Android绑定服务（二）：创建绑定…
- 核心：提供一个定义了客户端到服务端交互编程接口的IBinder
- 定义接口的三条途径：
-- 扩展Binder类
> 服务归应用私有，且跑在同一个进程
> 实例由onBind()返回
> 若服务仅仅为本应用的后台工作，那么推荐使用
> 除非服务被其它应用或进程调用
-- 使用Messenger
> 接口需要跨进程工作
> Handler：定义在服务中，Messenger基于之而向客户端分享IBinder，使客户端能钩运用消息对象向服务发送命令
> 客户端亦可自定义一个Messenger，这样服务可以回传消息
> 这是IPC的最简便途径。Messenger将所有请求排队到同一个线程——这样就无需考虑线程安全问题。
> 基于AIDL实现
-- 使用AIDL
> AIDL: Android Interface Definition Language
> 将对象解构为操作系统识别的原始类型，跨进成传送、重组以执行IPC
> 同时处理多个请求应处理线程安全问题
> 创建.aidl文件以定义接口，安卓SDK工具生成一个实现该接口的抽象类来处理IPC。
> 多数应用无必要使用。
扩展Binder类
- 如何创建？
1. 在服务中创建一个Binder类的实例，以下任一方式皆可：
> 包含供客户端调用的公共方法
> 返回当前服务的实例，它包含客户端可以调用的公共方法
> 返回一个服务包含的内部类的实例，由它提供客户端调用的方法
2. 从onBinder()方法返回这个Binder实例
public class LocalService extends Service {
// Binder given to clients
private final IBinder mBinder = new LocalBinder();
// Random number generator
private final Random mGenerator = new Random();
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;
}
public int getRandomNumber() {
return mGenerator.nextInt(100);
}
}
3. 在客户端，从oonServiceConnected()方法接收到Binder， 然后根据需要调用方法。
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;
}
}
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();
}
}
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;
}
};
}
- 关键组件：Service, Binder, ServiceConnection
使用Messenger
- 如何运用Messenger？
1. 服务实现一个Handler来接收从客户端来的回调
2. Handler被用来创建Messenger对象（Handler的引用）
3. Messenger创建onBind()方法返回的IBinder
public class MessengerService extends Service {
static final int MSG_SAY_HELLO = 1;
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);
}
}
}
final Messenger mMessenger = new Messenger(new IncomingHandler());
@Override
public IBinder onBind(Intent intent) {
Toast.makeText(getApplicationContext(), "binding", Toast.LENGTH_SHORT).show();
return mMessenger.getBinder();
}
}
4. 客户端使用IBinder来实例化Messenger，用它来向服务发送消息对象
5. 服务的Handler接收到各个消息对象，具体地说，在handleMessage()方法中
- 此方法中，没有供客户端调用的“方法”。取而代之的是由客户端传递消息给服务进行处理
public class ActivityMessenger extends Activity {
Messenger mService = null;
boolean mBound;
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;
}
}
}
- 注意，本例中没有展示服务器如何响应客户端。你需要在客户端中也穿件Messenger。当客户端收到onServiceConnected()回调时，向服务端发送一条消息，将客户端的Messenger作为send()方法的replyTo参数