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TimedEventQueue分析
AwesomePlayer里面定义了一个TimedEventQueue,通过向它发事件消息,来驱动player的运转。
定义在AwesomePlayer.h里面,如下:
TimedEventQueue mQueue;
TimedEventQueue中维护了一个队列,外面通过调用其提供的方法postEvent, postEventWithDelay等等来向队列添加事件,在执行完mQueue.start()后,TimedEventQueue将启动一个线程,用来取出队列中的事件,并执行之。
先看mQueue.start()
void TimedEventQueue::start() { if (mRunning) { return; } mStopped = false; pthread_attr_t attr; pthread_attr_init(&attr); pthread_attr_setdetachstate(&attr, PTHREAD_CREATE_JOINABLE); // 创建线程,线程函数为ThreadWrapper pthread_create(&mThread, &attr, ThreadWrapper, this); pthread_attr_destroy(&attr); mRunning = true; } // static void *TimedEventQueue::ThreadWrapper(void *me) { androidSetThreadPriority(0, ANDROID_PRIORITY_FOREGROUND); // 线程函数又调用了自己的threadEntry() static_cast<TimedEventQueue *>(me)->threadEntry(); return NULL; } void TimedEventQueue::threadEntry() { prctl(PR_SET_NAME, (unsigned long)"TimedEventQueue", 0, 0, 0); for (;;) { int64_t now_us = 0; sp<Event> event; { Mutex::Autolock autoLock(mLock); if (mStopped) { break; } // 等待队列中有事件来 while (mQueue.empty()) { mQueueNotEmptyCondition.wait(mLock); } event_id eventID = 0; for (;;) { if (mQueue.empty()) { // The only event in the queue could have been cancelled // while we were waiting for its scheduled time. break; } // 往队列中添加事件是按照事件的时间先后顺序添加的 // 所以这里是取执行时间最靠前的事件 List<QueueItem>::iterator it = mQueue.begin(); eventID = (*it).event->eventID(); now_us = ALooper::GetNowUs(); int64_t when_us = (*it).realtime_us; int64_t delay_us; if (when_us < 0 || when_us == INT64_MAX) { delay_us = 0; } else { delay_us = when_us - now_us; } if (delay_us <= 0) { break; // 不需要等待 } static int64_t kMaxTimeoutUs = 10000000ll; // 10 secs bool timeoutCapped = false; // 等待时间超过10secs,则分成几次来等 if (delay_us > kMaxTimeoutUs) { ALOGW("delay_us exceeds max timeout: %lld us", delay_us); // We'll never block for more than 10 secs, instead // we will split up the full timeout into chunks of // 10 secs at a time. This will also avoid overflow // when converting from us to ns. delay_us = kMaxTimeoutUs; timeoutCapped = true; } // 等待相应的时长 status_t err = mQueueHeadChangedCondition.waitRelative( mLock, delay_us * 1000ll); if (!timeoutCapped && err == -ETIMEDOUT) { // We finally hit the time this event is supposed to // trigger. now_us = ALooper::GetNowUs(); break; } } // The event w/ this id may have been cancelled while we're // waiting for its trigger-time, in that case // removeEventFromQueue_l will return NULL. // Otherwise, the QueueItem will be removed // from the queue and the referenced event returned. event = removeEventFromQueue_l(eventID); } // 执行取出的事件 if (event != NULL) { // Fire event with the lock NOT held. event->fire(this, now_us); } } }
可见,mQueue.start()之后,TimedEventQueue就启动了一个线程,等待外面给它发event,然后取出消息,并通过调用event->fire(this, now_us)执行之。
接下来,就去看看AwesomePlayer往队列里发的event。
mAsyncPrepareEvent = new AwesomeEvent(
this, &AwesomePlayer::onPrepareAsyncEvent);
mQueue.postEvent(mAsyncPrepareEvent);
AwesomeEvent的定义如下:struct AwesomeEvent : public TimedEventQueue::Event { AwesomeEvent( AwesomePlayer *player, void (AwesomePlayer::*method)()) : mPlayer(player), mMethod(method) { } protected: virtual ~AwesomeEvent() {} virtual void fire(TimedEventQueue *queue, int64_t /* now_us */) { (mPlayer->*mMethod)(); } private: AwesomePlayer *mPlayer; void (AwesomePlayer::*mMethod)(); AwesomeEvent(const AwesomeEvent &); AwesomeEvent &operator=(const AwesomeEvent &); };
在构造函数中,mPlayer被赋值为this,即AwesomePlayer自己,mMethod被赋值为AwesomePlayer::onPrepareAsyncEvent。AwesomeEvent实现了fire方法,(mPlayer->*mMethod)(),综合前面对TimedEventQueue的了解,当这个AwesomeEvent被post到TimedEventQueue的队列后,就会在TimedEventQueue启动的线程函数中被取出,然后会调用event的fire函数,在这里就是调用AwesomePlayer的onPrepareAsyncEvent方法了。
AwesomePlayer里面定义的AwesomeEvent有:
mVideoEvent = new AwesomeEvent(this,&AwesomePlayer::onVideoEvent);
mStreamDoneEvent = new AwesomeEvent(this,&AwesomePlayer::onStreamDone);
mBufferingEvent = new AwesomeEvent(this,&AwesomePlayer::onBufferingUpdate);
mVideoLagEvent = new AwesomeEvent(this,&AwesomePlayer::onVideoLagUpdate);
mAsyncPrepareEvent = new AwesomeEvent(
this, &AwesomePlayer::onPrepareAsyncEvent);
等等……
到此,TimedEventQueue分析完毕。