IntentService源码解析
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我们先看下源码:
public abstract class IntentService extends Service { private volatile Looper mServiceLooper; private volatile ServiceHandler mServiceHandler; private String mName; private boolean mRedelivery; private final class ServiceHandler extends Handler { public ServiceHandler(Looper looper) { super(looper); } @Override public void handleMessage(Message msg) { onHandleIntent((Intent)msg.obj); stopSelf(msg.arg1); } } /** * Creates an IntentService. Invoked by your subclass's constructor. * * @param name Used to name the worker thread, important only for debugging. */ public IntentService(String name) { super(); mName = name; } /** * Sets intent redelivery preferences. Usually called from the constructor * with your preferred semantics. * * <p>If enabled is true, * {@link #onStartCommand(Intent, int, int)} will return * {@link Service#START_REDELIVER_INTENT}, so if this process dies before * {@link #onHandleIntent(Intent)} returns, the process will be restarted * and the intent redelivered. If multiple Intents have been sent, only * the most recent one is guaranteed to be redelivered. * * <p>If enabled is false (the default), * {@link #onStartCommand(Intent, int, int)} will return * {@link Service#START_NOT_STICKY}, and if the process dies, the Intent * dies along with it. */ public void setIntentRedelivery(boolean enabled) { mRedelivery = enabled; } @Override public void onCreate() { // TODO: It would be nice to have an option to hold a partial wakelock // during processing, and to have a static startService(Context, Intent) // method that would launch the service & hand off a wakelock. super.onCreate(); HandlerThread thread = new HandlerThread("IntentService[" + mName + "]"); thread.start(); mServiceLooper = thread.getLooper(); mServiceHandler = new ServiceHandler(mServiceLooper); } @Override public void onStart(@Nullable Intent intent, int startId) { Message msg = mServiceHandler.obtainMessage(); msg.arg1 = startId; msg.obj = intent; mServiceHandler.sendMessage(msg); } /** * You should not override this method for your IntentService. Instead, * override {@link #onHandleIntent}, which the system calls when the IntentService * receives a start request. * @see android.app.Service#onStartCommand */ @Override public int onStartCommand(@Nullable Intent intent, int flags, int startId) { onStart(intent, startId); return mRedelivery ? START_REDELIVER_INTENT : START_NOT_STICKY; } @Override public void onDestroy() { mServiceLooper.quit(); } /** * Unless you provide binding for your service, you don't need to implement this * method, because the default implementation returns null. * @see android.app.Service#onBind */ @Override @Nullable public IBinder onBind(Intent intent) { return null; } /** * This method is invoked on the worker thread with a request to process. * Only one Intent is processed at a time, but the processing happens on a * worker thread that runs independently from other application logic. * So, if this code takes a long time, it will hold up other requests to * the same IntentService, but it will not hold up anything else. * When all requests have been handled, the IntentService stops itself, * so you should not call {@link #stopSelf}. * * @param intent The value passed to {@link * android.content.Context#startService(Intent)}. * This may be null if the service is being restarted after * its process has gone away; see * {@link android.app.Service#onStartCommand} * for details. */ @WorkerThread protected abstract void onHandleIntent(@Nullable Intent intent);}
首先我们大致看到了,有looper,有handler,有Thread,我们想到了什么?当然是android的消息机制。
还看到了,该类帮我们处理了Service的实现方法,他的子类需要实现一个onHandleIntent()。这下就清晰了。因为Service本身是不支持执行耗时操作的。要想执行耗时操作,必须新建线程。所以,IntentService的出现就解决这个问题,它继承于Service,封装了多线程,耗时操作在子线程中操作。
我们看先这句:
@Override public void onCreate() { // TODO: It would be nice to have an option to hold a partial wakelock // during processing, and to have a static startService(Context, Intent) // method that would launch the service & hand off a wakelock. super.onCreate(); HandlerThread thread = new HandlerThread("IntentService[" + mName + "]"); thread.start(); mServiceLooper = thread.getLooper(); mServiceHandler = new ServiceHandler(mServiceLooper); }
熟悉android的消息机制的应该都会知道,当一个线程启动的时候他会创建一个looper对象,执行looper.loop(),用来循环获取MessageQueue队列的对象。当前的Looper对象是属于HandlerThread的,通过这句代码就可以得知ServiceHandler是也是运行在HandlerThread线程的。
mServiceHandler = new ServiceHandler(mServiceLooper);
我们在看看这段代码:
private final class ServiceHandler extends Handler { public ServiceHandler(Looper looper) { super(looper); } @Override public void handleMessage(Message msg) { onHandleIntent((Intent)msg.obj); stopSelf(msg.arg1); } }
说明onHandleMessage也是运行在HandleThread线程中。
所以我们在实现IntentService中onHandleIntent方法时,就可以直接进行耗时操作。
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