Timer with Lambda function

[SPlatten]

I am sending JSON messages between processes and I want start a timer when the message is sent, if no acknowledge is received then the timer will timeout and the same message will be written again, at least that's the intention.

I have a structure:

typedef struct {
    QJsonObject objMsg;
    QTimer* pTimer;
} tAckMsgTracking;

typedef std::map<qulonglong, tAckMsgTracking*> mmpAck;

The map is keyed by a message transaction ID which is uniquely generated when the message is sent. The problem I'm having is that the lambda slot does not get called:

tAckMsgTracking* pAckMsgTrkr = new tAckMsgTracking;
pAckMsgTrkr->objMsg = objJSON;
pAckMsgTrkr->pTimer = new QTimer(this);
QObject::connect(pAckMsgTrkr->pTimer, &QTimer::timeout, [this, pAckMsgTrkr]() {
//Re-send message
    emit write(pAckMsgTrkr->objMsg);
});
//Insert a unique message ID into the message
objJSON.insert(clsJSON::mscszMsgID, QString::number(++clsMsgSender::msulnglngMsgID));
//Insert entry into the map
clsMsgSender::msmpAcks.insert(std::make_pair(clsMsgSender::msulnglngMsgID, pAckMsgTrkr));
//Start the timer that will resend message if ack. not received
pAckMsgTrkr->pTimer->start(clsMsgSender::mscuint16AckTimeout);

The value of 'mscuint16AckTimeout' is 5000 (milliseconds). I've also tried:

pAckMsgTrkr->pTimer->start(std::chrono::milliseconds(clsMsgSender::mscuint16AckTimeout));

I have a break point in the lambda slot, it doesn't get hit.

[jsulm]

@SPlatten said in Timer with Lambda function:

pAckMsgTrkr->pTimer = new QTimer(this);

Not related, but: why do you allocate timer on the heap?

So, you don't get acknowledge and timeout slot is not called?

[SPlatten]

@jsulm, I create a timeout for each message that is sent as the queue doesn't wait one message to be sent successfully, before it sends another. So the timeout is for each message sent, the same queue can send to different hosts.

[jsulm]

@SPlatten I still don't see the need for heap allocation for pTimer...

[SPlatten]

@jsulm , what else would you suggest? I don't want to static allocate and I cannot use the stack.

[jsulm]

@SPlatten said in Timer with Lambda function:

what else would you suggest?

Make it member instead of a pointer? What's the point to have it as pointer allocate memory on the heap (which is slower) and then care to not to forget to delete it?

[JonB]

@SPlatten
Since the code looks OK (other than heap issue), try a qDebug() instead of relying on breakpoint, and move the timeout down to 0 to debug.

[KroMignon]

@SPlatten said in Timer with Lambda function:

I have a break point in the lambda slot, it doesn't get hit.

There are only 2 things why slot is not called:

• the thread in which the QTimer in living does not have a running event loop
• the event loop is locked (by a QThread::sleep() or a forever loop)

[SPlatten]

I've re-written the code instead of a structure:

    class clsMsgTrkr : QTimer {
    private:
        static const quint16 mscuint16AckTimeout = 5000;
        static mmpAck msmpAcks;

        QJsonObject mobjMsg;
        clsMsgSender* mpMsgSndr;

    public:
        clsMsgTrkr(clsMsgSender* pMsgSndr, const QJsonObject& crobjJSON) {
            setInterval(mscuint16AckTimeout);
            mobjMsg = crobjJSON;
            mpMsgSndr = pMsgSndr;
    //Add tracker to list
            clsMsgTrkr::msmpAcks.insert(std::make_pair(clsMsgSender::ulnglngGetMsgID(), this));

            QObject::connect(this, &QTimer::timeout, [this]() {
                qdbg() << "TIMEOUT!";
                //Re-send message
                emit mpMsgSndr->write(mobjMsg);
            });
            start();
        }
    };

I can see in the debugger the constructor is getting called and processed the timer is started but I don't get anything in the slot.

[jsulm]

@SPlatten Don't know why you are now subclassing QTimer...
You can add a destrcutor with debug output to see whether the instance is destroyed before timeout occurs.
Also check what others wrote.

[JonB]

@SPlatten
As @jsulm has just said. And as I suggested, put timeout down to 0 while you debug....

[SPlatten]

I added a call after call:

start();
qdbg() << this->isActive();

qdbg is just a macro I use which is:

#define qdbg()      qDebug().noquote().nospace()

In the Application Output I see true so the timer is active, but doesn't timeout. Also, I just set the timer interval to 0, still no change, no timeout signal occurs.

[KroMignon]

@SPlatten said in Timer with Lambda function:

In the Application Output I see true so the timer is active, but doesn't timeout. Also, I just set the timer interval to 0, still no change, no timeout signal occurs.

Do you have a forever loop in your code or do you use QThread::sleep()?
Are you sure QEventLoop of the used thread is working?

[SPlatten]

@KroMignon, the message transmission is in a thread and there very short sleep in the thread loop:

void clsMsgSender::run() {    
    QJsonObject objJSON;
    while( blnAnythingToDo(objJSON) == true ) {
    //Sleep to allow a small cap between transmission
        QThread::usleep(100);
    //Look for a module name in the message
        QJsonObject::iterator itrFound = objJSON.find(clsJSON::mscszMsgType);

        if ( itrFound != objJSON.end() ) {
            const QJsonValueRef crobjMsgType = itrFound.value();
            QString strMsgType(crobjMsgType.toString());

            if ( strMsgType.compare(clsJSON::mscszAck) != 0 ) {
    //Insert a unique message ID into the message
                objJSON.insert(clsJSON::mscszMsgID, QString::number(++clsMsgSender::msulnglngMsgID));
    //Create entry to monitor status of this message
                new clsMsgTrkr(this, objJSON);
            }
        }
    //Writes message to socket
        emit write(objJSON);
    }
    emit queueEmpty();
}

[jsulm]

@SPlatten You expect this to work?

[SPlatten]

@jsulm , ok, perhaps I'm to close to see the obvious, what have I done wrong? Its not complete yet, I haven't finished the code yet, but is there something that would explain why the timer isn't working?

[KroMignon]

@SPlatten I've told you many times to read basic Qt documentation.

this cannot work!!!

First, I suppose clsMsgSender is subclassing QThread. And you have create your own run() implementation. So there is no running QEventLoop. This means, all QObject which are running in this thread can NOT receive/emit signals.

Second, you have a forever loop, so even if there where a running QEventLoop, it will not be called!

[SPlatten]

@KroMignon , I'm impatient, I know there is benefit from reading the documentation, I just don't want to stop what I'm doing to spend what would seem a long time to digest the documentation.

[KroMignon]

@SPlatten said in Timer with Lambda function:

I'm impatient,

Perhaps you are impatient, but in fact you are losing days doing nonsense code which not working.
If that is the best way to work, I am pretty sure NO.

Reading this document would take you 1 or 2 hours, how many hours have you spend to create those non working code?

[jsulm]

@SPlatten said in Timer with Lambda function:

I just don't want to stop what I'm doing to spend what would seem a long time to digest the documentation

So, instead you waste your time here?
I really don't get the logic...