Use of QSpinBox::blockSignals. How to avoid infinite loop with valueChanged.

[Petross404_Petros S]

I leave this in case someone finds this useful.

Let's say I have a spinBox which is able to set directly an image displayed on a QLabel with valueChanged* signal. The same spinBox is updated ( -> it emits valueChanged again ) every time the image is changed from a pushButton with ::setValue(int)** member function.

• One way to alter this spinBox, is to interact directly it's arrows to control the image through image_update

//In the ctor of QtDice
connect(m_ui->spinBox, QOverload<int>::of(&QSpinBox::valueChanged),
    this, static_cast<void (QtDice::*)(int)>(&QtDice::reload));
//reload calls image_update

• In here the spinBox is just updated with image_number; no direct user interaction

//In a member function of QtDice ( void image_update(int) )
m_ui->spinBox->setValue(image_number);
//This triggers valueChanged signal leading valueChanged to call this again...

As I noticed each time the spinBox was altered, the image_update(int) was called and each time image_update(int) was called the spinBox was altered leading the program in an endless loop. So I had to found a way to instruct the spinBox to emit valueChanged only if direct user interaction with it took place.

But how do you instruct a spinBox to emit (kind of) conditionally ? As Chernobyl of StackOverflow pointed out, that's the way to go :

....
m_ui->spinBox->blockSignals(true); //Shush, block your signals
m_ui->spinBox->setValue(image_number);
m_ui->spinBox->blockSignals(false); //OK, emit from now on
...

That's all, I hope this will be useful to someone one day. Of course other more experienced users may add their their solutions.

[SGaist]

Hi,

To avoid signals storms you usually have something like this workflow:

void setMyProperty(int value)
{
    if (value == _myPropertyValue) {
        return;
    }
    _myPropertyValue = value;
    emit myPropertyChanged(value);
}

[Petross404_Petros S]

@SGaist QSpinBox emits automatically, so why should I do it again?

[SGaist]

Doing so ensure the proper behaviour. If you connect anything that doesn't do it, you'll still avoid the signal storm.