Android ApiDemos示例解析(44)：App->Service->Service Start Arguments Controller

本例是ApiDemos关于Service的最后一个例子，这个例子的主要目的是介绍如何向Service传递参数。前面的例子忽略了一个重要的问题： Service 的onStartCommand 或是 onStart(2.1版本之前）是使用调用它的Android组件（通常是Activity）同一个Thread来执行的，对应Activity来说，这个Thread通常是UI Thread，前面的Service例子都是提供非常简单的服务，对于UI 性能不会有很大影响，但如果在Service中使用了较费时的操作，如果网络访问，数据库查询，如果还是使用UI Thread来运行的话，就可能大大降低UI的响应性能，甚至出现ANR（Application Not Response）对话框，所以通常都是在Service新建一个线程来处理来自Client的请求。

Android.os 的Handler, HandlerThread, Loop, Message 常用于Service中，其中Handler介绍可以参见Android 系统Handler用法简介。

Android 中每个Thread都可以有一Message Queue，但除UI Thread外，Thread缺省情况下不带Message Queue，要为一个Thread 创建一个Message　Queue，可以参见下面代码：

[java]
class LooperThread extends Thread {
public Handler mHandler;
public void run() {
Looper.prepare();
mHandler = new Handler() {
public void handleMessage(Message msg) {
// process incoming messages here
}
};
Looper.loop();
}
}
class LooperThread extends Thread {
public Handler mHandler;
public void run() {
Looper.prepare();
mHandler = new Handler() {
public void handleMessage(Message msg) {
// process incoming messages here
}
};
Looper.loop();
}
}
Looper.prepare()用来创建一个Message Queue， Looper.loop() 处理消息直到Loop停止。在Thread在创建的Handler将会和Thread的Message Queue关联。Handler的handleMessage用来处理消息，其类型为Message类。

HandlerThread派生于Thread，用于方便创建一个带Looper的Thread。本例就是使用了HandlerThread.

[java]
private volatile Looper mServiceLooper;
private volatile ServiceHandler mServiceHandler;

private final class ServiceHandler extends Handler {
public ServiceHandler(Looper looper) {
super(looper);
}

@Override
public void handleMessage(Message msg) {
...
}

...
HandlerThread thread = new HandlerThread("ServiceStartArguments",
Process.THREAD_PRIORITY_BACKGROUND);
thread.start();

mServiceLooper = thread.getLooper();
mServiceHandler = new ServiceHandler(mServiceLooper);
private volatile Looper mServiceLooper;
private volatile ServiceHandler mServiceHandler;

private final class ServiceHandler extends Handler {
public ServiceHandler(Looper looper) {
super(looper);
}

@Override
public void handleMessage(Message msg) {
...
}

...
HandlerThread thread = new HandlerThread("ServiceStartArguments",
Process.THREAD_PRIORITY_BACKGROUND);
thread.start();

mServiceLooper = thread.getLooper();
mServiceHandler = new ServiceHandler(mServiceLooper);
ServiceStartArguments Service 使用一个新创建的Thread来处理来自Client的消息，这个Thread不是UI Thread，使用HandlerThread 创建一个带Looper的线程，而具体的消息处理是由ServiceHandler的handleMessage来处理。

Message类定义了消息，可以含有对消息的描述description，类型what，缺省带arg1,arg2, obj三个参数可以直接使用。并提供了obtain()静态函数来构造一个新的Message对象。

ServiceStartArguments.Controller 为ServiceStartArguments Service的Client。它定义了四个按钮，通过Intent的Extra向Service传递参数：

[java]
startService(new Intent(Controller.this,
ServiceStartArguments.class)
.putExtra("name", "One"));

startService(new Intent(Controller.this,
ServiceStartArguments.class)
.putExtra("name", "Two"));

startService(new Intent(Controller.this,
ServiceStartArguments.class)
.putExtra("name", "Three")
.putExtra("redeliver", true));

startService(new Intent(Controller.this,
ServiceStartArguments.class)
.putExtra("name", "Failure")
.putExtra("fail", true));
startService(new Intent(Controller.this,
ServiceStartArguments.class)
.putExtra("name", "One"));

startService(new Intent(Controller.this,
ServiceStartArguments.class)
.putExtra("name", "Two"));

startService(new Intent(Controller.this,
ServiceStartArguments.class)
.putExtra("name", "Three")
.putExtra("redeliver", true));

startService(new Intent(Controller.this,
ServiceStartArguments.class)
.putExtra("name", "Failure")
.putExtra("fail", true));
相当于向Service传递了name (string), redeliver(boolean) ,fail(boolean)几个参数。

StartService会触发Service的onStartCommand方法：

[java]
@Override
public int onStartCommand(Intent intent, int flags, int startId) {
Log.i("ServiceStartArguments",
"Starting #" + startId + ": " + intent.getExtras());
Message msg = mServiceHandler.obtainMessage();
msg.arg1 = startId;
msg.arg2 = flags;
msg.obj = intent.getExtras();
mServiceHandler.sendMessage(msg);
Log.i("ServiceStartArguments", "Sending: " + msg);

// For the start fail button, we will simulate the process dying
// for some reason in onStartCommand().
if (intent.getBooleanExtra("fail", false)) {
// Don't do this if we are in a retry... the system will
// eventually give up if we keep crashing.
if ((flags&START_FLAG_RETRY) == 0) {
// Since the process hasn't finished handling the command,
// it will be restarted with the command again, regardless of
// whether we return START_REDELIVER_INTENT.
Process.killProcess(Process.myPid());
}
}

// Normally we would consistently return one kind of result...
// however, here we will select between these two, so you can see
// how they impact the behavior. Try killing the process while it
// is in the middle of executing the different commands.
return intent.getBooleanExtra("redeliver", false)
? START_REDELIVER_INTENT : START_NOT_STICKY;
}
@Override
public int onStartCommand(Intent intent, int flags, int startId) {
Log.i("ServiceStartArguments",
"Starting #" + startId + ": " + intent.getExtras());
Message msg = mServiceHandler.obtainMessage();
msg.arg1 = startId;
msg.arg2 = flags;
msg.obj = intent.getExtra

补充：移动开发 , Android ,