How to get the value of a spin box just before it changes

Question

Using Qt, I need to get the value of a spin box (by a signal) just before it changes.

I've got:

QSpinBox spinBoxWidth:
QSpinBox spinBoxScale;

I want to connect a signal from spinBoxWidth to spinBoxScale, so that the value of spinBoxScale is always "the Value of spinBoxWidth after changing", to "its value before changing".

Scale = width_new / width_old

I didn't find any slot in Qt which returns the old value of a spin box while changing the value. Can I somehow write a slot for that?

Answer 1

There are two ways of doing this:

• Catch the change before it happens and store the old value using the event system (QKeyEvent, QMouseEvent). This is error-prone, as the value of spinBoxWidth can be set manually.
• Connect spinBoxWidth's valueChanged(int) signal to a slot and reference the last value it was called with. I recommend this method.

Try something like this:

class MonitoringObject : public QObject
{
    Q_OBJECT
    int lastValue;
    int currentValue;
    ...
public Q_SLOTS:
    void onValueChanged(int newVal)
    {
        lastValue = currentValue;
        currentValue = newVal;
        if (lastValue == 0) //catch divide-by-zero
            emit ratioChanged(0);
        else
            emit ratioChanged(currentValue/lastValue);
    }
Q_SIGNALS:
    void ratioChanged(int);

After your signals are connected, the flow should look like this:

• spinBoxWidth emits valueChanged(int)
• MonitoringObject::onValueChanged(int) is invoked, does its work and emits ratioChanged(int)
• spinBoxScale receives the signal in its setValue(int) slot and sets the appropriate value.