void SurfaceFlinger::queueTransaction(TransactionState& state) {
    Mutex::Autolock _l(mQueueLock);

    // If its TransactionQueue already has a pending TransactionState or if it is pending
    auto itr = mPendingTransactionQueues.find(state.applyToken);
    // if this is an animation frame, wait until prior animation frame has
    // been applied by SF
    if (state.flags & eAnimation) {
        while (itr != mPendingTransactionQueues.end()) {
            status_t err = mTransactionQueueCV.waitRelative(mQueueLock, s2ns(5));
            if (CC_UNLIKELY(err != NO_ERROR)) {
                ALOGW_IF(err == TIMED_OUT,
                         "setTransactionState timed out "
                         "waiting for animation frame to apply");
                break;
            }
            itr = mPendingTransactionQueues.find(state.applyToken);
        }
    }

如果当前是一个动画帧且该进程有未完成的事务，则等待完成，每个事务最多等 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));
    }

    mTransactionQueue.emplace(state);
    ATRACE_INT("TransactionQueue", mTransactionQueue.size());

    const auto schedule = [](uint32_t flags) {
        if (flags & eEarlyWakeupEnd) return TransactionSchedule::EarlyEnd;
        if (flags & eEarlyWakeupStart) return TransactionSchedule::EarlyStart;
        return TransactionSchedule::Late;
    }(state.flags);

    setTransactionFlags(eTransactionFlushNeeded, schedule, state.applyToken);
}