Qt/C++ - Lesson 026. Using CallBack function

callback функция, callback, Qt урок, callback c++, Qt callback

To transfer data in Qt uses signals and slots system, but this does not mean that you can not use the old proven method, namely the use CallBack functions. The fact that the use CallBack function is somewhat fast option than signals and slots. And also can be easier to use with regard to the fact that it is desirable to disconnect signals from the slots when an object sends a program signal is destroyed and is no longer used. This point is particularly relevant when one considers that, in C ++ there is no garbage collection, like Java or C# .

The principle of CallBack

The principle of CallBack To use CallBack functions in a class that will have to return a result, you must declare a pointer to a function with the same signature as the function that will be used as a CallBack function. But to place the pointer on the function you want to use a class method to apply this pointer. That is, this method is passed a pointer to a function, which is set in the CallBack class pointer, which will return the result of its activities. At the same time in this class, this pointer is used as an ordinary function that will perform the action specified in the class from which this feature has been installed as a CallBack function in the current class.

For example, the class will be used, which draws a square on the graphic scene and Control keys W, A, S, D. When moving square should send data on their coordinates in the class that was created. That is, should call this class function as its CallBack function.

Project Structure

Project Structure For a work function CallBack use project with the following structure:

• mainwindow.h - The header of the main window;
• mainwindow.cpp - source files of the main application window;
• square.h - Header class file, the object of which is to use the CallBack function.
• square.cpp - file source code of the class;

mainwindow.ui

In the main window in the designer throws a graphic scene , and objects QLineEdit class, which will display the coordinates manually create and set in this window. Since the data objects must be declared as static. The same condition should apply for CallBack function. It must also be declared as static .

mainwindow.h

Also, in the header of the main window to declare the object class Square.

#ifndef MAINWINDOW_H
#define MAINWINDOW_H
 
#include <QMainWindow>
#include <QGraphicsScene>
#include <QLineEdit>
 
#include <square.h>
 
namespace Ui {
class MainWindow;
}
 
class MainWindow : public QMainWindow
{
    Q_OBJECT
 
public:
    explicit MainWindow(QWidget *parent = 0);
    ~MainWindow();
 
private:
    Ui::MainWindow *ui;
    QGraphicsScene *scene;  
    Square *square;         // We declare the square, which will transmit the callback function
    static QLineEdit *line1;    // We declare a static QLineEdit, which will run the callback function
    static QLineEdit *line2;    // We declare a static QLineEdit, which will run the callback function
 
private:
    // Declare a callback function
    static void getPosition(QPointF point);
};
 
#endif // MAINWINDOW_H

mainwindow.cpp

In addition to static objects QLineEdit their ads more necessary and implemented as functions in the source file, otherwise the compiler will announce an error. The fact that static objects necessarily need to be initialized.

#include "mainwindow.h"
#include "ui_mainwindow.h"
 
MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow)
{
    ui->setupUi(this);
 
    // Инициализируем объекты QLineEdit
    line1 = new QLineEdit();
    line2 = new QLineEdit();
 
    // Устанавлвиваем их в gridLayout
    ui->gridLayout->addWidget(line1,0,1);
    ui->gridLayout->addWidget(line2,0,2);
 
    scene = new QGraphicsScene();       // Init graphics scene
    ui->graphicsView->setScene(scene);  // Set scene into the graphicsView
    scene->setSceneRect(0,0,300,300);   
    square = new Square();              
    square->setCallbackFunc(getPosition);   // Set callback function into th square
    square->setPos(100,100);            
    scene->addItem(square);             
}
 
MainWindow::~MainWindow()
{
    delete ui;
}
 
/* callback function receives a square position and places it in a position line1 and line2
 * */
void MainWindow::getPosition(QPointF point)
{
    line1->setText(QString::number(point.x()));
    line2->setText(QString::number(point.y()));
}
 
QLineEdit * MainWindow::line1;
QLineEdit * MainWindow::line2;

square.h

This class is inherited from QGraphicsItem and it is declared a pointer for the CallBack function and function for its installation. The function return values must be specified without fail.

#ifndef SQUARE_H
#define SQUARE_H
 
#include <QObject>
#include <QGraphicsItem>
#include <QPainter>
#include <QTimer>
#include <QPointF>
 
class Square : public QObject, public QGraphicsItem
{
    Q_OBJECT
public:
    explicit Square(QObject *parent = 0);
    ~Square();
    // Function for setting of callback function
    void setCallbackFunc(void (*func) (QPointF point));
 
signals:
 
public slots:
 
protected:
    QRectF boundingRect() const;
    void paint(QPainter *painter, const QStyleOptionGraphicsItem *option, QWidget *widget);
 
private:
    QTimer *timer;  
    // Pointer to callback function
    void (*callbackFunc)(QPointF point);
 
private slots:
    void slotTimer();   
};
 
#endif // SQUARE_H

square.cpp

This example is similar in mechanics to the game, so no wonder the presence of the game timer. In the slot, connected to the signal from timer we implement movement square in the graphic scene and to transfer its coordinates in the CallBack function. And to check the status of target use functional buttons WinAPI .

#include "square.h"
#include <windows.h>
 
Square::Square(QObject *parent) :
    QObject(parent), QGraphicsItem()
{
    timer = new QTimer();
    connect(timer, &QTimer::timeout, this, &Square::slotTimer);
    timer->start(1000/33);
}
 
Square::~Square()
{
 
}
 
QRectF Square::boundingRect() const
{
    return QRectF(-15,-15,30,30);
}
 
void Square::paint(QPainter *painter, const QStyleOptionGraphicsItem *option, QWidget *widget)
{
    painter->setPen(Qt::black);
    painter->setBrush(Qt::green);
    painter->drawRect(-15,-15,30,30);
 
    Q_UNUSED(option);
    Q_UNUSED(widget);
}
 
void Square::slotTimer()
{
    // Depending on the button pressed move the square around the stage
    if(GetAsyncKeyState('A')){
        this->setX(this->x() - 2);
    }
    if(GetAsyncKeyState('D')){
        this->setX(this->x() + 2);
    }
    if(GetAsyncKeyState('W')){
        this->setY(this->y() - 2);
    }
    if(GetAsyncKeyState('S')){
        this->setY(this->y() + 2);
    }
    // Call the callback function to transmit the coordinates of a square
    callbackFunc(this->pos());
}
 
void Square::setCallbackFunc(void (*func)(QPointF point))
{
    // Set the pointer to the callback function
    callbackFunc = func;
}

Result

As a result, you should get the app, which made management a green square, and through the CallBack feature available data on its coordinates in the class of the main window.

Video