Qt-2-signal and slot

Signal and slot

Signals and slots are Qt stay C++ New features based on , Be similar to Java、C# Callback mechanism , But it is more convenient to use , It is easy to make passive function calls between different components , It's a communication mechanism .

A signal is a function

Slots are also a function

To use signal functions and slot functions , There are two prerequisites ：

• The object of communication must be from QObject derived

• Class must be added O_OBJECT macro

The function prototype

QObject::connect(const QObject * sender,

const char * signal,

const QObject * receiver,

const char * method) [static]

Parameter one ： Launcher , Is the calling object of the signal function

Parameter two ： Signal function , It is a function automatically triggered by the system under certain conditions , Put it on the outside SIGNAL()

Parameter 3 ： The receiver , Is the object that executes the slot function .

Parameter 4 ： Slot function , A function to be executed when a signal is transmitted , Put it on the outside SLOT()

The receiver binds the sender's signal , Once the transmitter sends this signal , The receiver automatically executes the slot function .

The signal slot can be connected , You can also disconnect , After disconnection, the previously connected signal slot will not take effect . There are two common ways to disconnect .

• Automatic disconnection

When the sender or receiver object is destroyed , The signal slot will also be automatically disconnected .

• Manual disconnection

have access to disconnect Function to disconnect the previously connected signal slot , Its parameters and connect equally .

Three binding methods

The following is an explanation of three signal slot binding methods .

1. Own signal → With groove

Regardless of the signal function , Or slot function , You don't need to write it yourself , Just look up the documentation and find out .

【 Example 】 Click button , close window .

Launcher ： Button object

Signal function ：clicked

The receiver ： Window object

Slot function ：close

dialog.h

#ifndef DIALOG_H
#define DIALOG_H

#include <QDialog>
#include <QPushButton>

class Dialog : public QDialog
{
    Q_OBJECT

public:
    Dialog(QWidget *parent = 0);
    ~Dialog();

private:
    QPushButton *btn;
};

#endif // DIALOG_H

dialog.cpp

#include "dialog.h"

Dialog::Dialog(QWidget *parent)
    : QDialog(parent)
{
    btn = new QPushButton(" close ",this);
    btn->move(200,200);

    //  Click button , close window 
    connect(btn,SIGNAL(clicked()),this,SLOT(close()));
    //  disconnect 
//    disconnect(btn,SIGNAL(clicked()),this,SLOT(close()));
}

Dialog::~Dialog()
{
    delete btn;
}

2. Own signal → Custom slot function

The signal function needs to consult the documentation to find out , Slot functions need to be written by yourself .

Slot function is a special member function , Just add... To the declaration slots Key words can be used .

【 Example 】 Click button , Move the window and print out its current coordinates .

Launcher ： Button object

Signal function ：clicked

The receiver ： Custom window class object

Slot function ： Custom slot function

dialog.h

#ifndef DIALOG_H
#define DIALOG_H

#include <QDialog>
#include <QPushButton>
#include <QDebug>

class Dialog : public QDialog
{
    Q_OBJECT

public:
    Dialog(QWidget *parent = 0);
    ~Dialog();

private:
    QPushButton *btn;

//  Private slot function declaration 
private slots:
    void mySlot();
};

#endif // DIALOG_H

dialog.cpp

#include "dialog.h"

Dialog::Dialog(QWidget *parent)
    : QDialog(parent)
{
    resize(200,200);
    btn = new QPushButton(" Trigger ",this);
    btn->move(100,100);

    connect(btn,SIGNAL(clicked()),this,SLOT(mySlot()));
}

void Dialog::mySlot()
{
    // Get the current coordinates first 
    int x = this->x();
    int y = this->y();
    // Lower right 
    x += 10;
    y += 10;
    //  Move 
    move(x,y);
    qDebug() << x << "," << y;
}

Dialog::~Dialog()
{
    delete btn;
}

3. Custom signal → Custom slot function / With slot function

The signal function needs to be written by yourself , Slot functions can be written and viewed by themselves .

Be careful , Signal functions are not authorized , And only the declaration has no definition .

【 Example 】 Click button , Trigger custom slot function 1, In the custom slot function 1 Transmit custom signal in ; Use a custom signal to trigger a custom slot function 2.

Click button → Slot function 1（ Sending custom signals ）

Custom signal → Slot function 2

dialog.h

#ifndef DIALOG_H
#define DIALOG_H

#include <QDialog>
#include <QPushButton>
#include <QDebug>

class Dialog : public QDialog
{
    Q_OBJECT

public:
    Dialog(QWidget *parent = 0);
    ~Dialog();

private:
    QPushButton *btn;

private slots:
    //  Slot function 1
    void mySlot1();
    void mySlot2();

    //  Custom signal function 
signals:
    void mySignal();
};

#endif // DIALOG_H

dialog.cpp

#include "dialog.h"

Dialog::Dialog(QWidget *parent)
    : QDialog(parent)
{
    btn = new QPushButton(" Sending custom signals ",this);
    btn->move(200,200);

    //  Click button , Sending custom signals 
    connect(btn,SIGNAL(clicked()),this,SLOT(mySlot1()));
    connect(this,SIGNAL(mySignal()),this,SLOT(mySlot2()));
}

void Dialog::mySlot1()
{
    //  Sending custom signals 
    emit mySignal();
}

void Dialog::mySlot2()
{
    qDebug() << " Received custom signal ！";
}

Dialog::~Dialog()
{
    delete btn;
}

Signal slot transmission parameters

In addition to the basic communication function, the signal slot , You can also carry data for transmission .

dialog.h

#ifndef DIALOG_H
#define DIALOG_H

#include <QDialog>
#include <QPushButton>

class Dialog : public QDialog
{
    Q_OBJECT

public:
    Dialog(QWidget *parent = 0);
    ~Dialog();

private:
    QPushButton *btn;

private slots:
    //  Custom slot function 1
    void mySlot1();
    //  Custom slot function 2： receive int Parameters 
    void mySlot2(int);

signals:
    //  Parameters are carried data , Can be emitted to the slot function 
    void mySignal(int);
};

#endif // DIALOG_H

dialog.cpp

#include "dialog.h"

Dialog::Dialog(QWidget *parent)
    : QDialog(parent)
{
    resize(400,400);
    btn = new QPushButton("0",this);
    connect(btn,SIGNAL(clicked()),this,SLOT(mySlot1()));
    connect(this,SIGNAL(mySignal(int)),this,SLOT(mySlot2(int)));
}

void Dialog::mySlot1()
{
    //  Count 
    static int count = 0;
    count++;
    //  Send out the number of times through the signal function 
    emit mySignal(count);
}

void Dialog::mySlot2(int count)
{
    // QString QString::number(int n, int base = 10) [static]
    QString text = QString::number(count);
    //  Display the times on the window title 
    setWindowTitle(text);
}

Dialog::~Dialog()
{
    delete btn;
}

matters needing attention ：

1. You can pass any number of parameters

2. Parameter type must match

3. The number of parameters of the signal can be greater than or equal to the number of parameters of the slot function

If the above code is written according to the normal development method , It can be optimized to ：

dialog.h

#ifndef DIALOG_H
#define DIALOG_H

#include <QDialog>
#include <QPushButton>

class Dialog : public QDialog
{
    Q_OBJECT

public:
    Dialog(QWidget *parent = 0);
    ~Dialog();

private:
    QPushButton *btn;
    int count = 0;

private slots:
    void mySlot();
};

#endif // DIALOG_H

dialog.cpp

#include "dialog.h"

Dialog::Dialog(QWidget *parent)
    : QDialog(parent)
{
    resize(300,300);
    btn = new QPushButton("0",this);
    connect(btn,SIGNAL(clicked()),this,SLOT(mySlot()));
}

void Dialog::mySlot()
{
    setWindowTitle(QString::number(++count));
}

Dialog::~Dialog()
{
    delete btn;
}