CallbackId TransactionCompletedListener::addCallbackFunction(
        const TransactionCompletedCallback& callbackFunction,
        const std::unordered_set<sp<SurfaceControl>, SurfaceComposerClient::SCHash>&
                surfaceControls,
        CallbackId::Type callbackType) {
    std::lock_guard<std::mutex> lock(mMutex);
    startListeningLocked();

    CallbackId callbackId(getNextIdLocked(), callbackType);
    mCallbacks[callbackId].callbackFunction = callbackFunction;
    auto& callbackSurfaceControls = mCallbacks[callbackId].surfaceControls;

    for (const auto& surfaceControl : surfaceControls) {
        callbackSurfaceControls[surfaceControl->getHandle()] = surfaceControl;
    }

    return callbackId;
}

• TransactionCompletedCallback 是函数类型。

  using TransactionCompletedCallback =
          std::function<void(nsecs_t /*latchTime*/, const sp<Fence>& /*presentFence*/,
                             const std::vector<SurfaceControlStats>& /*stats*/)>;
  

• TransactionCompletedListener::startListeningLocked() 开启 binder 线程。

• 新建一个 CallbackId，调用 TransactionCompletedListener::getNextIdLocked() 分配一个 id。

• callbackId 加入 TransactionCompletedListener::mCallbacks，同时 callbackFunction 和 surfaceControls 存入 value，类型是 TransactionCompletedListener::CallbackTranslation。

TransactionCompletedListener 是一个进程一个对象，mCallbacks 中存着 CallbackId。当 SurfaceFlinger 回调 ITransactionCompletedListener 接口时，通过 CallbackId 来找到 callbackFunction。例如，对于 BLASTBufferQueue::acquireNextBufferLocked() 中调用 Transaction::setBuffer() 的事务，callbackFunction 是 transactionCallbackThunk()。