status_t SurfaceFlinger::setTransactionState(
        const FrameTimelineInfo& frameTimelineInfo, const Vector<ComposerState>& states,
        const Vector<DisplayState>& displays, uint32_t flags, const sp<IBinder>& applyToken,
        const InputWindowCommands& inputWindowCommands, int64_t desiredPresentTime,
        bool isAutoTimestamp, const client_cache_t& uncacheBuffer, bool hasListenerCallbacks,
        const std::vector<ListenerCallbacks>& listenerCallbacks, uint64_t transactionId) {
    uint32_t permissions =
            callingThreadHasUnscopedSurfaceFlingerAccess() ? Permission::ACCESS_SURFACE_FLINGER : 0;
    ...
    constint64_t postTime = systemTime();

    IPCThreadState* ipc = IPCThreadState::self();
    constint originPid = ipc->getCallingPid();
    constint originUid = ipc->getCallingUid();
    TransactionState state{frameTimelineInfo,  states,
                           displays,           flags,
                           applyToken,         inputWindowCommands,
                           desiredPresentTime, isAutoTimestamp,
                           uncacheBuffer,      postTime,
                           permissions,        hasListenerCallbacks,
                           listenerCallbacks,  originPid,
                           originUid,          transactionId};
    ...
    queueTransaction(state);

    // Check the pending state to make sure the transaction is synchronous.
    if (state.transactionCommittedSignal) {
        waitForSynchronousTransaction(*state.transactionCommittedSignal);
    }

    return NO_ERROR;
}

• 新建 TransactionState 结构体。

• 调用 SurfaceFlinger::queueTransaction() 把 TransactionState 加入队列。

• 在 SurfaceFlinger::queueTransaction() 中判断是否是同步事务，如果是同步事务则等待完成，最多 5 秒。

  void SurfaceFlinger::queueTransaction(TransactionState& state) {
      ...
      // Generate a CountDownLatch pending state if this is a synchronous transaction.
      if ((state.flags & eSynchronous) || state.inputWindowCommands.syncInputWindows) {
          state.transactionCommittedSignal = std::make_shared<CountDownLatch>(
                  (state.inputWindowCommands.syncInputWindows
                           ? (CountDownLatch::eSyncInputWindows | CountDownLatch::eSyncTransaction)
                           : CountDownLatch::eSyncTransaction));
      }