IntentService是什么？
IntentService是继承并处理异步请求的一个类，在IntentService内有一个工作线程来处理耗时操作，启动IntentService方法和启动传统的Service一样，同时，当任务执行完后，IntentService会自动停止，而不需要我们手动的stopSelf()。另外，可以启动IntentService多次，而每个耗时操作会以工作队列的方式在IntentService的onHandleIntent回调方法中执行，并且，每次只会执行一个线程，执行完第一个再执行第二个。
- 它本质上是一种特殊的Service，继承自Service并且本身是一个抽象类。
- 它内部是通过HandlerThread和Handler实现异步操作的。
IntentService使用方法：
关于它的使用可以参考之前写的博客：
下面总结一下：创建IntentService时，只需要实现onHandleIntent()和构造方法，onHandleIntent()为异步方法，可以执行耗时操作。
IntentService源码解析：
跟HandlerThread的源代码类似，代码也不多，如下：
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);
}而首先先看一下它的onCreate()方法：
首先会创建一个HandlerThread,然后再根据它的Looper来创建一个Handler，如下：
此时就可以在Handler中执行异步任务了。那IntentService是如何启动异步任务的呢？这里可以看一下onStrartCommand()方法：
又调用了onStart()方法：
接着就会到handler的handleMessage()方法中去处理了：
而onHandleIntent()方法是个抽象方法，由咱们自己来实现：
而其中调用停止任务时传了一个参数：
这时就会等待所有任务处理完之后才会停止，而不是立马停止。也就是如果有很多个任务都要由IntentService处理完之后，才会去让IntentService去停止，而非执行完一个就立马停止了。
它的本质就是一个封装了HandlerThread和handler的异步框架。
