在上篇文章中，我们讲到了应用通过createSurface()在surfaceflinger内生成了一个Layer，并且获取到了该Layer的Handle且将其和其他信息保存到了SurfaceControl。应用拿到了这个SurfaceControl，那么接下来就要创建应用端的surface了。完成这个任务的接口就是getSurface()。
SurfaceControl::getSurface()
cpp
//frameworks/native/libs/gui/SurfaceControl.cpp
sp<Surface> SurfaceControl::getSurface()
{
Mutex::Autolock _l(mLock);
if (mSurfaceData == nullptr) {
return generateSurfaceLocked();
}
return mSurfaceData;
}
sp<Surface> SurfaceControl::generateSurfaceLocked()
{
uint32_t ignore;
auto flags = mCreateFlags & (ISurfaceComposerClient::eCursorWindow |
ISurfaceComposerClient::eOpaque);
mBbqChild = mClient->createSurface(String8("bbq-wrapper"), 0, 0, mFormat,
flags, mHandle, {}, &ignore);
mBbq = sp<BLASTBufferQueue>::make("bbq-adapter", mBbqChild, mWidth, mHeight, mFormat);
// This surface is always consumed by SurfaceFlinger, so the
// producerControlledByApp value doesn't matter; using false.
mSurfaceData = mBbq->getSurface(true);
return mSurfaceData;
}我们在这个函数里发现两个点，这是Android12与之前版本的不同之处，一个是这里再次调用并创建一个SurfaceControl，另一个是创建一个BLASTBufferQueue。个人感觉这个设计应该还是一个过渡性的内容，因为里面的内容多多少少感觉有些冗余性。
下面逐一看看generateSurfaceLocked()内部要做的事情：
createSurface()
这里不再赘述了，上一篇文章已经讲解过了。这里生成的SurfaceControl保存在了当前SurfaceControl的mBbqChild变量中。值得注意的是，这里创建Layer时传入的宽和高都为0。
创建BLASTBufferQueue
这是Android12新加入的内容，其实就是对BufferQueue的一个封装。之前应用的BufferQueue都是在surfaceflinger内部，现在Google把它移到了应用进程内自己管理。这个可能是想参考Wayland的设计吧。
言归正传，来看看BLASTBufferQueue的构造过程：
cpp
//frameworks/native/libs/gui/BLASTBufferQueue.cpp
BLASTBufferQueue::BLASTBufferQueue(const std::string& name, const sp<SurfaceControl>& surface,
int width, int height, int32_t format)
: mSurfaceControl(surface),
mSize(width, height),
mRequestedSize(mSize),
mFormat(format),
mNextTransaction(nullptr) {
createBufferQueue(&mProducer, &mConsumer);
// since the adapter is in the client process, set dequeue timeout
// explicitly so that dequeueBuffer will block
mProducer->setDequeueTimeout(std::numeric_limits<int64_t>::max());
// safe default, most producers are expected to override this
mProducer->setMaxDequeuedBufferCount(2);
mBufferItemConsumer = new BLASTBufferItemConsumer(mConsumer,
GraphicBuffer::USAGE_HW_COMPOSER |
GraphicBuffer::USAGE_HW_TEXTURE,
1, false);
static int32_t id = 0;
mName = name + "#" + std::to_string(id);
auto consumerName = mName + "(BLAST Consumer)" + std::to_string(id);
mQueuedBufferTrace = "QueuedBuffer - " + mName + "BLAST#" + std::to_string(id);
id++;
mBufferItemConsumer->setName(String8(consumerName.c_str()));
mBufferItemConsumer->setFrameAvailableListener(this);
mBufferItemConsumer->setBufferFreedListener(this);
mBufferItemConsumer->setDefaultBufferSize(mSize.width, mSize.height);
mBufferItemConsumer->setDefaultBufferFormat(convertBufferFormat(format));
mBufferItemConsumer->setBlastBufferQueue(this);
ComposerService::getComposerService()->getMaxAcquiredBufferCount(&mMaxAcquiredBuffers);
mBufferItemConsumer->setMaxAcquiredBufferCount(mMaxAcquiredBuffers);
mTransformHint = mSurfaceControl->getTransformHint();
mBufferItemConsumer->setTransformHint(mTransformHint);
SurfaceComposerClient::Transaction()
.setFlags(surface, layer_state_t::eEnableBackpressure,
layer_state_t::eEnableBackpressure)
.setApplyToken(mApplyToken)
.apply();
mNumAcquired = 0;
mNumFrameAvailable = 0;
BQA_LOGV("BLASTBufferQueue created width=%d height=%d format=%d mTransformHint=%d", width,
height, format, mTransformHint);
}构造函数还是有点东西的，但是不多。我们逐一拆解：
createBufferQueue()
cpp
//frameworks/native/libs/gui/BLASTBufferQueue.cpp
void BLASTBufferQueue::createBufferQueue(sp<IGraphicBufferProducer>* outProducer,
sp<IGraphicBufferConsumer>* outConsumer) {
LOG_ALWAYS_FATAL_IF(outProducer == nullptr, "BLASTBufferQueue: outProducer must not be NULL");
LOG_ALWAYS_FATAL_IF(outConsumer == nullptr, "BLASTBufferQueue: outConsumer must not be NULL");
sp<BufferQueueCore> core(new BufferQueueCore());
LOG_ALWAYS_FATAL_IF(core == nullptr, "BLASTBufferQueue: failed to create BufferQueueCore");
sp<IGraphicBufferProducer> producer(new BBQBufferQueueProducer(core));
LOG_ALWAYS_FATAL_IF(producer == nullptr,
"BLASTBufferQueue: failed to create BBQBufferQueueProducer");
sp<BufferQueueConsumer> consumer(new BufferQueueConsumer(core));
consumer->setAllowExtraAcquire(true);
LOG_ALWAYS_FATAL_IF(consumer == nullptr,
"BLASTBufferQueue: failed to create BufferQueueConsumer");
*outProducer = producer;
*outConsumer = consumer;
}很简单，
- 创建了一个BufferQueue，即BufferQueueCore。
- 基于BufferQueue创建了一个生产者，即BBQBufferQueueProducer。
- 基于BufferQueue创建了一个消费者，即BufferQueueConsumer。
BLASTBufferItemConsumer
cpp
//frameworks/native/include/gui/BLASTBufferQueue.h
BLASTBufferItemConsumer(const sp<IGraphicBufferConsumer>& consumer, uint64_t consumerUsage,
int bufferCount, bool controlledByApp)
: BufferItemConsumer(consumer, consumerUsage, bufferCount, controlledByApp),
mCurrentlyConnected(false),
mPreviouslyConnected(false),
mBLASTBufferQueue(nullptr) {}
//frameworks/native/libs/gui/BufferItemConsumer.cpp
BufferItemConsumer::BufferItemConsumer(
const sp<IGraphicBufferConsumer>& consumer, uint64_t consumerUsage,
int bufferCount, bool controlledByApp) :
ConsumerBase(consumer, controlledByApp)
{
status_t err = mConsumer->setConsumerUsageBits(consumerUsage);
LOG_ALWAYS_FATAL_IF(err != OK,
"Failed to set consumer usage bits to %#" PRIx64, consumerUsage);
if (bufferCount != DEFAULT_MAX_BUFFERS) {
err = mConsumer->setMaxAcquiredBufferCount(bufferCount);
LOG_ALWAYS_FATAL_IF(err != OK,
"Failed to set max acquired buffer count to %d", bufferCount);
}
}
//frameworks/native/libs/gui/ConsumerBase.cpp
ConsumerBase::ConsumerBase(const sp<IGraphicBufferConsumer>& bufferQueue, bool controlledByApp) :
mAbandoned(false),
mConsumer(bufferQueue),
mPrevFinalReleaseFence(Fence::NO_FENCE) {
// Choose a name using the PID and a process-unique ID.
mName = String8::format("unnamed-%d-%d", getpid(), createProcessUniqueId());
// Note that we can't create an sp<...>(this) in a ctor that will not keep a
// reference once the ctor ends, as that would cause the refcount of 'this'
// dropping to 0 at the end of the ctor. Since all we need is a wp<...>
// that's what we create.
wp<ConsumerListener> listener = static_cast<ConsumerListener*>(this);
sp<IConsumerListener> proxy = new BufferQueue::ProxyConsumerListener(listener);
status_t err = mConsumer->consumerConnect(proxy, controlledByApp);
if (err != NO_ERROR) {
CB_LOGE("ConsumerBase: error connecting to BufferQueue: %s (%d)",
strerror(-err), err);
} else {
mConsumer->setConsumerName(mName);
}
}可以看到三者之间的继承关系：BLASTBufferItemConsumer->BufferItemConsumer->ConsumerBase。在ConsumerBase构造函数内会创建一个ProxyConsumerListener，并且通过BufferQueueConsumer将其保存到BufferQueueCore内。
setFrameAvailableListener()
将BLASTBufferQueue本身设置到ConsumerBase的mFrameAvailableListener对象，当有新的帧到来时，它会被通知到。
setBufferFreedListener()
将BLASTBufferQueue本身设置到BufferItemConsumer的mBufferFreedListener对象，当有旧的buffer在被释放时，它会被通知到。
Transaction
SurfaceComposerClient::Transaction()设置Layer对应的属性到surfaceflinger，这里设置了layer_state_t和applyToken两种属性。下一篇文章将详细讲解Transaction的原理。
到此，BLASTBufferQueue创建完毕，接下来就是真正创建应用端surface的时候了。
BLASTBufferQueue::getSurface()
cpp
//frameworks/native/libs/gui/BLASTBufferQueue.cpp
sp<Surface> BLASTBufferQueue::getSurface(bool includeSurfaceControlHandle) {
std::unique_lock _lock{mMutex};
sp<IBinder> scHandle = nullptr;
if (includeSurfaceControlHandle && mSurfaceControl) {
scHandle = mSurfaceControl->getHandle();
}
return new BBQSurface(mProducer, true, scHandle, this);
}这里首先看mSurfaceControl->getHandle()，它的mSurfaceControl是指向当前SurfaceControl的子SurfaceControl的，所以用来获取子SurfaceControl的Handle。
cpp
//frameworks/native/libs/gui/SurfaceControl.cpp
sp<IBinder> SurfaceControl::getHandle() const {
if (mBbqChild != nullptr) {
return mBbqChild->getHandle();
}
return getLayerStateHandle();
}
sp<IBinder> SurfaceControl::getLayerStateHandle() const
{
return mHandle;
}看getHandle()的实现很明显的看出，这个函数永远获取子SurfaceControl所指向的mHandle。
最后是new一个BBQSurface，它继承自Surface：
cpp
//frameworks/native/libs/gui/BLASTBufferQueue.cpp
BBQSurface(const sp<IGraphicBufferProducer>& igbp, bool controlledByApp,
const sp<IBinder>& scHandle, const sp<BLASTBufferQueue>& bbq)
: Surface(igbp, controlledByApp, scHandle), mBbq(bbq) {}这里用BLASTBufferQueue本身、它的BBQBufferQueueProducer、Layer的Handle构建出了一个BBQSurface。
简单的画个BLASTBufferQueue的生产者消费者模型则如下：
总结
通过本节内容，应用程序搭建完成了生产者消费者模型，获取到了Surface接口端，下面就可以生产buffer送显了。但是在这之前还要设置surface的各项属性，通过Transaction来达成目的，这个在下节讲解，这里提一嘴。
老规矩，还是通过框架图的形式总结下本节的内容，注意这里没有再去画出来surfaceflinger内部生成的内容了，因为上篇文章已经展示过了。
