In Qt how do I get a button press to set a spinbox to a certain value?
-
03-07-2019 - |
Question
I'm trying to get to grips with Qt's signal and slots mechanism. I have an app with a QPushButton and a QSpinBox. When I click the button I want the spinbox to change to 20. What signal and slot do I need to set up?
The code below shows the app, the connect function is the one I am having trouble with. As I understand it the setValue(int) slot of QSpinBox will not work here because the clicked() signal of QPushButton has a different signature, and anyway how would I pass the value 20 to the spinbox? Do I need to write some sort of auxiliary function to act as a slot which calls spinbox->setValue(20)? If so, what form would that take?
int main(int argc, char *argv[])
{
QApplication app(argc, argv);
QWidget *window = new QWidget;
QSpinBox *spinbox = new QSpinBox;
QPushButton *button = new QPushButton("Set to 20");
spinbox->setRange(0, 100);
// What should I put below?
QObject::connect(button, SIGNAL(clicked()), spinbox, SLOT(???????));
QLayout *layout = new QHBoxLayout;
layout->addWidget(spinbox);
layout->addWidget(button);
window->setLayout(layout);
window->show();
return app.exec();
}
Solution
You can either do:
class AuxSignals : public QObject
{
Q_OBJECT
...
signals:
void valueChanged(int);
public slots:
void buttonClicked() { emit valueChanged(20); }
};
...
// On main.cpp
AuxSignals *auxSignals = new AuxSignals;
QObject::connect(button, SIGNAL(clicked()), auxSignal, SLOT(buttonClicked));
QObject::connect(auxSignals, SIGNAL(valueChanged(int)), spinbox, SLOT(setValue(int)));
or
class AuxSignals : public QObject
{
Q_OBJECT
...
QSpinBox *m_spinBox;
public:
AuxSignals(QSpinBox *spinBox) : m_spinBox(spinBox) {}
public slots:
void buttonClicked() { m_spinBox->setValue(20); }
};
...
// On main.cpp
AuxSignals *auxSignals = new AuxSignals(spinBox);
QObject::connect(button, SIGNAL(clicked()), auxSignals, SLOT(buttonClicked()));
I prefer the first option because it doesn't require the AuxSignals class to have a pointer to a specific QWidget.
OTHER TIPS
I think you are looking at a custom SLOT here. A QPushButton::clicked
signal will give a boolean (true|false
) event. If you catch it using QSpinBox::setValue
you won't go very far. The QSpinBox::setValue
expects an int and converts the input boolean to 0 or 1 as the case maybe and your spinbox increments by only 1 unit. If you were to write a custom SLOT
you can actually set the exact slider value with far more control.