题外话
我竟然已经写了这个系列的十一篇文章了，虽然内容很浅显，虽然内容很枯燥，虽然内容也许没营养，但我为自己的坚持点赞！
必读：Android 12(S) 图像显示系统 - 开篇
一、前言
前面的两篇文章，分别讲解了Producer的处理逻辑和queue buffer后通过FrameAvailableListener通知到Consumer的基本过程。
流程已经走到了BufferQueueConsumer::acquireBuffer中，所以这篇文章聚焦Consumer的一些处理逻辑。
还是把流程图贴上来
从流程图中看，这篇文章就是讲解右半部分的内容。
二、消费者-Consumer的相关逻辑
了解了 BufferQueueCore 和 BufferQueueProducer，接着看 BufferQueue 的最后一个元素：BufferQueueConsumer。
BufferQueueConsumer作为消费者的一个代表元素通过 acquireBuffer 来获取图像缓冲区，通过 releaseBuffer 来释放该缓冲区。
下面就分别看看 BufferQueueConsumer 中 acquireBuffer 和 releaseBuffer 两个操作的具体流程。
2.1 代码位置
/frameworks/native/libs/gui/BufferQueueConsumer.cpp
2.2 acquireBuffer的逻辑
先看 acquireBuffer 的过程，上源码
status_t BufferQueueConsumer::acquireBuffer(BufferItem* outBuffer,
nsecs_t expectedPresent, uint64_t maxFrameNumber) {
ATRACE_CALL();
int numDroppedBuffers = 0;
sp<IProducerListener> listener;
{
std::unique_lock<std::mutex> lock(mCore->mMutex);
// Check that the consumer doesn't currently have the maximum number of
// buffers acquired. We allow the max buffer count to be exceeded by one
// buffer so that the consumer can successfully set up the newly acquired
// buffer before releasing the old one.
// 检查acquire的buffer的数量是否超出了限制
int numAcquiredBuffers = 0;
for (int s : mCore->mActiveBuffers) {
if (mSlots[s].mBufferState.isAcquired()) {
++numAcquiredBuffers;
}
}
const bool acquireNonDroppableBuffer = mCore->mAllowExtraAcquire &&
numAcquiredBuffers == mCore->mMaxAcquiredBufferCount + 1;
if (numAcquiredBuffers >= mCore->mMaxAcquiredBufferCount + 1 &&
!acquireNonDroppableBuffer) {
BQ_LOGE("acquireBuffer: max acquired buffer count reached: %d (max %d)",
numAcquiredBuffers, mCore->mMaxAcquiredBufferCount);
return INVALID_OPERATION;
}
bool sharedBufferAvailable = mCore->mSharedBufferMode &&
mCore->mAutoRefresh && mCore->mSharedBufferSlot !=
BufferQueueCore::INVALID_BUFFER_SLOT;
// In asynchronous mode the list is guaranteed to be one buffer deep,
// while in synchronous mode we use the oldest buffer.
// 检查BufferQueueCore中的mQueue队列是否为空
if (mCore->mQueue.empty() && !sharedBufferAvailable) {
return NO_BUFFER_AVAILABLE;
}
// 获取BufferQueueCore中的mQueue队列的迭代器
BufferQueueCore::Fifo::iterator front(mCore->mQueue.begin());
// If expectedPresent is specified, we may not want to return a buffer yet.
// If it's specified and there's more than one buffer queued, we may want
// to drop a buffer.
// Skip this if we're in shared buffer mode and the queue is empty,
// since in that case we'll just return the shared buffer.
if (expectedPresent != 0 && !mCore->mQueue.empty()) {
// expectedPresent表示期望这个buffer什么时候显示到屏幕上。
// 如果buffer的期望显示时间小于expectedPresent，我们会acquire and return这个buffer
// 如果我们不想显示它直到expectedPresent之后，可以返回PRESENT_LATER
// The 'expectedPresent' argument indicates when the buffer is expected
// to be presented on-screen. If the buffer's desired present time is
// earlier (less) than expectedPresent -- meaning it will be displayed
// on time or possibly late if we show it as soon as possible -- we
// acquire and return it. If we don't want to display it until after the
// expectedPresent time, we return PRESENT_LATER without acquiring it.
//
// 安全起见，如果expectedPresent超过了buffer的期望显示时间1秒，我们会推迟acquire
// To be safe, we don't defer acquisition if expectedPresent is more
// than one second in the future beyond the desired present time
// (i.e., we'd be holding the buffer for a long time).
//
// NOTE: Code assumes monotonic time values from the system clock
// are positive.
// 检查是否需要丢弃一些帧,主要是判断timestamps & expectedPresent
// Start by checking to see if we can drop frames. We skip this check if
// the timestamps are being auto-generated by Surface. If the app isn't
// generating timestamps explicitly, it probably doesn't want frames to
// be discarded based on them.
while (mCore->mQueue.size() > 1 && !mCore->mQueue[0].mIsAutoTimestamp) {
const BufferItem& bufferItem(mCore->mQueue[1]);
// If dropping entry[0] would leave us with a buffer that the
// consumer is not yet ready for, don't drop it.
if (maxFrameNumber && bufferItem.mFrameNumber > maxFrameNumber) {
break;
}
// If entry[1] is timely, drop entry[0] (and repeat). We apply an
// additional criterion here: we only drop the earlier buffer if our
// desiredPresent falls within +/- 1 second of the expected present.
// Otherwise, bogus desiredPresent times (e.g., 0 or a small
// relative timestamp), which normally mean "ignore the timestamp
// and acquire immediately", would cause us to drop frames.
//
// We may want to add an additional criterion: don't drop the
// earlier buffer if entry[1]'s fence hasn't signaled yet.
nsecs_t desiredPresent = bufferItem.mTimestamp;
// desiredPresent比expectedPresent小了1 second多，或desiredPresent大于expectedPresent
if (desiredPresent < expectedPresent - MAX_REASONABLE_NSEC ||
desiredPresent > expectedPresent) {
// This buffer is set to display in the near future, or
// desiredPresent is garbage. Either way we don't want to drop
// the previous buffer just to get this on the screen sooner.
BQ_LOGV("acquireBuffer: nodrop desire=%" PRId64 " expect=%"
PRId64 " (%" PRId64 ") now=%" PRId64,
desiredPresent, expectedPresent,
desiredPresent - expectedPresent,
systemTime(CLOCK_MONOTONIC));
break;
}
BQ_LOGV("acquireBuffer: drop desire=%" PRId64 " expect=%" PRId64
" size=%zu",
desiredPresent, expectedPresent, mCore->mQueue.size());
// 处理要drop的buffer
if (!front->mIsStale) {
// Front buffer is still in mSlots, so mark the slot as free
// 对应的BufferSlot设置为FREE状态
mSlots[front->mSlot].mBufferState.freeQueued();
// After leaving shared buffer mode, the shared buffer will
// still be around. Mark it as no longer shared if this
// operation causes it to be free.
if (!mCore->mSharedBufferMode &&
mSlots[front->mSlot].mBufferState.isFree()) {
mSlots[front->mSlot].mBufferState.mShared = false;
}
// mActiveBuffers ：绑定了GraphicBuffer且状态为非FREE的BufferSlot集合;
// mFreeBuffers ：绑定了GraphicBuffer且状态为FREE的BufferSlot集合；
// Don't put the shared buffer on the free list
if (!mSlots[front->mSlot].mBufferState.isShared()) {
mCore->mActiveBuffers.erase(front->mSlot); // 从mActiveBuffers删除
mCore->mFreeBuffers.push_back(front->mSlot);// 添加进mFreeBuffers
}
if (mCore->mBufferReleasedCbEnabled) {
listener = mCore->mConnectedProducerListener; // 设置生产者的监听器
}
++numDroppedBuffers; // 计数加1，记录drop了几个buffer
}
mCore->mQueue.erase(front);// 从mQueue中删除
front = mCore->mQueue.begin();// 重置front，进入下一次while循环
}
// See if the front buffer is ready to be acquired
nsecs_t desiredPresent = front->mTimestamp;
bool bufferIsDue = desiredPresent <= expectedPresent ||
desiredPresent > expectedPresent + MAX_REASONABLE_NSEC;
bool consumerIsReady = maxFrameNumber > 0 ?
front->mFrameNumber <= maxFrameNumber : true;
if (!bufferIsDue || !consumerIsReady) {
BQ_LOGV("acquireBuffer: defer desire=%" PRId64 " expect=%" PRId64
" (%" PRId64 ") now=%" PRId64 " frame=%" PRIu64
" consumer=%" PRIu64,
desiredPresent, expectedPresent,
desiredPresent - expectedPresent,
systemTime(CLOCK_MONOTONIC),
front->mFrameNumber, maxFrameNumber);
ATRACE_NAME("PRESENT_LATER");
return PRESENT_LATER;
}
BQ_LOGV("acquireBuffer: accept desire=%" PRId64 " expect=%" PRId64 " "
"(%" PRId64 ") now=%" PRId64, desiredPresent, expectedPresent,
desiredPresent - expectedPresent,
systemTime(CLOCK_MONOTONIC));
}
// 走到这里就说明：该丢弃的已经都丢弃了，余下的就可以拿去显示了。
int slot = BufferQueueCore::INVALID_BUFFER_SLOT;
if (sharedBufferAvailable && mCore->mQueue.empty()) {
// make sure the buffer has finished allocating before acquiring it
// 共享Buffer模式下处理
mCore->waitWhileAllocatingLocked(lock);
slot = mCore->mSharedBufferSlot;
// Recreate the BufferItem for the shared buffer from the data that
// was cached when it was last queued.
outBuffer->mGraphicBuffer = mSlots[slot].mGraphicBuffer;
outBuffer->mFence = Fence::NO_FENCE;
outBuffer->mFenceTime = FenceTime::NO_FENCE;
outBuffer->mCrop = mCore->mSharedBufferCache.crop;
outBuffer->mTransform = mCore->mSharedBufferCache.transform &
~static_cast<uint32_t>(
NATIVE_WINDOW_TRANSFORM_INVERSE_DISPLAY);
outBuffer->mScalingMode = mCore->mSharedBufferCache.scalingMode;
outBuffer->mDataSpace = mCore->mSharedBufferCache.dataspace;
outBuffer->mFrameNumber = mCore->mFrameCounter;
outBuffer->mSlot = slot;
outBuffer->mAcquireCalled = mSlots[slot].mAcquireCalled;
outBuffer->mTransformToDisplayInverse =
(mCore->mSharedBufferCache.transform &
NATIVE_WINDOW_TRANSFORM_INVERSE_DISPLAY) != 0;
outBuffer->mSurfaceDamage = Region::INVALID_REGION;
outBuffer->mQueuedBuffer = false;
outBuffer->mIsStale = false;
outBuffer->mAutoRefresh = mCore->mSharedBufferMode &&
mCore->mAutoRefresh;
} else if (acquireNonDroppableBuffer && front->mIsDroppable) {
BQ_LOGV("acquireBuffer: front buffer is not droppable");
return NO_BUFFER_AVAILABLE;
} else {
// 从front获取对应的slot index
slot = front->mSlot;
*outBuffer = *front;
}
ATRACE_BUFFER_INDEX(slot);
BQ_LOGV("acquireBuffer: acquiring { slot=%d/%" PRIu64 " buffer=%p }",
slot, outBuffer->mFrameNumber, outBuffer->mGraphicBuffer->handle);
if (!outBuffer->mIsStale) {
mSlots[slot].mAcquireCalled = true;
// Don't decrease the queue count if the BufferItem wasn't
// previously in the queue. This happens in shared buffer mode when
// the queue is empty and the BufferItem is created above.
if (mCore->mQueue.empty()) {
mSlots[slot].mBufferState.acquireNotInQueue();
} else {
// 将BufferState状态改为acquire
mSlots[slot].mBufferState.acquire();
}
mSlots[slot].mFence = Fence::NO_FENCE;
}
// If the buffer has previously been acquired by the consumer, set
// mGraphicBuffer to NULL to avoid unnecessarily remapping this buffer
// on the consumer side
if (outBuffer->mAcquireCalled) {
outBuffer->mGraphicBuffer = nullptr;
}
//将该Buffer从mQueue中移除
mCore->mQueue.erase(front);
// We might have freed a slot while dropping old buffers, or the producer
// may be blocked waiting for the number of buffers in the queue to
// decrease.
mCore->mDequeueCondition.notify_all();
ATRACE_INT(mCore->mConsumerName.string(),
static_cast<int32_t>(mCore->mQueue.size()));
#ifndef NO_BINDER
mCore->mOccupancyTracker.registerOccupancyChange(mCore->mQueue.size());
#endif
VALIDATE_CONSISTENCY();
}
// 回调，通知生产者
if (listener != nullptr) {
for (int i = 0; i < numDroppedBuffers; ++i) {
listener->onBufferReleased();
}
}
return NO_ERROR;
}
acquireBuffer 函数中的逻辑也非常的清晰，源码中也做了详细注释。
主要就是这几件事情：
- 判断 BufferQueueCore 中的 mQueue 是否为空，mQueue 就是前面 BufferQueueProducer 调用 queueBuffer 函数时，将缓冲区入队的容器；
- 取出对应的 BufferSlot（会有一些判断规则，舍弃一些buffer）；
- 将 BufferState 改为 acquire 状态；
- 将该 Buffer 从 mQueue 中移除；
2.3 消费者acquire拿到buffer后又是怎样通知release buffer呢？
要回答这个问题，我们需要在回到调用acquireBuffer的地方，即 BLASTBufferQueue::processNextBufferLocked 函数中，先看其代码：
void BLASTBufferQueue::processNextBufferLocked(bool useNextTransaction) {
......
SurfaceComposerClient::Transaction localTransaction;
bool applyTransaction = true;
SurfaceComposerClient::Transaction* t = &localTransaction;
// acquireBuffer获取要处理的buffer
BufferItem bufferItem;
status_t status =
mBufferItemConsumer->acquireBuffer(&bufferItem, 0 /* expectedPresent */, false);
......
// 拿到了实际的GraphicBuffer了
auto buffer = bufferItem.mGraphicBuffer;
mNumFrameAvailable--;
// 某些情况下，直接releaseBuffer而无需送SurfaceFlinger合成显示
mLastAcquiredFrameNumber = bufferItem.mFrameNumber;
ReleaseCallbackId releaseCallbackId(buffer->getId(), mLastAcquiredFrameNumber);
mSubmitted[releaseCallbackId] = bufferItem;
....
// Ensure BLASTBufferQueue stays alive until we receive the transaction complete callback.
incStrong((void*)transactionCallbackThunk);
// release buffer的回到函数
auto releaseBufferCallback =
std::bind(releaseBufferCallbackThunk, wp<BLASTBufferQueue>(this) /* callbackContext */,
std::placeholders::_1, std::placeholders::_2, std::placeholders::_3,
std::placeholders::_4);
t->setBuffer(mSurfaceControl, buffer, releaseCallbackId, releaseBufferCallback);
......
if (applyTransaction) {
t->setApplyToken(mApplyToken).apply();
}
}上述代码，做了比较多的简化，只保留我认为比较重要的部分。
- 调用acquireBuffer获取一个BufferItem;
- 取出GraphicBuffer -- auto buffer = bufferItem.mGraphicBuffer;
- 通过事务Transaction来向SurfaceFlinger提交Buffer与图层的属性；
t->setBuffer(mSurfaceControl, buffer, releaseCallbackId, releaseBufferCallback);对于setBuffer，就是设置传递给SF的buffer，并指定了一个releaseBufferCallback，暂时可以理解为SF消费完这个buffer，就会通过这个callback通知来释放这个buffer。
本文作者@二的次方 2022-03-23 发布于博客园
在acquireBuffer中加入log，打印调用堆栈信息，如下：
11-13 01:23:59.275 3016 3030 E BufferQueueConsumer: stackdump:#00 pc 000580ff /system/lib/libgui.so (android::BufferQueueConsumer::releaseBuffer(int, unsigned long long, android::sp<android::Fence> const&, void*, void*)+130)
11-13 01:23:59.275 3016 3030 E BufferQueueConsumer: stackdump:#01 pc 00059117 /system/lib/libgui.so (android::BufferQueueConsumer::releaseBuffer(int, unsigned long long, void*, void*, android::sp<android::Fence> const&)+30)
11-13 01:23:59.275 3016 3030 E BufferQueueConsumer: stackdump:#02 pc 00076d27 /system/lib/libgui.so (android::ConsumerBase::releaseBufferLocked(int, android::sp<android::GraphicBuffer>, void*, void*)+134)
11-13 01:23:59.275 3016 3030 E BufferQueueConsumer: stackdump:#03 pc 0007580d /system/lib/libgui.so (android::BufferItemConsumer::releaseBuffer(android::BufferItem const&, android::sp<android::Fence> const&)+140)
11-13 01:23:59.275 3016 3030 E BufferQueueConsumer: stackdump:#04 pc 0006c467 /system/lib/libgui.so (android::BLASTBufferQueue::releaseBufferCallback(android::ReleaseCallbackId const&, android::sp<android::Fence> const&, unsigned int, unsigned int)+1362)
11-13 01:23:59.275 3016 3030 E BufferQueueConsumer: stackdump:#05 pc 0006d827 /system/lib/libgui.so (android::releaseBufferCallbackThunk(android::wp<android::BLASTBufferQueue>, android::ReleaseCallbackId const&, android::sp<android::Fence> const&, unsigned int, unsigned int)+62)
11-13 01:23:59.276 3016 3030 E BufferQueueConsumer: stackdump:#06 pc 0007039b /system/lib/libgui.so (std::__1::__function::__func<std::__1::__bind<void (&)(android::wp<android::BLASTBufferQueue>, android::ReleaseCallbackId const&, android::sp<android::Fence> const&, unsigned int, unsigned int), android::wp<android::BLASTBufferQueue>, std::__1::placeholders::__ph<1> const&, std::__1::placeholders::__ph<2> const&, std::__1::placeholders::__ph<3> const&, std::__1::placeholders::__ph<4> const&>, std::__1::allocator<std::__1::__bind<void (&)(android::wp<android::BLASTBufferQueue>, android::ReleaseCallbackId const&, android::sp<android::Fence> const&, unsigned int, unsigned int), android::wp<android::BLASTBufferQueue>, std::__1::placeholders::__ph<1> const&, std::__1::placeholders::__ph<2> const&, std::__1::placeholders::__ph<3> const&, std::__1::placeholders::__ph<4> const&> >, void (android::ReleaseCallbackId const&, android::sp<android::Fence> const&, unsigned int, unsigned int)>::operator()(android::ReleaseCallbackId const&, android::sp<android::Fence> const&,
11-13 01:23:59.276 3016 3030 E BufferQueueConsumer: stackdump:#07 pc 000a7d47 /system/lib/libgui.so (android::TransactionCompletedListener::onTransactionCompleted(android::ListenerStats)+3382)
11-13 01:23:59.276 3016 3030 E BufferQueueConsumer: stackdump:#08 pc 000925a5 /system/lib/libgui.so (int android::SafeBnInterface<android::ITransactionCompletedListener>::callLocalAsync<void (android::ITransactionCompletedListener::*)(android::ListenerStats)>(android::Parcel const&, android::Parcel*, void (android::ITransactionCompletedListener::*)(android::ListenerStats))+204)
11-13 01:23:59.276 3016 3030 E BufferQueueConsumer: stackdump:#09 pc 00028ddb /system/lib/libbinder.so (android::BBinder::transact(unsigned int, android::Parcel const&, android::Parcel*, unsigned int)+162)
看上面的调用栈，是不是一目了然，从遥远的Binder来的神秘信息触发了这一些列的事件：
>>> releaseBufferCallbackThunk
>>> BLASTBufferQueue::releaseBufferCallback
>>> BufferItemConsumer::releaseBuffer
>>> ConsumerBase::releaseBufferLocked
>>> BufferQueueConsumer::releaseBuffer
2.4 releaseBuffer的逻辑
老规矩，直接看代码，是不是很简单啊！
status_t BufferQueueConsumer::releaseBuffer(int slot, uint64_t frameNumber,
const sp<Fence>& releaseFence, EGLDisplay eglDisplay,
EGLSyncKHR eglFence) {
ATRACE_CALL();
ATRACE_BUFFER_INDEX(slot);
if (slot < 0 || slot >= BufferQueueDefs::NUM_BUFFER_SLOTS ||
releaseFence == nullptr) {
BQ_LOGE("releaseBuffer: slot %d out of range or fence %p NULL", slot,
releaseFence.get());
return BAD_VALUE;
}
sp<IProducerListener> listener;
{ // Autolock scope
std::lock_guard<std::mutex> lock(mCore->mMutex);
// If the frame number has changed because the buffer has been reallocated,
// we can ignore this releaseBuffer for the old buffer.
// Ignore this for the shared buffer where the frame number can easily
// get out of sync due to the buffer being queued and acquired at the
// same time.
if (frameNumber != mSlots[slot].mFrameNumber &&
!mSlots[slot].mBufferState.isShared()) {
return STALE_BUFFER_SLOT;
}
if (!mSlots[slot].mBufferState.isAcquired()) {
BQ_LOGE("releaseBuffer: attempted to release buffer slot %d "
"but its state was %s", slot,
mSlots[slot].mBufferState.string());
return BAD_VALUE;
}
mSlots[slot].mEglDisplay = eglDisplay;
mSlots[slot].mEglFence = eglFence;
mSlots[slot].mFence = releaseFence;
mSlots[slot].mBufferState.release();//置为FREE状态
// After leaving shared buffer mode, the shared buffer will
// still be around. Mark it as no longer shared if this
// operation causes it to be free.
if (!mCore->mSharedBufferMode && mSlots[slot].mBufferState.isFree()) {
mSlots[slot].mBufferState.mShared = false;
}
// Don't put the shared buffer on the free list.
if (!mSlots[slot].mBufferState.isShared()) {
mCore->mActiveBuffers.erase(slot);// 从mActiveBuffers中删除
mCore->mFreeBuffers.push_back(slot);//加入到mFreeBuffers中
}
if (mCore->mBufferReleasedCbEnabled) {
listener = mCore->mConnectedProducerListener; / 设置listener
}
BQ_LOGV("releaseBuffer: releasing slot %d", slot);
// 唤醒等待的线程
mCore->mDequeueCondition.notify_all();
VALIDATE_CONSISTENCY();
} // Autolock scope
// Call back without lock held
if (listener != nullptr) {
listener->onBufferReleased(); //通知producer
}
return NO_ERROR;
}releaseBuffer方法的流程相对简单：
-
slot就是需要释放的BufferSlot的序号;
-
Buffer的FrameNumber变了，可能Buffer已经重新分配，这个是不用管;
-
只能释放acquire状态的buffer序号，释放后是Buffer放会mFreeBuffers中;
-
releaseFence，从consumer那边传过来，producer可以dequeue mFreeBuffers中的buffer，但是只有releaseFence发信号出来后，consumer才真正用完，producer才可以写;
-
最后通过listener通知producer。
2.5 ProducerListener是怎样工作的？
在前面的讲解中，有几处都有出现 listener->onBufferReleased() ，意思是通知producer有buffer释放了。这个listener是在哪里设置的？onBufferReleased又做了哪些工作呢？接下来分析
定义：/frameworks/native/libs/gui/include/gui/IProducerListener.h
首先我们看到这张类图：
1. 在BufferQueueCore中有成员 sp<IProducerListener> mConnectedProducerListener，它就是用来处理onBufferReleased事件的；
2. mConnectedProducerListener是在哪里被设置的呢？答案是 BufferQueueProducer::connect；
3. 根据调用栈来追踪：
11-13 01:20:19.388 2955 3013 E BufferQueueProducer: stackdump:#00 pc 0005e667 /system/lib/libgui.so (android::BufferQueueProducer::connect(android::sp<android::IProducerListener> const&, int, bool, android::IGraphicBufferProducer::QueueBufferOutput*)+1018)
11-13 01:20:19.388 2955 3013 E BufferQueueProducer: stackdump:#01 pc 0006ee41 /system/lib/libgui.so (android::BBQBufferQueueProducer::connect(android::sp<android::IProducerListener> const&, int, bool, android::IGraphicBufferProducer::QueueBufferOutput*)+176)
11-13 01:20:19.388 2955 3013 E BufferQueueProducer: stackdump:#02 pc 000a268b /system/lib/libgui.so (android::Surface::connect(int, android::sp<android::IProducerListener> const&, bool)+138)
11-13 01:20:19.388 2955 3013 E BufferQueueProducer: stackdump:#03 pc 0009dd61 /system/lib/libgui.so (android::Surface::hook_perform(ANativeWindow*, int, ...)+128)4. 可以肯定，是执行 native_window_api_connect() 一路走下来的。沿着这条路，看看listener是哪里产生的呢？
看起来是这个位置：
int Surface::connect(int api) {
static sp<IProducerListener> listener = new StubProducerListener();
return connect(api, listener);
}5. 不过，奇怪的事情发生了，StubProducerListener看起来没有任何操作，乖乖（这一点还是很奇怪的）
6. BLASTBufferQueue中还有使用AsyncProducerListener做一层封装，实现异步处理；
class StubProducerListener : public BnProducerListener {
public:
virtual ~StubProducerListener();
virtual void onBufferReleased() {}
virtual bool needsReleaseNotify() { return false; }
};
三、小结
BufferQueue的运作流程到这里就算讲完了。生产者做了什么事情？消费者做了什么事情？图形缓存是怎样流转的？状态是怎样变化的？在几篇文章中基本上都有做了或简单或详细的介绍。通过BufferQueue的几篇文章，帮助自己建立起基本的逻辑框架，为我们后续研究和分析问题奠定基础。