1.前言
想必大家应该都使用过Service组件，为什么有service，还需要IntentService呢。它们有什么不同，带着这些问题，下面我们分析下源码。
2.IntentService源码分析
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);
}
2.1 onCreate方法
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);
}
public class HandlerThread extends Thread
可以看出在onCreate方法中HandlerThread变量。可以看出HandlerThread其实就是一个线程。在此启动一个线程。同时定义一个ServiceHandler的Handler，由于接收处理onStart发出的Message。接下来看看onStartCommand&onStart方法。
2.2 onStartCommand和onStart方法
@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 onStart(@Nullable Intent intent, int startId) {
Message msg = mServiceHandler.obtainMessage();
msg.arg1 = startId;
msg.obj = intent;
mServiceHandler.sendMessage(msg);
}
在启动这个service的时候会调用onStartCommand方法，onStartCommand方法又会调用onStart方法，onStart干了什么事情呢？其实就是发送Message给到ServiceHandler中。
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);
}
}
然后处理onHandlerIntent()这个抽象方法。onHandlerIntent方法跑在onCreate方法开启的线程中。然后stopSelf()。
2.3 onDestroy方法
@Override
public void onDestroy() {
mServiceLooper.quit();
}
服务停止后会执行onDestroy方法，在onDestroy方法中会清除消息队列Message。所以IntentService多次启动，如果服务停止了，会清除所有消息队列的任务,后面的任务不会执行。因为都跑在一个消息队列中，多次启动服务不会执行onCreate方法，所以不会创建多个消息队列。
2.4 总结
由此可以看出Service和IntentService的区别
1.Service与IntentService本质上是一个Service组件。
2.Service不能处理耗时过长的任务，会引起ANR, 因为service运行在UI线程上。
3.Service需要处理耗时任务必须自己开启一个线程。
4.IntentService可以处理耗时任务，并且不需要关心是否开启工作线程，继承它就可以了，实现onHandleIntent抽象方法处理耗时操作就行了。
5.使用IntentService不需要关心关闭Service。在任务完成后会自动stopSelf掉。
6.由源码看出，IntentService不适合用bindService启动，因为onBind返回null，同时用bindService IntentService也不会回调onHandleIntent方法，也不会在service任务完成后停止。如果使用bindService启动IntentService，那仅仅是一个普通的Service。
3.如何使用
3.1继承IntentService实现onHandleIntent方法
/**
* Created by zw on 2019/7/7 11:31
*/
public class MyIntentService extends IntentService {
/**
* Creates an IntentService. Invoked by your subclass's constructor.
*
* @param name Used to name the worker thread, important only for debugging.
*/
public MyIntentService(String name) {
// 可以定义工作线程名称
super(name);
}
@Override
protected void onHandleIntent(@Nullable Intent intent) {
// 处理耗时任务
}
}
3.2 AndriodManifest.xml中注册这个Service
<service android:name=".MyIntentService"/>
3.3 启动服务
startService(new Intent(this, MyIntentService.class));
That's All
IntentService源码onCreate方法里出现的Looper，Message, Handler另外MessageQueue将在接下来分析它们。
