Signals and slots: mutual connections - How to avoid feedback?

[borrisso]

Greetings!

In my application I need a pair of widgets (to specify: dial and spin box) to work in a way, that changing value in one of them will automaticly cause value change in the other one. If I make these connections via Qt Designer (Edit Signals/Slots mode) it works, but unfortunatly I need special handling for values that are going to be set.

Here are these mutual connection, that I make in a constructor of my widget's class:
@
QObject::connect( ui->FreqDial, SIGNAL( valueChanged(int) ),
this, SLOT( SetFreqSpinBoxValue(int) ) );
QObject::connect( ui->FreqSpinBox, SIGNAL( valueChanged(int) ),
this, SLOT( SetFreqDialValue(int) ) );
@

And here's code of my functions:
@
void GeneratorWindow::SetFreqSpinBoxValue(int value)
{
double spinBoxValue = 0;

/* Calculations... */

ui->FreqSpinBox->setValue( static_cast<int>(spinBoxValue) );

}
@

@
void GeneratorWindow::SetFreqDialValue(int value)
{
double dialValue = 0;

/* Calculations... */

ui->FreqDial->setValue( static_cast<int>(dialValue) );

}
@

Of course it generates some kind of feedback - calling one of these functions emits a signal, the other functions is called, it emits it's own signal and that keeps going on forever. My question is how can I prevent that from happening?

Thank you for your answers,
Boris

[lgeyer]

You could block signal emission during the manual update.
@
void GeneratorWindow::SetFreqSpinBoxValue(int value)
{
double spinBoxValue = 0;

/* Calculations... */

bool signalsBlocked = ui->FreqSpinBox->blockSignals(true);
ui->FreqSpinBox->setValue( static_cast<int>(spinBoxValue) );
ui->FreqSpinBox->blockSignals(signalsBlocked);

}
@

[Rahul Das]

What kind of widgets are these two ?? I suppose, both are [[doc: QSpinBox]] . And i suppose there is something else causing the trouble. Because, i have used in criss cross way, and worked fine.

[borrisso]

Thank you very much, Lukas - it works perfectly :).

[mlong]

A good design principal in writing a setter is to make sure that the new value is different than the old value before doing any kind of assignment and signaling...

@
void SomeClass::someSetter(int value) {
if (value != the_current_value)
{
the_current_value = value;
emit theValueChanged(value);
}
}
@

This will prevent those kind of infinite loops without having to block signals.