Service的高级应用

下面是android Service的高级应用的一个例子，主要内容是在Service中spawn一个thread 来执行一写 CPU intensive的任务，以便让UI thread 能够更好的运行。

这里我们使用到了Looper 和 Handler 两个特殊的类。

Looper??? 是一个用来运行一个线程的Message循环?，每一个线程都默认的有一个和他们相关联的Looper。

Handler 是Looper 用来处理这个线程的消息循环的类

?

整个流程的概括，调用Service的client 用Itent来调用这个Service，然后就会运行public void onStart(Intent intent, int startId)， 我们通过Intent中不同的action 来确定client想要完成的工作。?根据不同的action给thread的消息队列发送不同的消息?

???????? Message msg = Message.obtain();
???????? msg.what = MSG_UPDATE;
???????? handler.sendMessage(msg);

下面是实现的详细代码

?public class MyService extends Service {

??? private static final int MSG_UPDATE= 0;
??? private static final int MSG_CANCLE = 1;
??? /**
???? * intent to update

???? */
??? public static final String ACTION_UPDATE =? "UPDATE";
???
??? /**
???? * intent to cancel???? */
??? public static final String ACTION_CANCLE = "CANCLE";
???????
??? private volatile Looper mServiceLooper;
??? private volatile ServiceHandler handler;
???
??? private final class ServiceHandler extends Handler {
???????????????
??????? public ServiceHandler(Looper looper) {
?? ???super(looper);
? ??}

? ??@Override
??????? public void handleMessage(Message msg)
??????? {
???????? switch (msg.what) {
??????????? case MSG_UPDATE:
?????Log.v("TAG", "Update");???????????????
??????????????? break;
??????????? case MSG_CANCLE:
?????Log.v("TAG", "Cancle");
???????????? break;
??????? }
??? };

?
??? @Override
??? public void onCreate() {
??????? super.onCreate();
??????? log.info("ReadingService.onCreate()");
???????
??????? // Start up the thread running the service.? Note that we create a
??????? // separate thread because the service normally runs in the process's
??????? // main thread, which we don't want to block.? We also make it
??????? // background priority so CPU-intensive work will not disrupt our UI.
??????? HandlerThread thread = new HandlerThread("service",
??????????????? Process.THREAD_PRIORITY_BACKGROUND);
??????? thread.start();
??//获取thread默认的Looper
??????? mServiceLooper = thread.getLooper();
??????? //用Looper来构造这个 Handler 这样的话 Looper就可以用这个Handler来处理thread的消息循环
??????? handler = new ServiceHandler(mServiceLooper);
??? }

??? // This is the old onStart method that will be called on the pre-2.0
??? // platform.? On 2.0 or later we override onStartCommand() so this
??? // method will not be called.
??? @Override
??? public void onStart(Intent intent, int startId) {
??????? handleCommand(intent);
??? }

??? //@Override
??? public int onStartCommand(Intent intent, int flags, int startId) {
??????? handleCommand(intent);
??????? // We want this service to continue running until it is explicitly
??????? // stopped, so return sticky.
??????? return 1; // START_STICKY
??? }

??? private void handleCommand(Intent intent) {
???? if (intent == null || (intent.getAction() == null)) {
????? return;
???? }
????
???? String action = intent.getAction();
??????? if (ACTION_UPDATE.equals(action)) {
?????? //Return a new Message instance from the global pool
???????? Message msg = Message.obtain();
??????? //Message还可以携带信息
???????? msg.setData(intent.getExtras());
???????? msg.what = MSG_UPDATE;
???????? handler.sendMessage(msg);
??????? } else if (ACTION_CANCLE.equals(action)) {
??????? //从消息队列中删掉消息
???????? handler.removeMessages(MSG_UPDATE);
??????? }
??? }

??? @Override
??? public void onDestroy() {
??? //退出消息循环
???? ?mServiceLooper.quit();
??????? super.onDestroy();
??? }

??? @Override
??? public IBinder onBind(Intent intent) {
??????? return null;
??? }
}