// Adds listener and callbackIds in case there are no SurfaceControls that are supposed
// to be included in the callback. This functions should be call before attempting to register
// any callback handles.
status_t TransactionCallbackInvoker::startRegistration(const ListenerCallbacks& listenerCallbacks) {
    std::lock_guard lock(mMutex);

    auto [itr, inserted] = mRegisteringTransactions.insert(listenerCallbacks);
    auto& [listener, callbackIds] = listenerCallbacks;

    if (inserted) {
        if (mCompletedTransactions.count(listener) == 0) {
            status_t err = listener->linkToDeath(mDeathRecipient);
            if (err != NO_ERROR) {
                ALOGE("cannot add callback because linkToDeath failed, err: %d", err);
                return err;
            }
        }
        auto& transactionStatsDeque = mCompletedTransactions[listener];
        transactionStatsDeque.emplace_back(callbackIds);
    }

    return NO_ERROR;
}

• mRegisteringTransactions 是一个 unordered_set。如果写入成功，则继续写入 mCompletedTransactions。
• mCompletedTransactions value 类型是 TransactionStats，它的构造函数接收 CallbackId 向量。
• 在 TransactionCallbackInvoker::endRegistration() 从 mRegisteringTransactions 移除。