QTimer inside a for loop

n_moosavi

Dear all
I'm new to QT and face some problems with Qtimer.
I want to have a loop that in each iteration sends a packet with a serial port.
There is a problem with Qtimer inside the loop since it only works for the last iteration and just sends the last packet through serialport?
what can I do to fix it?
here is the for loop:
for(int i=1;i<5;i++)
{
send[2]=LightState(i);
connect(timer, SIGNAL(timeout()), this, SLOT(senddata()));
timer->setSingleShot(true);
timer->start(timestate[i]);
}
void Trafficlight::senddata()
{
if(serialport->isOpen()){
serialport->write(send);
qDebug() <<"sending data:"<<send.toHex();
}else{
qDebug() <<"Port is not open, data:" << send.toHex();
}
}

JonB

@n_moosavi
Hello and welcome.

You have a single timer instance, and you start(timestate[i]); it 4 times with a different timestate[i] each time. This will end up with just the one timer started on the final timestate[i] value.

It is hard to know what you are wanting to achieve here. Do you want 4 distinct QTimers for each of the timestate[i] values? Does each timer want to count its time (delay) from when the first one was started, or does each timestate value want to count/delay from when the previous one timed out?

n_moosavi

@JonB Thanks for your reply.
Actually i want a timer that for example after 1000s sends 1, then after 2000s from when the first one was started, sends2.
so the timestate is [1000, 2000,3000,4000] and the packet is [1 2 3 4].

JonB

@n_moosavi
So you can implement in one of two ways, as you choose:

Create 4 separate QTimer (singleshot) instances, all connected to same senddata() slot, each one with each of the timeout durations in your timestate array.
Create one QTimer (singleshot). Keep a member variable to count through timestate so you know where you have got to. On the timeout, apart from doing the sendata(), advance the counter, if it's not at end calculate the difference between the value in timestate for this time's value and the previous one, re-load the single QTimer instance with that (all would be 1000 given the values you presently show in timestate).

n_moosavi

@JonB For the first way. I tried as follows. I have created 3 separate QTimer:{timer,timerr,timerrr}
.But still, only the last timer works and sends data.

        timer = new QTimer(this);
        send=1;
        connect(timer, SIGNAL(timeout()), this, SLOT(senddata()));
        timer->setSingleShot(true);
        timer->start(timestate[0]);

        timerr = new QTimer(this);
        send=2;
        connect(timerr, SIGNAL(timeout()), this, SLOT(senddata()));
        timerr->setSingleShot(true);
        timerr->start(timestate[1]);

        timerrr = new QTimer(this);
        send=3;
        connect(timerrr, SIGNAL(timeout()), this, SLOT(senddata()));
        timerrr->setSingleShot(true);
        timerrr->start(timestate[2]);

JonB

@n_moosavi
These should work. Put qDebug() << "senddata()"; as the first statement in senddata() and see how many times it outputs.

mpergand

@n_moosavi

QTimer as a counter using dynamic property:

MainWindow::MainWindow(QWidget *parent)
    : QMainWindow(parent)
{
    auto* timer=new QTimer;
    timer->setProperty("count",5);
    connect(timer,&QTimer::timeout,this, &MainWindow::timeout);
    timer->start(1000);
}

void MainWindow::timeout()
{
    QTimer* timer=qobject_cast<QTimer*>(sender());
    if(timer==nullptr) return; // error wrong sender

    int count=timer->property("count").toInt()-1;
    timer->setProperty("count",count);

    qDebug()<<__FUNCTION__<<count;
    
    // call sendData() here

    if(count<=0)
        {
        timer->stop();
        delete timer;
        }
}

n_moosavi

@JonB Dear JohB.
I added the qDebug() << "senddata()"; to the senddata. Then i run
It outputs 3 times and each time with the last packet. It sends the last packet 3 times.
I tested it with switch/case but it still sends the last packet .
for (int i = 0; i < 4; i++)
{
switch(timestate[i]) {
case 1000:
send[2]=LightState(0);
connect(timer, SIGNAL(timeout()), this, SLOT(senddata()));
timer->setSingleShot(true);
timer->start(timestate[0]);
case 2000:
send[2]=LightState(1);
connect(timerr, SIGNAL(timeout()), this, SLOT(senddata()));
timerr->setSingleShot(true);
timerr->start(timestate[1]);
case 3000:
send[2]=LightState(2);
connect(timerrr, SIGNAL(timeout()), this, SLOT(senddata()));
timerrr->setSingleShot(true);
timerrr->start(timestate[2]);
}
}

Kent-Dorfman

there really needs to be a "required reading" section for Qt, before people ask questions. As we see again, trying to use sequential/procedural programming instead of the event driven model.

staggering timer events only needs a single timer implementation. in the callback from Qtimer(1000ms)::TimerExpired() execute a switch sstatement based on a static var that is incremented every time the timer expires. in the "case" segments put your light (on/off) logic.

JonB

@n_moosavi
So you know your senddata() slot is being hit 3 times, which was the objective of the exercise. What you do in senddata() is up to you, you seem to write(send);. And that will be in whatever state you left it after your for ... switch loop, which perhaps means send[2]=LightState(2);, and that is what you are complaining about.

Since the loop runs when you are setting the timers up, not when the timers expire, if you change send[] there that will affect when all the timers expire, which is perhaps not what you want. So you need to alter code in that light.

n_moosavi

@Kent-Dorfman thanks for your help, I'm new to QT, and I know that I should work harder.

n_moosavi

@mpergand thanks for your help

n_moosavi

@JonB thanks for your help