How to add a tick mark to a slider if it cannot inherit QSlider - qt

Question

I have a Qt dialog and there is a slider in it, when the dialog is initialized the slider will be set a value. In order to remind the user what is the default value, I want to add a mark to the slider, just draw a line or a triangle above the handle. Here, the slider should be of QSlider type, that means I can't implement a customized control derived from QSlider. Is there any way to realize it ?

Answer 1

I'm not clear why you can't derive a control from QSlider. You can still treat it like a QSlider, just override the paintEvent method. The example below is pretty cheesy, visually speaking, but you could use the methods from QStyle to make it look more natural:
#include <QtGui>
class DefaultValueSlider : public QSlider {
Q_OBJECT
public:
DefaultValueSlider(Qt::Orientation orientation, QWidget *parent = NULL)
: QSlider(orientation, parent),
default_value_(-1) {
connect(this, SIGNAL(valueChanged(int)), SLOT(VerifyDefaultValue(int)));
}
protected:
void paintEvent(QPaintEvent *ev) {
int position = QStyle::sliderPositionFromValue(minimum(),
maximum(),
default_value_,
width());
QPainter painter(this);
painter.drawLine(position, 0, position, height());
QSlider::paintEvent(ev);
}
private slots:
void VerifyDefaultValue(int value){
if (default_value_ == -1) {
default_value_ = value;
update();
}
}
private:
int default_value_;
};
int main(int argc, char *argv[]) {
QApplication app(argc, argv);
DefaultValueSlider *slider = new DefaultValueSlider(Qt::Horizontal);
slider->setValue(30);
QWidget *w = new QWidget;
QVBoxLayout *layout = new QVBoxLayout;
layout->addWidget(slider);
layout->addStretch(1);
w->setLayout(layout);
QMainWindow window;
window.setCentralWidget(w);
window.show();
return app.exec();
}
#include "main.moc"

Answer 2

Easiest way I can think off is:
Add QSlider to QSlider (like you do it with layouts and QFrames). Slider above will be your current slider (clickable one). Slider below will be your "default tick position" value.
#include <QApplication>
#include <QSlider>
#include <QVBoxLayout>
int main(int argc, char * argv[])
{
QApplication app(argc, argv);
QSlider * defaultValueSlider = new QSlider();
QSlider * valueSlider = new QSlider(defaultValueSlider);
QVBoxLayout * lay = new QVBoxLayout(defaultValueSlider);
lay->setContentsMargins(0, 0, 0, 0);
lay->setSpacing(0);
lay->addWidget(valueSlider);
defaultValueSlider->setRange(0, 100);
valueSlider->setRange(0, 100);
defaultValueSlider->setValue(30);
defaultValueSlider->show();
return app.exec();
}

Answer 3

Why do you need to inherit a QSlider to access its public methods?
http://doc.trolltech.com/4.7/qslider.html
You can just call its setTickPosition() in your app.