Android 接口定义语言 (AIDL)

Android 接口定义语言 (AIDL) 与您可能使用过的其他接口语言 (IDL) 类似。您可以利用它定义客户端与服务均认可的编程接口,以便二者使用进程间通信 (IPC) 进行相互通信。在 Android 中,一个进程通常无法访问另一个进程的内存。因此,为进行通信,进程需将其对象分解成可供操作系统理解的原语,并将其编组为可供您操作的对象。编写执行该编组操作的代码较为繁琐,因此 Android 会使用 AIDL 为您处理此问题。

注意:只有在需要不同应用的客户端通过 IPC 方式访问服务,并且希望在服务中进行多线程处理时,您才有必要使用 AIDL。如果您无需跨不同应用执行并发 IPC,则应通过实现 Binder 来创建接口;或者,如果您想执行 IPC,但需要处理多线程,请使用 Messenger 来实现接口。无论如何,在实现 AIDL 之前,请您务必理解绑定服务

在开始设计 AIDL 接口之前,请注意,AIDL 接口的调用是直接函数调用。您无需对发生调用的线程做任何假设。实际情况的差异取决于调用是来自本地进程中的线程,还是远程进程中的线程。具体而言:

  • 来自本地进程的调用在发起调用的同一线程内执行。如果该线程是您的主界面线程,则其将继续在 AIDL 接口中执行。如果该线程是其他线程,则其便是在服务中执行代码的线程。因此,只有在本地线程访问服务时,您才能完全控制哪些线程在服务中执行(但若出现此情况,您根本无需使用 AIDL,而应通过实现 Binder 类来创建接口)。
  • 远程进程的调用分派自线程池,且平台会在您自己的进程内部维护该线程池。您必须为来自未知线程,且多次调用同时发生的传入调用做好准备。换言之,AIDL 接口的实现必须基于完全的线程安全。如果调用来自同一远程对象上的某个线程,则该调用将依次抵达接收器端。
  • oneway 关键字用于修改远程调用的行为。使用此关键字后,远程调用不会屏蔽,而只是发送事务数据并立即返回。最终接收该数据时,接口的实现会将其视为来自 Binder 线程池的常规调用(普通的远程调用)。如果 oneway 用于本地调用,则不会有任何影响,且调用仍为同步调用。

定义 AIDL 接口

您必须在 .aidl 文件中使用 Java 编程语言的语法定义 AIDL 接口,然后将其保存至应用的源代码(在 src/ 目录中)内,这类应用会托管服务或与服务进行绑定。

在构建每个包含 .aidl 文件的应用时,Android SDK 工具会生成基于该 .aidl 文件的 IBinder 接口,并将其保存到项目的 gen/ 目录中。服务必须视情况实现 IBinder 接口。然后,客户端应用便可绑定到该服务,并调用 IBinder 中的方法来执行 IPC。

如要使用 AIDL 创建绑定服务,请执行以下步骤:

  1. 创建 .aidl 文件

    此文件定义带有方法签名的编程接口。

  2. 实现接口

    Android SDK 工具会基于您的 .aidl 文件,使用 Java 编程语言生成接口。此接口拥有一个名为 Stub 的内部抽象类,用于扩展 Binder 类并实现 AIDL 接口中的方法。您必须扩展 Stub 类并实现这些方法。

  3. 向客户端公开接口

    实现 Service 并重写 onBind(),从而返回 Stub 类的实现。

注意:如果您在首次发布 AIDL 接口后对其进行更改,则每次更改必须保持向后兼容性,以免中断其他应用使用您的服务。换言之,由于只有在将您的 .aidl 文件复制到其他应用后,才能使其访问服务接口,因而您必须保留对原始接口的支持。

1. 创建 .aidl 文件

AIDL 使用一种简单语法,允许您通过一个或多个方法(可接收参数和返回值)来声明接口。参数和返回值可为任意类型,甚至是 AIDL 生成的其他接口。

您必须使用 Java 编程语言构建 .aidl 文件。每个 .aidl 文件均须定义单个接口,并且只需要接口声明和方法签名。

默认情况下,AIDL 支持下列数据类型:

  • Java 编程语言中的所有原语类型(如 intlongcharboolean 等)
  • String
  • CharSequence
  • List

    List 中的所有元素必须是以上列表中支持的数据类型,或者您所声明的由 AIDL 生成的其他接口或 Parcelable 类型。您可选择将 List 用作“泛型”类(例如,List<String>)。尽管生成的方法旨在使用 List 接口,但另一方实际接收的具体类始终是 ArrayList

  • Map

    Map 中的所有元素必须是以上列表中支持的数据类型,或者您所声明的由 AIDL 生成的其他接口或 Parcelable 类型。不支持泛型 Map(如 Map<String,Integer> 形式的 Map)。尽管生成的方法旨在使用 Map 接口,但另一方实际接收的具体类始终是 HashMap

即使您在与接口相同的包内定义上方未列出的附加类型,亦须为其各自加入一条 import 语句。

定义服务接口时,请注意:

  • 方法可带零个或多个参数,返回值或空值。
  • 所有非原语参数均需要指示数据走向的方向标记。这类标记可以是 inoutinout(见下方示例)。

    原语默认为 in,不能是其他方向。

    注意:您应将方向限定为真正需要的方向,因为编组参数的开销较大。

  • 生成的 IBinder 接口内包含 .aidl 文件中的所有代码注释(import 和 package 语句之前的注释除外)。
  • 您可以在 ADL 接口中定义 String 常量和 int 字符串常量。例如:const int VERSION = 1;
  • 方法调用由 transact() 代码分派,该代码通常基于接口中的方法索引。由于这会增加版本控制的难度,因此您可以向方法手动配置事务代码:void method() = 10;
  • 使用 @nullable 注释可空参数或返回类型。

以下是 .aidl 文件示例:

// IRemoteService.aidl
package com.example.android

// Declare any non-default types here with import statements
/** Example service interface */
internal interface IRemoteService {
    /** Request the process ID of this service, to do evil things with it. */
    val pid:Int

    /** Demonstrates some basic types that you can use as parameters
     * and return values in AIDL.
     */
    fun basicTypes(anInt:Int, aLong:Long, aBoolean:Boolean, aFloat:Float,
                 aDouble:Double, aString:String)
}

您只需将 .aidl 文件保存至项目的 src/ 目录内,这样在构建应用时,SDK 工具便会在项目的 gen/ 目录中生成 IBinder 接口文件。生成文件的名称与 .aidl 文件的名称保持一致,区别在于其使用 .java 扩展名(例如,IRemoteService.aidl 生成的文件名是 IRemoteService.java)。

如果您使用 Android Studio,增量构建几乎会立即生成 Binder 类。如果您不使用 Android Studio,则 Gradle 工具会在您下一次开发应用时生成 Binder 类。因此,在编写完 .aidl 文件后,您应立即使用 gradle assembleDebug(或 gradle assembleRelease)构建项目,以便您的代码能够链接到生成的类。

2. 实现接口

当您构建应用时,Android SDK 工具会生成以 .aidl 文件命名的 .java 接口文件。生成的接口包含一个名为 Stub 的子类(例如,YourInterface.Stub),该子类是其父接口的抽象实现,并且会声明 .aidl 文件中的所有方法。

注意:Stub 还会定义几个辅助方法,其中最值得注意的是 asInterface(),该方法会接收 IBinder(通常是传递给客户端 onServiceConnected() 回调方法的参数),并返回 Stub 接口的实例。如需了解如何进行此转换的更多详情,请参阅调用 IPC 方法部分。

如要实现 .aidl 生成的接口,请扩展生成的 Binder 接口(例如,YourInterface.Stub),并实现继承自 .aidl 文件的方法。

以下示例展示使用匿名实例实现 IRemoteService 接口(由以上 IRemoteService.aidl 示例定义)的过程:

Kotlin

private val binder = object : IRemoteService.Stub() {

    override fun getPid(): Int =
            Process.myPid()

    override fun basicTypes(
            anInt: Int,
            aLong: Long,
            aBoolean: Boolean,
            aFloat: Float,
            aDouble: Double,
            aString: String
    ) {
        // Does nothing
    }
}

Java

private final IRemoteService.Stub binder = 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
    }
};

现在,binderStub 类的一个实例(一个 Binder),其定义了服务的远程过程调用 (RPC) 接口。在下一步中,我们会向客户端公开此实例,以便客户端能与服务进行交互。

在实现 AIDL 接口时,您应注意遵守以下规则:

  • 由于无法保证在主线程上执行传入调用,因此您一开始便需做好多线程处理的准备,并对您的服务进行适当构建,使其达到线程安全的标准。
  • 默认情况下,RPC 调用是同步调用。如果您知道服务完成请求的时间不止几毫秒,则不应从 Activity 的主线程调用该服务,因为这可能会使应用挂起(Android 可能会显示“Application is Not Responding”对话框)— 通常,您应从客户端内的单独线程调用服务。
  • 您引发的任何异常都不会回传给调用方。

3. 向客户端公开接口

在为服务实现接口后,您需要向客户端公开该接口,以便客户端进行绑定。如要为您的服务公开该接口,请扩展 Service 并实现 onBind(),从而返回实现生成的 Stub 的类实例(如前文所述)。以下是向客户端公开 IRemoteService 示例接口的服务示例。

Kotlin

class RemoteService : Service() {

    override fun onCreate() {
        super.onCreate()
    }

    override fun onBind(intent: Intent): IBinder {
        // Return the interface
        return binder
    }


    private val binder = object : IRemoteService.Stub() {
        override fun getPid(): Int {
            return Process.myPid()
        }

        override fun basicTypes(
                anInt: Int,
                aLong: Long,
                aBoolean: Boolean,
                aFloat: Float,
                aDouble: Double,
                aString: String
        ) {
            // Does nothing
        }
    }
}

Java

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

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

    private final IRemoteService.Stub binder = 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
        }
    };
}

现在,当客户端(如 Activity)调用 bindService() 以连接此服务时,客户端的 onServiceConnected() 回调会接收服务的 onBind() 方法所返回的 binder 实例。

客户端还必须拥有接口类的访问权限,因此如果客户端和服务在不同应用内,则客户端应用的 src/ 目录内必须包含 .aidl 文件(该文件会生成 android.os.Binder 接口,进而为客户端提供 AIDL 方法的访问权限)的副本。

当客户端在 onServiceConnected() 回调中收到 IBinder 时,它必须调用 YourServiceInterface.Stub.asInterface(service),以将返回的参数转换成 YourServiceInterface 类型。例如:

Kotlin

var iRemoteService: IRemoteService? = null

val mConnection = object : ServiceConnection {

    // Called when the connection with the service is established
    override fun onServiceConnected(className: ComponentName, service: IBinder) {
        // Following the example above for an AIDL interface,
        // this gets an instance of the IRemoteInterface, which we can use to call on the service
        iRemoteService = IRemoteService.Stub.asInterface(service)
    }

    // Called when the connection with the service disconnects unexpectedly
    override fun onServiceDisconnected(className: ComponentName) {
        Log.e(TAG, "Service has unexpectedly disconnected")
        iRemoteService = null
    }
}

Java

IRemoteService iRemoteService;
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
        iRemoteService = 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");
        iRemoteService = null;
    }
};

如需查看更多示例代码,请参阅 ApiDemos 中的 RemoteService.java 类。

通过 IPC 传递对象

您可以通过 IPC 接口,将某个类从一个进程发送至另一个进程。不过,您必须确保 IPC 通道的另一端可使用该类的代码,并且该类必须支持 Parcelable 接口。支持 Parcelable 接口很重要,因为 Android 系统能通过该接口将对象分解成可编组至各进程的原语。

如要创建支持 Parcelable 协议的类,您必须执行以下操作:

  1. 让您的类实现 Parcelable 接口。
  2. 实现 writeToParcel,它会获取对象的当前状态并将其写入 Parcel
  3. 为您的类添加 CREATOR 静态字段,该字段是实现 Parcelable.Creator 接口的对象。
  4. 最后,创建声明 Parcelable 类的 .aidl 文件(遵照下文 Rect.aidl 文件所示步骤)。

    如果您使用的是自定义编译进程,请勿在您的构建中添加 .aidl 文件。此 .aidl 文件与 C 语言中的头文件类似,并未经过编译。

AIDL 会在其生成的代码中使用这些方法和字段,以对您的对象进行编组和解编。

例如,下方的 Rect.aidl 文件可创建 Parcelable 类型的 Rect 类:

package android.graphics;

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

以下示例展示 Rect 类如何实现 Parcelable 协议。

Kotlin

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

class Rect() : Parcelable {
    var left: Int = 0
    var top: Int = 0
    var right: Int = 0
    var bottom: Int = 0

    companion object CREATOR : Parcelable.Creator<Rect> {
        override fun createFromParcel(parcel: Parcel): Rect {
            return Rect(parcel)
        }

        override fun newArray(size: Int): Array<Rect> {
            return Array(size) { Rect() }
        }
    }

    private constructor(inParcel: Parcel) : this() {
        readFromParcel(inParcel)
    }

    override fun writeToParcel(outParcel: Parcel, flags: Int) {
        outParcel.writeInt(left)
        outParcel.writeInt(top)
        outParcel.writeInt(right)
        outParcel.writeInt(bottom)
    }

    private fun readFromParcel(inParcel: Parcel) {
        left = inParcel.readInt()
        top = inParcel.readInt()
        right = inParcel.readInt()
        bottom = inParcel.readInt()
    }

    override fun describeContents(): Int {
        return 0
    }
}

Java

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, int flags) {
        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();
    }

    public int describeContents() {
        return 0;
    }
}

Rect 类中的编组相当简单。请查看 Parcel 的其他相关方法,了解您可以向 Parcel 写入哪些其他类型的值。

警告:请勿忘记从其他进程中接收数据的安全问题。在本例中,RectParcel 读取四个数字,但您需确保:无论调用方目的为何,这些数字均在可接受的值范围内。如需详细了解如何防止应用受到恶意软件侵害、保证应用安全,请参阅安全与权限

带软件包参数(包含 Parcelable 类型)的方法

如果您的 AIDL 接口包含接收软件包作为参数(预计包含 Parcelable 类型)的方法,则在尝试从软件包读取之前,请务必通过调用 Bundle.setClassLoader(ClassLoader) 设置软件包的类加载器。否则,即使您在应用中正确定义 Parcelable 类型,也会遇到 ClassNotFoundException。例如,

如果您有 .aidl 文件:

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

/** Example service interface */
interface IRectInsideBundle {
    /** Rect parcelable is stored in the bundle with key "rect" */
    void saveRect(in Bundle bundle);
}
如下方实现所示,在读取 Rect 之前,ClassLoader 已在 Bundle 中完成显式设置

Kotlin

private val binder = object : IRectInsideBundle.Stub() {
    override fun saveRect(bundle: Bundle) {
      bundle.classLoader = classLoader
      val rect = bundle.getParcelable<Rect>("rect")
      process(rect) // Do more with the parcelable
    }
}

Java

private final IRectInsideBundle.Stub binder = new IRectInsideBundle.Stub() {
    public void saveRect(Bundle bundle){
        bundle.setClassLoader(getClass().getClassLoader());
        Rect rect = bundle.getParcelable("rect");
        process(rect); // Do more with the parcelable.
    }
};

调用 IPC 方法

如要调用通过 AIDL 定义的远程接口,调用类必须执行以下步骤:

  1. 在项目的 src/ 目录中加入 .aidl 文件。
  2. 声明一个 IBinder 接口实例(基于 AIDL 生成)。
  3. 实现 ServiceConnection
  4. 调用 Context.bindService(),从而传入您的 ServiceConnection 实现。
  5. onServiceConnected() 实现中,您将收到一个 IBinder 实例(名为 service)。调用 YourInterfaceName.Stub.asInterface((IBinder)service),以将返回的参数转换为 YourInterface 类型。
  6. 调用您在接口上定义的方法。您应始终捕获 DeadObjectException 异常,系统会在连接中断时抛出此异常。您还应捕获 SecurityException 异常,当 IPC 方法调用中两个进程的 AIDL 定义发生冲突时,系统会抛出此异常。
  7. 如要断开连接,请使用您的接口实例调用 Context.unbindService()

有关调用 IPC 服务的几点说明:

  • 对象是跨进程计数的引用。
  • 您可以方法参数的形式发送匿名对象。

如需了解有关绑定服务的详细信息,请阅读绑定服务文档。

以下示例代码摘自 ApiDemos 项目的远程服务示例,展示如何调用 AIDL 创建的服务。

Kotlin

private const val BUMP_MSG = 1

class Binding : Activity() {

    /** The primary interface we will be calling on the service.  */
    private var mService: IRemoteService? = null

    /** Another interface we use on the service.  */
    internal var secondaryService: ISecondary? = null

    private lateinit var killButton: Button
    private lateinit var callbackText: TextView
    private lateinit var handler: InternalHandler

    private var isBound: Boolean = false

    /**
     * Class for interacting with the main interface of the service.
     */
    private val mConnection = object : ServiceConnection {

        override fun onServiceConnected(className: ComponentName, service: IBinder) {
            // 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)
            killButton.isEnabled = true
            callbackText.text = "Attached."

            // We want to monitor the service for as long as we are
            // connected to it.
            try {
                mService?.registerCallback(mCallback)
            } catch (e: RemoteException) {
                // 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(
                    this@Binding,
                    R.string.remote_service_connected,
                    Toast.LENGTH_SHORT
            ).show()
        }

        override fun onServiceDisconnected(className: ComponentName) {
            // This is called when the connection with the service has been
            // unexpectedly disconnected -- that is, its process crashed.
            mService = null
            killButton.isEnabled = false
            callbackText.text = "Disconnected."

            // As part of the sample, tell the user what happened.
            Toast.makeText(
                    this@Binding,
                    R.string.remote_service_disconnected,
                    Toast.LENGTH_SHORT
            ).show()
        }
    }

    /**
     * Class for interacting with the secondary interface of the service.
     */
    private val secondaryConnection = object : ServiceConnection {

        override fun onServiceConnected(className: ComponentName, service: IBinder) {
            // Connecting to a secondary interface is the same as any
            // other interface.
            secondaryService = ISecondary.Stub.asInterface(service)
            killButton.isEnabled = true
        }

        override fun onServiceDisconnected(className: ComponentName) {
            secondaryService = null
            killButton.isEnabled = false
        }
    }

    private val mBindListener = View.OnClickListener {
        // 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.
        val intent = Intent(this@Binding, RemoteService::class.java)
        intent.action = IRemoteService::class.java.name
        bindService(intent, mConnection, Context.BIND_AUTO_CREATE)
        intent.action = ISecondary::class.java.name
        bindService(intent, secondaryConnection, Context.BIND_AUTO_CREATE)
        isBound = true
        callbackText.text = "Binding."
    }

    private val unbindListener = View.OnClickListener {
        if (isBound) {
            // If we have received the service, and hence registered with
            // it, then now is the time to unregister.
            try {
                mService?.unregisterCallback(mCallback)
            } catch (e: RemoteException) {
                // There is nothing special we need to do if the service
                // has crashed.
            }

            // Detach our existing connection.
            unbindService(mConnection)
            unbindService(secondaryConnection)
            killButton.isEnabled = false
            isBound = false
            callbackText.text = "Unbinding."
        }
    }

    private val killListener = View.OnClickListener {
        // 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.
        try {
            secondaryService?.pid?.also { pid ->
                // 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)
                callbackText.text = "Killed service process."
            }
        } catch (ex: RemoteException) {
            // Recover gracefully from the process hosting the
            // server dying.
            // Just for purposes of the sample, put up a notification.
            Toast.makeText(this@Binding, 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 val mCallback = object : 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.
         */
        override fun valueChanged(value: Int) {
            handler.sendMessage(handler.obtainMessage(BUMP_MSG, value, 0))
        }
    }

    /**
     * Standard initialization of this activity.  Set up the UI, then wait
     * for the user to poke it before doing anything.
     */
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)

        setContentView(R.layout.remote_service_binding)

        // Watch for button clicks.
        var button: Button = findViewById(R.id.bind)
        button.setOnClickListener(mBindListener)
        button = findViewById(R.id.unbind)
        button.setOnClickListener(unbindListener)
        killButton = findViewById(R.id.kill)
        killButton.setOnClickListener(killListener)
        killButton.isEnabled = false

        callbackText = findViewById(R.id.callback)
        callbackText.text = "Not attached."
        handler = InternalHandler(callbackText)
    }

    private class InternalHandler(
            textView: TextView,
            private val weakTextView: WeakReference<TextView> = WeakReference(textView)
    ) : Handler() {
        override fun handleMessage(msg: Message) {
            when (msg.what) {
                BUMP_MSG -> weakTextView.get()?.text = "Received from service: ${msg.arg1}"
                else -> super.handleMessage(msg)
            }
        }
    }
}

Java

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 secondaryService = null;

    Button killButton;
    TextView callbackText;

    private InternalHandler handler;
    private boolean isBound;

    /**
     * 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(unbindListener);
        killButton = (Button)findViewById(R.id.kill);
        killButton.setOnClickListener(killListener);
        killButton.setEnabled(false);

        callbackText = (TextView)findViewById(R.id.callback);
        callbackText.setText("Not attached.");
        handler = new InternalHandler(callbackText);
    }

    /**
     * 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);
            killButton.setEnabled(true);
            callbackText.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;
            killButton.setEnabled(false);
            callbackText.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 secondaryConnection = new ServiceConnection() {
        public void onServiceConnected(ComponentName className,
                IBinder service) {
            // Connecting to a secondary interface is the same as any
            // other interface.
            secondaryService = ISecondary.Stub.asInterface(service);
            killButton.setEnabled(true);
        }

        public void onServiceDisconnected(ComponentName className) {
            secondaryService = null;
            killButton.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.
            Intent intent = new Intent(Binding.this, RemoteService.class);
            intent.setAction(IRemoteService.class.getName());
            bindService(intent, mConnection, Context.BIND_AUTO_CREATE);
            intent.setAction(ISecondary.class.getName());
            bindService(intent, secondaryConnection, Context.BIND_AUTO_CREATE);
            isBound = true;
            callbackText.setText("Binding.");
        }
    };

    private OnClickListener unbindListener = new OnClickListener() {
        public void onClick(View v) {
            if (isBound) {
                // 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(secondaryConnection);
                killButton.setEnabled(false);
                isBound = false;
                callbackText.setText("Unbinding.");
            }
        }
    };

    private OnClickListener killListener = 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 (secondaryService != null) {
                try {
                    int pid = secondaryService.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);
                    callbackText.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) {
            handler.sendMessage(handler.obtainMessage(BUMP_MSG, value, 0));
        }
    };

    private static final int BUMP_MSG = 1;

    private static class InternalHandler extends Handler {
        private final WeakReference<TextView> weakTextView;

        InternalHandler(TextView textView) {
            weakTextView = new WeakReference<>(textView);
        }

        @Override
        public void handleMessage(Message msg) {
            switch (msg.what) {
                case BUMP_MSG:
                    TextView textView = weakTextView.get();
                    if (textView != null) {
                        textView.setText("Received from service: " + msg.arg1);
                    }
                    break;
                default:
                    super.handleMessage(msg);
            }
        }
    }
}