Qt Signals and Slots A Small Example
Example
Signals and slots are used for communication between objects. The signals and slots mechanism is a central feature of Qt and probably the part that differs most from the features provided by other frameworks.
The minimal example requires a class with one signal, one slot and one connection:
counter.h
#ifndef COUNTER_H
#define COUNTER_H
#include <QWidget>
#include <QDebug>
class Counter : public QWidget
{
/*
* All classes that contain signals or slots must mention Q_OBJECT
* at the top of their declaration.
* They must also derive (directly or indirectly) from QObject.
*/
Q_OBJECT
public:
Counter (QWidget *parent = 0): QWidget(parent)
{
m_value = 0;
/*
* The most important line: connect the signal to the slot.
*/
connect(this, &Counter::valueChanged, this, &Counter::printvalue);
}
void setValue(int value)
{
if (value != m_value) {
m_value = value;
/*
* The emit line emits the signal valueChanged() from
* the object, with the new value as argument.
*/
emit valueChanged(m_value);
}
}
public slots:
void printValue(int value)
{
qDebug() << "new value: " << value;
}
signals:
void valueChanged(int newValue);
private:
int m_value;
};
#endif
The main sets a new value. We can check how the slot is called, printing the value.
#include <QtGui>
#include "counter.h"
int main(int argc, char *argv[])
{
QApplication app(argc, argv);
Counter counter;
counter.setValue(10);
counter.show();
return app.exec();
}
Finally, our project file:
SOURCES = \
main.cpp
HEADERS = \
counter.h
