status_t TransactionCallbackInvoker::registerPendingCallbackHandle(
        const sp<CallbackHandle>& handle) {
    std::lock_guard lock(mMutex);

    // If we can't find the transaction stats something has gone wrong. The client should call
    // startRegistration before trying to register a pending callback handle.
    TransactionStats* transactionStats;
    status_t err = findTransactionStats(handle->listener, handle->callbackIds, &transactionStats);
    if (err != NO_ERROR) {
        ALOGE("cannot find transaction stats");
        return err;
    }

    mPendingTransactions[handle->listener][handle->callbackIds]++;
    return NO_ERROR;
}

• 调用 TransactionCallbackInvoker::findTransactionStats() 从 mCompletedTransactions 查找 listener 和 callbackids 对应的 TransactionStats。
• mCompletedTransactions 是在 TransactionCallbackInvoker::startRegistration() 中写入的。
• 如果找到了，mPendingTransactions 中对应 listener 和 callbackIds 计数加一。