IntentService也算比较常用的一个组件,所以有必要增加对它的了解。
首先看看源码
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;
}
public void setIntentRedelivery(boolean enabled) {
mRedelivery = enabled;
}
@Override
public void onCreate() {
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);
}
@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();
}
@Override
@Nullable
public IBinder onBind(Intent intent) {
return null;
}
@WorkerThread
protected abstract void onHandleIntent(@Nullable Intent intent);
IntentService 四个成员变量
volatile大家都懂的,内存可见性,禁止编译器优化代码重排。
private volatile Looper mServiceLooper; //子线程的looper
private volatile ServiceHandler mServiceHandler; //静态内部类,继承handler
private String mName;//作为工作线程名
private boolean mRedelivery;//是否重投递
介绍下方法
@WorkerThread
protected abstract void onHandleIntent(@Nullable Intent intent);
入参是intent,在子线程种执行,也就是大家要做的耗时操作都在这里做,本身是个抽象方法,需要我们实现的。
@Override
@Nullable
public IBinder onBind(Intent intent) {
return null;
}
并没有bindservice 这种操作,直接返回 null;
@Override
public void onDestroy() {
mServiceLooper.quit();
}
销毁时,退出looper循环,资源回收
@Override
public int onStartCommand(@Nullable Intent intent, int flags, int startId) {
onStart(intent, startId);
return mRedelivery ? START_REDELIVER_INTENT : START_NOT_STICKY;
}
public void setIntentRedelivery(boolean enabled) {
mRedelivery = enabled;
}
值得注意的是在onStartCommand中返回值,常用的返回值有:START_NOT_STICKY、START_SICKY和START_REDELIVER_INTENT,这三个都是静态常理值。
START_NOT_STICKY:
表示当Service运行的进程被Android系统强制杀掉之后,不会重新创建该Service,如果想重新实例化该Service,就必须重新调用startService来启动。
使用场景:表示当Service在执行工作中被中断几次无关紧要或者对Android内存紧张的情况下需要被杀掉且不会立即重新创建这种行为也可接受的话,这是可以在onStartCommand返回值中设置该值。如在Service中定时从服务器中获取最新数据
START_STICKY:
表示Service运行的进程被Android系统强制杀掉之后,Android系统会将该Service依然设置为started状态(即运行状态),但是不再保存onStartCommand方法传入的intent对象,然后Android系统会尝试再次重新创建该Service,并执行onStartCommand回调方法,这时onStartCommand回调方法的Intent参数为null,也就是onStartCommand方法虽然会执行但是获取不到intent信息。
使用场景:如果你的Service可以在任意时刻运行或结束都没什么问题,而且不需要intent信息,那么就可以在onStartCommand方法中返回START_STICKY,比如一个用来播放背景音乐功能的Service就适合返回该值。
START_REDELIVER_INTENT:
表示Service运行的进程被Android系统强制杀掉之后,与返回START_STICKY的情况类似,Android系统会将再次重新创建该Service,并执行onStartCommand回调方法,但是不同的是,Android系统会再次将Service在被杀掉之前最后一次传入onStartCommand方法中的Intent再次保留下来并再次传入到重新创建后的Service的onStartCommand方法中,这样我们就能读取到intent参数。
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);
}
}
静态内部类 ServiceHandler, 这个是子线程的handler,覆写 handleMessage ,做了两件事,一个调用我们实现的onHandleIntent 执行我们的耗时操作,之后
stopSelf(msg.arg1);检查是不是最后一个任务ID,如果是就stopself.
@Override
public void onCreate() {
super.onCreate();
HandlerThread thread = new HandlerThread("IntentService[" + mName + "]");
thread.start();
mServiceLooper = thread.getLooper();
mServiceHandler = new ServiceHandler(mServiceLooper);
}
创建HandlerThread 并启动,取出looper关连mServiceHandler。
@Override
public void onStart(@Nullable Intent intent, int startId) {
Message msg = mServiceHandler.obtainMessage();
msg.arg1 = startId;
msg.obj = intent;
mServiceHandler.sendMessage(msg);
}
onStartCommand 调用onStart ,onStart 中创建消息用mServiceHandler.sendMessage发送到子线程的消息队列,在子线程的looper中调用mServiceHandler 的 handleMessage处理这个消息。handler looper message机制已经被说烂了,这里就不详谈。
public class HandlerThread extends Thread {
int mPriority;
int mTid = -1;
Looper mLooper;
public HandlerThread(String name) {
super(name);
mPriority = Process.THREAD_PRIORITY_DEFAULT;
}
public HandlerThread(String name, int priority) {
super(name);
mPriority = priority;
}
/**
* Call back method that can be explicitly overridden if needed to execute some
* setup before Looper loops.
*/
protected void onLooperPrepared() {
}
@Override
public void run() {
mTid = Process.myTid();
Looper.prepare();
synchronized (this) {
mLooper = Looper.myLooper();
notifyAll();
}
Process.setThreadPriority(mPriority);
onLooperPrepared();
Looper.loop();
mTid = -1;
}
public Looper getLooper() {
if (!isAlive()) {
return null;
}
// If the thread has been started, wait until the looper has been created.
synchronized (this) {
while (isAlive() && mLooper == null) {
try {
wait();
} catch (InterruptedException e) {
}
}
}
return mLooper;
}
public boolean quit() {
Looper looper = getLooper();
if (looper != null) {
looper.quit();
return true;
}
return false;
}
public boolean quitSafely() {
Looper looper = getLooper();
if (looper != null) {
looper.quitSafely();
return true;
}
return false;
}
/**
* Returns the identifier of this thread. See Process.myTid().
*/
public int getThreadId() {
return mTid;
}
}
官方实现的一个子线程用handler looper message机制。也简单易懂。