Qt-SIGNAL＆SLOT沒有從工人階級的主窗口中更新我的QLabel

Question

我在Qt 5.7中應用了一個使用線程的C ++示例。 除以下兩點外，其他一切都很好：

1-我使用Signal＆Slot來更新主表單中的標簽。 問題是那沒有效果。 真的，我不知道問題出在哪里。

2-循環工作正常，但是當我退出程序時（通過“應用程序輸出”），我看到該循環仍然有效（我認為這與啟動的線程有關）。

這是我的小例子：

---

mainwindow.h

#ifndef MAINWINDOW_H
#define MAINWINDOW_H

#include <QMainWindow>

namespace Ui {
class MainWindow;
}

class MainWindow : public QMainWindow
{
    Q_OBJECT

public:
    explicit MainWindow(QWidget *parent = 0);
    ~MainWindow();  

private slots:
    void on_pushButton_clicked();
    void updateLabelText(const QString TheString);

private:
    Ui::MainWindow *ui;

};

#endif // MAINWINDOW_H

主窗口

#include "mainwindow.h"
#include "ui_mainwindow.h"
#include "worker.h"

//#include <QDebug>
#include <QThread>

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

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

void MainWindow::on_pushButton_clicked()
{
    QThread *workerThread = new QThread;
    Worker *worker  = new Worker;
    worker->moveToThread(workerThread);
    connect(worker, SIGNAL(sendText(const QString)), this, SLOT(updateLabelText(QString)));
    workerThread->start();
}

void  MainWindow::updateLabelText(const QString TheString)
{
    ui->label->setText(TheString);
}

工人

#ifndef WORKER_H
#define WORKER_H

#include <QObject>

class Worker : public QObject
{
    Q_OBJECT

public:
    explicit Worker();

public slots:
    void doWork();

signals:
  void sendText(const QString);

};

#endif // WORKER_H

worker.cpp

#include "worker.h"
#include "mainwindow.h"
#include "ui_mainwindow.h"

#include <QDebug>
#include <QThread>
#include <QMessageBox>
#include <QApplication>

Worker::Worker()
{
    doWork();
}

void Worker::doWork()
{
    for (int i = 0; i<999999; i++) {
        emit sendText(QString::number(i));
        qDebug() << "The number is : " + QString::number(i);
        qApp->processEvents();
        //QThread::msleep(5);
    }
}

---

我怎樣才能解決這個問題？

謝謝。

Answer 1

在您的代碼中，從Worker的構造函數調用doWork()函數，該構造函數在主線程中調用（在Worker* worker= new Worker; ）。

當然，這不是您要執行的操作，因為這將導致主線程甚至在進入connect調用之前在doWork()執行for循環。

而不是從Worker的構造函數調用doWork() ，您應該將線程的started()信號連接到doWork()插槽，然后在將Worker對象移動到新線程之后調用thread->start() 。 這將利用Qt跨線程信號在新線程啟動時立即調用doWork() 。

代碼如下所示：

#include <QtWidgets>

//QThread wrapper for safe destruction
//see http://stackoverflow.com/a/19666329
class Thread : public QThread{
    using QThread::run; //final
public:
    Thread(QObject* parent= nullptr): QThread(parent){}
    ~Thread(){ quit(); wait();}

};

class Worker : public QObject{
    Q_OBJECT
public:
    explicit Worker(QObject* parent= nullptr): QObject(parent){}
    ~Worker()= default;

    Q_SIGNAL void sendText(QString text);
    Q_SIGNAL void workFinished();

    Q_SLOT void doWork(){
        for (int i = 0; i<1000; i++) {
            emit sendText(QString::number(i));
            QThread::msleep(5);
        }
        emit workFinished();
    }
};

class Widget : public QWidget{
    Q_OBJECT
public:
    explicit Widget(QWidget* parent= nullptr): QWidget(parent){
        layout.addWidget(&buttonWork);
        layout.addWidget(&label);

        connect(&buttonWork, &QPushButton::clicked,
                this, &Widget::buttonWorkClicked);

    }
    ~Widget()= default;

    Q_SLOT void buttonWorkClicked(){
        Thread* thread= new Thread(this);
        Worker* worker= new Worker;
        worker->moveToThread(thread);
        //invoke doWork as soon as the thread is started
        connect(thread, &QThread::started, worker, &Worker::doWork);
        connect(worker, &Worker::sendText, this, &Widget::updateLabelText);
        //quit the thread when work is finished
        connect(worker, &Worker::workFinished, thread, &QThread::quit);
        //destroy thread and worker object when work is finished
        connect(thread, &QThread::finished, thread, &QObject::deleteLater);
        connect(thread, &QThread::finished, worker, &QObject::deleteLater);
        //start the thread
        thread->start();
    }

    Q_SLOT void updateLabelText(QString text){
        label.setText(text);
    }

private:
    QVBoxLayout layout{this};
    QPushButton buttonWork{"Work"};
    QLabel label{"No work yet"};
};

int main(int argc, char* argv[]){
    QApplication a(argc, argv);

    Widget w;
    w.show();

    return a.exec();
}

#include "main.moc"

Answer 2

我按照@Mike先生的評論解決了我的問題。

簡而言之，我從講師中刪除了doWork（）函數。 然后，我在線程的started（）信號和工作者的doWork（）插槽之間添加了新的連接。

這些就是變化：

---

主窗口

#include "mainwindow.h"
#include "ui_mainwindow.h"
#include "worker.h"

#include <QThread>

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

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

void MainWindow::on_pushButton_clicked()
{
    QThread *workerThread = new QThread;
    Worker *worker  = new Worker;
    connect(workerThread, SIGNAL(started()), worker, SLOT(doWork()),Qt::QueuedConnection);
    connect(worker, SIGNAL(sendText(const QString)), this, SLOT(updateLabelText(const QString)),Qt::QueuedConnection);
    worker->moveToThread(workerThread);
    workerThread->start();
}
void  MainWindow::updateLabelText(const QString TheString)
{
    ui->label->setText(TheString);
}

worker.cpp

#include "worker.h"
#include "mainwindow.h"
#include "ui_mainwindow.h"

#include <QDebug>
#include <QThread>
#include <QMessageBox>
#include <QApplication>

Worker::Worker()
{
    doWork();
}

void Worker::doWork()
{
    for (int i = 0; i<999999; i++) {
        emit sendText(QString::number(i));
        qDebug() << "The number is : " + QString::number(i);
        qApp->processEvents();
        QThread::msleep(5);
    }
}