Audio & Video Notes (Day 4): QT Development Fundamentals - Signals & Slots

First, basic use

Signal: Clicking a button, for example, sends a click Signal
Slot: Also called Slot function, it is used to process signals
Official documentation: Signals & Slots

The image above looks like this:
- Object1 sends signal signal1 to sloT1 and SLOT2 of Object2 for processing
  - Object1 is the sender of the signal, and Object2 is the receiver of the signal
- Object1 sends signal signal2 to sloT1 of Object4 for processing
  - Object1 is the sender of the signal and Object4 is the receiver of the signal
- Object3 sends signal signal1 to slot SLOt3 of Object4 for processing
  - Object3 is the signal sender and Object4 is the signal receiver
- One signal can be processed by multiple slots, and one slot can process multiple signals
throughconnectThe function can take the signalThe sender,signal,Receiver of a signal,slotJoin together.

connect(sender of a signal, signal, receiver of a signal, slot)Copy the code

1. Use cases

Add one in the main windowShut downbutton

#include "mainwindow.h"
#include <QPushButton>

MainWindow::MainWindow(QWidget *parent)
    : QMainWindow(parent)
{
    QPushButton *btn = new QPushButton;
    btn->setText("Closed");
    btn->setFixedSize(100.50);
    btn->setParent(this);

    this->setFixedSize(500.500);
}
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After running, the picture is as follows

useCommand + Left mouse buttonClick on theQPushButton, you can viewQPushButtonThe header file

inQPushButtonWas not found in the header filesignalThe definition of
Continue to look at the parent classQAbstractButtonThe header file

inQAbstractButtonIn the header file, scroll down to see that four are definedSignal (Q_SIGNALS)

Using the same method, can be inQMainWindowThe parent classQWidgetFound in theSlot function (Q_SLOTS)

throughconnectFunctions, linksQPushButtonandsignalwithQMainWindowtheChannel function

#include "mainwindow.h" #include <QDebug> #include <QPushButton> MainWindow::MainWindow(QWidget *parent) : QMainWindow(parent) { QPushButton *btn = new QPushButton; BTN - > setText (" close "); btn->setFixedSize(100, 50); btn->setParent(this); this->setFixedSize(500, 500); connect(btn, &QPushButton::clicked, this, &MainWindow::close); } MainWindow::~MainWindow() {qDebug(" MainWindow is closed "); }Copy the code

Run the program again

Click the close button and you can seeThe main windowIs closed with a printout

Can be achieved bydisconnectFunction to disconnectSignals and slotsA link to the

// Disconnect can be used to disconnect
disconnect(btn, &QPushButton::clicked, this, &MainWindow::close);
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Two, custom signal and slot

Both sender and receiver of a signal must inherit fromQObjectControls in Qt are ultimately inherited from QObject, such as QMainWindow and QPushButton.

1. The sender of the signal

The new filesenderTo defineSenderClass as aThe sender of a signal

View the generated file

sender.h
- Q_OBJECT supports custom signals and slots
- Custom signals need to be written insignals:The following
- Custom signals only need to be declared, not implemented

#ifndef SENDER_H
#define SENDER_H

#include <QObject>

class Sender : public QObject
{
    Q_OBJECT
public:
    explicit Sender(QObject *parent = nullptr);
    
signals:
    // To define a signal, just add a declaration, no need to add an implementation
    void exit(a);
};

#endif // SENDER_H
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sender.cpp

#include "sender.h"

Sender::Sender(QObject *parent) : QObject(parent)
{

}
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2. The receiver of the signal

createreceiverTo defineReceiverClass, asReceiver of a signal, andsenderThe file creation methods are the same

receiver.h
- You are advised to write custom slots inpublic slots:The following

#ifndef RECEIVER_H
#define RECEIVER_H

#include <QObject>

class Receiver : public QObject
{
    Q_OBJECT
public:
    explicit Receiver(QObject *parent = nullptr);
    
// Custom slot
public slots:
    void handleExit(a);
};

#endif // RECEIVER_H
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receiver.cpp
- Custom slot function, must be written implementation

#include "receiver.h"
#include <QDebug>

Receiver::Receiver(QObject *parent) : QObject(parent)
{

}

// Implement slot function, write signal processing code
void Receiver::handleExit(a)
{
    qDebug() < <"Receiver::handleExit()";
}
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3, connection

useconnectFunction to connectsenderandreceiver
mainwindow.cpp

#include "mainwindow.h"
#include "sender.h"
#include "receiver.h"

MainWindow::MainWindow(QWidget *parent)
    : QMainWindow(parent)
{
    Sender *sender = new Sender;
    Receiver *receiver = new Receiver;

    connect(sender, &Sender::exit, receiver, &Receiver::handleExit);

    // sender sends a signal
    emit sender->exit(a); } MainWindow::~MainWindow() {}Copy the code

Run, view print

If you’re not using sender and Receiver, remember to destroy the object

#include "mainwindow.h"
#include "sender.h"
#include "receiver.h"

MainWindow::MainWindow(QWidget *parent)
    : QMainWindow(parent)
{
    Sender *sender = new Sender;
    Receiver *receiver = new Receiver;

    connect(sender, &Sender::exit, receiver, &Receiver::handleExit);

    // sender sends a signal
    emit sender->exit(a);// If you are not using sender and receiver, remember to destroy the object
    delete sender;
    delete receiver;
}
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Parameter and return value

Both signals and slots can have parameters and return values:
- The parameters of the signal are passed to the slot
- The return value of the slot is returned to the signal location

// Custom signal
signals:
    int exit(int a, int b);
 
// Custom slot
public slots:
    int handleExit(int a, int b);
 
int Receiver::handleExit(int a, int b)
{
    // Receiver::handleExit() 10 20
    qDebug() < <"Receiver::handleExit()" << a << b;
    return a + b;
}
 
// Send a signal
int a = emit sender->exit(10.20);
/ / 30
qDebug() << a;
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Note that the number of signal parameters must be greater than or equal to the number of slot parameters. For example, the following is incorrect:

// Custom signal
signals:
    void exit(int a);
 
// Custom slot
public slots:
    void handleExit(int a, int b);
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Connect two signals
- For example, Signal 1A for Object 1 and Signal 2A for Object 2 are connected
- When Object 1 sends Signal 1A, Slot X and Slot Y are triggered
- When Object 2 sends Signal 2A, only Slot Y is fired, not Slot X

createsender2File, definitionSender2Class as the sender of the second signal

Add the signal

#ifndef SENDER2_H
#define SENDER2_H

#include <QObject>

class Sender2 : public QObject
{
    Q_OBJECT
public:
    explicit Sender2(QObject *parent = nullptr);

signals:
    // To define a signal, just add a declaration, no need to add an implementation
    int exit2(int a, int b);
};

#endif // SENDER2_H
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receiver.hAdd a new slot function to the

#ifndef RECEIVER_H
#define RECEIVER_H

#include <QObject>

class Receiver : public QObject
{
    Q_OBJECT
public:
    explicit Receiver(QObject *parent = nullptr);

// Custom slot
public slots:
    int handleExit(int num1, int num2);
    int handleExit2(int num1, int num2);
};

#endif // RECEIVER_H
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receiver.cppAdd the implementation of slot function

#include "receiver.h"
#include <QDebug>

Receiver::Receiver(QObject *parent) : QObject(parent)
{

}

// Implement slot function, write signal processing code
int Receiver::handleExit(int num1, int num2)
{
    qDebug() < <"Receiver::handleExit()" << num1 << num2;
    return num1 + num2;
}

int Receiver::handleExit2(int num1, int num2)
{
    qDebug() < <"Receiver::handleExit2()" << num1 << num2;
    return num1 - num2;
}
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inmainwindow.cppIn the connectionsenderandsender2The signal of

#include "mainwindow.h"
#include "sender.h"
#include "sender2.h"
#include "receiver.h"
#include <QDebug>

MainWindow::MainWindow(QWidget *parent)
    : QMainWindow(parent)
{
    Sender *sender = new Sender;
    Sender2 *sender2 = new Sender2;
    Receiver *receiver = new Receiver;

    // connect sender to sender2
    connect(sender, &Sender::exit, sender2, &Sender2::exit2);
    // Connect sender and receiver
    connect(sender, &Sender::exit, receiver, &Receiver::handleExit);
    // Connect sender2 to receiver
    connect(sender2, &Sender2::exit2, receiver, &Receiver::handleExit2);

    // sender sends a signal
    int res = emit sender->exit(100.200);

    qDebug() << res;

    // If you are not using sender, sender2, and receiver, remember to destroy the object
    delete sender;
    delete sender2;
    delete receiver;
}
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senderSend a signal

5, Lambda

LambdaIs a block of function code that can be used for event handling

connect(sender, &Sender::exit, [](int a, int b) {
    qDebug() < <"lambda handle exit";
    return a + b;
});
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#include "mainwindow.h"
#include "sender.h"
#include "sender2.h"
#include "receiver.h"
#include <QDebug>

MainWindow::MainWindow(QWidget *parent)
    : QMainWindow(parent)
{
    Sender *sender = new Sender;
    connect(sender, &Sender::exit, [](int a, int b) {
        qDebug() < <"lambda handle exit";
        return a + b;
    });

    int res = emit sender->exit(10.20);
    qDebug() << res;
}
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3. Signals and slots in UI

1. UI binding slot

Create a new project and select UI

Build in projectmainwindow.uifile

Double click on themainwindow.uifile

Add two buttons labeled asThe loginandregistered

The right mouse buttonClick the Login button

chooseGo to the bath

chooseclicked, can be inmainwindow.handmainwindow.cppThe file automatically generates slot functions

inChannel functionTo add a print code

#include "mainwindow.h"
#include "ui_mainwindow.h"
#include <QDebug>

MainWindow::MainWindow(QWidget *parent)
    : QMainWindow(parent)
    , ui(new Ui::MainWindow)
{
    ui->setupUi(this);
}

MainWindow::~MainWindow()
{
    delete ui;
}


void MainWindow::on_pushButton_clicked(a)
{
    qDebug() < <"Login";
}
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To execute the program, clickThe loginButton, you can see that the console has printout

2. Naming rules of UI binding slots

Naming rules for UI binding slots:Variable name of on_ control _ event name
The aboveThe loginButton, which in the UI file is calledpushButton, the selected event isclicked

Modify theregisteredThe name of the button in UI isregisterButton

inmainwindow.hFile, manually addregisterButtonTriggered slot function declaration

inmainwindow.cppFile, manually addregisterButtonTriggered slot function implementation

To execute the program, clickregisteredButton to see a printout from the console

The controls in the UI file will automatically connect to the slot functions that follow the naming rules.

3. Get the controls in the UI file by code

Controls in UI files can be passed through codeThe UI - > variable namesAccess.

#include "mainwindow.h"
#include "ui_mainwindow.h"
#include <QDebug>

MainWindow::MainWindow(QWidget *parent)
    : QMainWindow(parent)
    , ui(new Ui::MainWindow)
{
    ui->setupUi(this);

    // Access the 2 buttons in the UI file via UI ->
    ui->pushButton->setFixedSize(200.50);
    ui->registerButton->setFixedSize(200.50);
}
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Audio & Video Notes (Day 4): QT Development Fundamentals – Signals & Slots

First, basic use

1. Use cases

Two, custom signal and slot

1. The sender of the signal

2. The receiver of the signal

3, connection

Parameter and return value

5, Lambda

3. Signals and slots in UI

1. UI binding slot

2. Naming rules of UI binding slots

3. Get the controls in the UI file by code

Audio & Video Notes (Day 4): QT Development Fundamentals – Signals & Slots

First, basic use

1. Use cases

Two, custom signal and slot

1. The sender of the signal

2. The receiver of the signal

3, connection

Parameter and return value

5, Lambda

3. Signals and slots in UI

1. UI binding slot

2. Naming rules of UI binding slots

3. Get the controls in the UI file by code

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