Question

I’m trying to trigger a distinct signal from a QSlider for when it increases or decreases. Unfortunately due to restrictions in the program i’m creating outside of my control I cannot track the value of the slider and just do a comparison.

Is there a way to do this?

Answer 1

If I understand you correctly, you can’t use valueChanged(int) to decide if the value increased or decreased, since you can’t track the value. However, you need to do this, since QSlider doesn’t have distinct signals for an increased / decreased value.

Use this helper class to keep track of the current value. It remaps the signal valueChanged(int) of the provided target object to the new signals valueIncreased(int) and valueDecreased(int) telling the (absolute) difference.

class ValueChangeTracker : public QObject
{
    Q_OBJECT
    int v;
public:
    ValueChangeTracker(QObject *target) :
        QObject(target)
    {
        connect(target, SIGNAL(valueChanged(int)), SLOT(changed(int)));
        v = target->property("value"); // or ->value() if you specifically use QSlider* instead of QObject* as the target type
    }
signals:
    void valueIncreased(int);
    void valueDecreased(int);
private slots:
    void changed(int newValue) {
        int diff = newValue - v;
        v = newValue;
        if(diff > 0) emit valueIncreased(diff);
        if(diff < 0) emit valueDecreased(-diff);
    }
};

To use it, simply create a new instance of this class and connect to its signals. In the QObject tree, it becomes a child of the target object, so you don’t have to keep the pointer to an instance.

QSlider *mySlider = ...
ValueChangeTracker *tracker = new ValueChangeTracker(mySlider);
connect(tracker, SIGNAL(valueIncreased(int)), ...);
connect(tracker, SIGNAL(valueDecreased(int)), ...);