SurfaceFlinger Transaction机制原理

概念：

Transaction机制作用是把App的window属性传递给display（保存在Surfaceflinger.layer的mDrawingState中,可以保证原子性）。例如：把App的TransactionCompletedListener等属性传递给的SurfaceFlinger（createLayer过程调用SurfaceComposerClient::apply方法可以实现）

TransactionCompletedListener保存到mCompletedTransactions中。然后在surfaceflinger在handleMessageInvalidate()中处理完transaction后调用TransactionCallbackInvoker的sendCallbacks()去回调TransactionCompletedListener的onTransactionCompleted()函数，通知blastbufferqueue release掉buffer

创建layer transaction流程：

SurfaceFlinger::createLayer(

SurfaceFlinger::addClientLayer

SurfaceFlinger::setTransactionState(->SurfaceFlinger::queueTransaction(state)

等待vsync刷新回调

SurfaceFlinger::onMessageInvalidate

{

   refreshNeeded = handleMessageTransaction()

//flushTransactionQueues()+commit的目的是更新绘制状态mCurrentState->mDrawin