；

*List：List里包含的必须是此处列举的数据类型、其他AIDL生成的接口、或声明的Parcelable。可以使用泛类型(如List<String>)；即使函数中使用的是List接口，在实体类中实际接收到的通常为ArrayList。

*Map：Map里包含的必须是此处列举的数据类型、其他AIDL生成的接口、或声明的Parcelable。不支持泛型Map;即使函数中使用的是Map接口,在实体类中实际接收到的通常为HashMap。

创建.aidl文件时的注意事项：

*里面的方法可以接收0或多个参数，可以有返回值或返回void；

*所有的非基本类型的参数需要一个定向标签(in、out或inout)，指示数据的去向；基本数据类型默认只能是in。(下面示例代码中未见到此标签的使用，不理解其具体作用。)

*.aidl文件中所有的注释都会被包含在生成的IBinder接口中(除了import和package语句前的注释)；

*与普通interface不同的是，AIDL中不能包含静态域。

下面一个.aidl文件的示例：

// IRemoteService.aidl
package com.example.android;
// 在此import非默认支持的数据类型
/** 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);
}

把此文件保存在工程的ser/文件夹下，然后SDK工具就会自动在gen/文件夹下生成相应的IBinder接口文件；两个文件的名称相同，后缀不同，前者为.aidl，后者为.java。

2、实现接口

在完成上面一步之后，SDK工具自动生成了一个接口文件，此文件中有一个内部抽象类Stub，它继承了Binder并抽象实现了父类接口；所谓"实现接口"就是要实现此Stub类。

示例代码如下：(使用了匿名机制，实际上包含了实现Stub类、并创建其实例两个过程)

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
}
};

此处得到的mBinder就是一个Binder实例(因为Stub继承了Binder)，是客户端跟service通信的桥梁。

实现AIDL接口的注意事项：

*要考虑多线程的问题，确保service是线程安全的；

*默认情况下，RPC(远程进程调用)是同步的，如果service需要较长时间完成对请求的响应，就不应该在activity的主线程内调用该service；应该在客户端另起一个线程来调用此service，以避免ANR；

*无法向客户端抛异常。

3、向客户端暴露接口

即实现service的onBind()方法，并返回Stub的实例。

示例代码如下：

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
}
};
}

此时，当客户端调用bindService()绑定此service时，会在客户端的onServiceConnected()方法中得到由onBind()返回的mBinder；在onServiceConnected()方法中要记得调用Stub的asInterface()方法，对得到的IBinder对象进行类型转化。示例代码如下：

IRemoteService mIRemoteService;
private ServiceConnection mConnection = new ServiceConnection() {
// Called when the connection with the service is established
public void onServiceConnected(ComponentName className, IBinder service) {
// 将IBinder向下转型为IRemoteService(因为此处的IBinder实际类型为Stub，而Stub实现了IRemoteService接口，所以可以转型成功)
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;
}
};

当然，客户端必须拥有Stub抽象类的访问权；因此，如果客户端与service不在同应用程序中(通常如此)，必须把上面创建的.aidl文件复制到客户端的ser/文件夹下，这样SDK工具会同样在客户端生成AIDL接口，客户端就可以使用Stub的方法了。

通过IPC传递对象

可以在IPC间传递类对象，但是此类需要实现Parcelable接口，并且要确保IPC的另一端拥有此类的源代码。

创建支持Parcelable协议的类的步骤如下：

1、让类实现Parcelable接口；

2、实现Parcelable的writeToParcel()方法，此方法用于把当前类写进Parcel；

3、添加一个实现了Parcelable.Creator接口、名称为CREATOR的域；

4、创建一个.aidl文件，在其中声明此类是parcelable的。

实例代码如下：

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();
}
}

下面是Rect.aidl文件，声明Rect类是parcelable的：

package android.graphics;
// Declare Rect so AIDL can find it and knows that it implements
// the parcelable protocol.
parcelable Rect;//注意此处的parcelable为小写

调用IPC另一端的方法

要调用AIDL中定义的远程接口中的方法，客户端需要完成以下几步：

1、在ser/文件夹下包含相应的.aidl文件，系统会基于此文件生成包含IBinder接口的文件；

2、声明上述IBinder接口类型的成员变量；(不用初始化，赋值操作在下述的onServiceConnected()中完成)

3、实现ServiceConnection；

4、调用Context.bindService()，并把ServiceConnection的实例传递给这个函数；

5、之后会在ServiceConnection的onServiceConnected()方法中得到一个IBinder对象，要记得调用AIDL接口中的asInterface()方法对其进行类型转化。

6、接下来就可以通过得到的IBinder对象调用远程接口中的方法了，但是此时一定要记得捕获DeadObjectException(此异常会在连接意外中断时抛出，是由远程方法抛出的唯一异常)；

7、要解除绑定，调用Context.unbindService()。

还有以下两点需注意：

*Objects are reference counted across processes.

*可以向函数传递匿名对象。

示例代码如下：

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);
}
}

};
}

此部分的所有示例代码均节选自官方的APIDemos中的Remote Service示例。