Use of multithreading in QT

Multithreading scenario

Desktop development in progress , The main thread is responsible for the window class , Other threads are responsible for application processing , Reduce the occurrence of window jams . Multithreading mechanism just meets this requirement , Improve user experience and code execution efficiency .

Using multithreading , There are some things that require extra attention ：

• The default thread is Qt It is called window thread in , Also called main thread , Responsible for window event processing or window control data update

• The sub thread is responsible for the business logic processing in the background , No operation can be done on the window object in the child thread , These things need to be handled by the window thread

• If you want to transfer data between the main thread and the sub thread , Need to use Qt Medium Signal slot Mechanism

quote : Da C who loves programming

Step into the theme ：

Multithreading implementation mechanism ：

The second implementation mechanism of multithreading is directly used here ： Although this way of writing will be relatively complicated , But it is more flexible to use , The specific operation steps are as follows ：

1、 Create a new class

Create a new class , Let this class start from QObject The derived

class MyWork:public QObject
{
    
    .......
}

2、 Add public member function

Add public member functions to the class , The function body is the business logic executed by the sub thread

class MyWork:public QObject
{
    
public:
    .......
    //  Specify the function name yourself ,  Whatever it's called ,  Parameters can be added according to actual requirements 
    void working();
}

3、 establish QThread object

Create a... In the main thread QThread object , This is the object of the subthread

QThread* sub = new QThread;

4、 Create working class objects

Create the working class object in the main thread （ Never assign a parent to a created object ）

MyWork* work = new MyWork(this);    // error
MyWork* work = new MyWork;          // ok

5、 take MyWork Object to the created child thread object

take MyWork Object to the created child thread object , Need to call QObject Class provides the moveToThread() Method

// void QObject::moveToThread(QThread *targetThread);
//  If work The parent object is specified ,  This function call fails 
//  Tips ： QObject::moveToThread: Cannot move objects with a parent
work->moveToThread(sub);	//  Move to a child thread to work 

6、 Start the child thread

Start the child thread , call start(), Now the thread starts , But the object moved to the thread doesn't work

7、 call MyWork Class object's working function

call MyWork Class object's working function , Let this function start executing , At this time, it is running in the sub thread moved to

Sample code

mywork.h

#ifndef MYWORK_H
#define MYWORK_H

#include <QObject>

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

    //  Working function 
    void working();

signals:
    void curNumber(int num);

public slots:
};

#endif // MYWORK_H

mywork.cpp

#include "mywork.h"
#include <QDebug>
#include <QThread>

MyWork::MyWork(QObject *parent) : QObject(parent)
{
    

}

void MyWork::working()
{
    
    qDebug() << " The address of the current thread object : " << QThread::currentThread();

    int num = 0;
    while(1)
    {
    
        emit curNumber(num++);
        if(num == 10000000)
        {
    
            break;
        }
        QThread::usleep(1);
    }
    qDebug() << "run()  completion of enforcement ,  The child thread exits ...";
}

mainwindow.cpp

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

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

    qDebug() << " The address of the main thread object : " << QThread::currentThread();

    //  Creating thread objects 
    QThread* sub = new QThread;
    //  Create working class objects 
    //  Never assign a parent to a created object 
    //  If you specify : QObject::moveToThread: Cannot move objects with a parent
    MyWork* work = new MyWork;
    //  Move the working class object to the child thread object created 
    work->moveToThread(sub);
    //  Start thread 
    sub->start();
    //  Let the object of work start working ,  Click the start button ,  start-up 
    connect(ui->startBtn, &QPushButton::clicked, work, &MyWork::working);
    //  Display the data 
    connect(work, &MyWork::curNumber, this, [=](int num)
    {
    
        ui->label->setNum(num);
    });
}

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

The phenomenon

After clicking the start button , Start counting operation

Using this multithreading approach , Suppose there are multiple unrelated business processes that need to be processed , Then you can create multiple objects like MyWork Class , Put business processes into common member functions of multiple classes , Then move the instance object of this business class to the corresponding sub thread moveToThread() That's all right. , This makes the program more flexible , More readable , Easier to maintain .