为什么QThread::完成信号没有发出

Why is QThread::finished signal not being emitted?

本文关键字：QThread 为什么 信号      更新时间：2023-10-16

我创建了自己的TestService，它在单独的QThread上运行，但是当MainLoop终止时，QThread::finished信号不会发出。我看到了一个类似的问题，但那里的问题略有不同，因为 OP QThread过载，而我只是将我的类移动到线程上。

请注意，我不重载QThread类，我只重载QObject基于以下示例：http://mayaposch.wordpress.com/2011/11/01/how-to-really-truly-use-qthreads-the-full-explanation/

这是我TestService课：

#include <QObject>
#include <QThread>
#include <QMutex>
#include <QWaitCondition>
#include <iostream>
using namespace std;
class TestService: public QObject
{
    Q_OBJECT;
private:
    volatile int _count;
    QWaitCondition _monitor;
    QMutex _mutex;
    QThread* _thread;
public:
    TestService(int numSeconds)
    {
        _count = numSeconds;
        _thread = NULL;
        cout << "TestService()" << endl;
    }
    virtual ~TestService()
    {
        cout << "~TestService()" << endl;
    }
    void Start()
    {
        QMutexLocker locker(&_mutex);
        if(_thread == NULL)
        {
            _thread = new QThread;
            // Move this service to a new thread
            this->moveToThread(_thread);
            // The main loop will be executed when the thread
            // signals that it has started.
            connect(_thread, SIGNAL(started()), this, SLOT(MainLoop()));
            // Make sure that we notify ourselves when the thread
            // is finished in order to correctly clean-up the thread.
            connect(_thread, SIGNAL(finished()), this, SLOT(OnFinished()));
            // The thread will quit when the sercives
            // signals that it's finished.
            connect(this, SIGNAL(Finished()), _thread, SLOT(quit()));
            // The thread will be scheduled for deletion when the 
            // service signals that it's finished
            connect(this, SIGNAL(Finished()), _thread, SLOT(deleteLater()));
            // Start the thread
            _thread->start();
        }
    }
    void Stop()
    {
        _count = 0;
        _monitor.wakeAll();
    }
private slots:
    void MainLoop()
    {
        cout << "MainLoop() Entered" << endl;
        while(_count > 0)
        {
            cout << "T minus " << _count << " seconds." << endl;
            QMutexLocker locker(&_mutex);
            _monitor.wait(&_mutex, 1000);
            _count--;
        }
        cout << "MainLoop() Finished" << endl;
        emit Finished();
    }
    virtual void OnFinished()
    {
        cout << "OnFinished()" << endl;
    }
signals:
    void Finished();
};

以下是测试代码：

void ServiceTest()
{
    cout << "Press q to quit." << endl;
    cout << "Press s to start." << endl;
    cout << "Press t to stop." << endl;
    QSharedPointer<TestService> testService(new TestService(10));
    char in = 'a';
    while( in != 'q' )
    {
        switch(tolower(in))
        {
        case 's':
            testService->Start();
            break;
        case 't':
            testService->Stop();
            break;
        default:
            break;
        }
        cin.get(in);
        in = tolower(in);
    }
}
int main(int argc, char *argv[])
{
    QCoreApplication a(argc, argv);
    ServiceTest();
    QTimer::singleShot(0, &a, SLOT(quit()));
    return a.exec();
}

输出为： 

Press q to quit.
Press s to start.
Press t to stop.
TestService()
s
MainLoop() Entered
T minus 10 seconds.
T minus 9 seconds.
T minus 8 seconds.
t
MainLoop() Finished
q
~TestService()
Press any key to continue . . .

谁能解释为什么finished没有发出我该如何解决它？

信号 done（） 被发出原因，但你没有抓住它。

这里：

connect(_thread, SIGNAL(finished()), this, SLOT(OnFinished()));

使用Qt::QueuedConnection，因为_thread和this（服务）位于不同的线程中。

当finished()发出时，_thread的事件循环已经执行完毕，因此信号不会传递到插槽。

您可以显式使用 Qt::DirectConnection 。

编辑：

QTherad的工作原理是这样的：

QThread::start()
{
    emit started();
    run();
    emit finished();
}
QThread::run()
{
     eventloop->exec();
}

因此，当发出finished时，事件循环已经停止执行。当你service移动到_thread时，服务的事件循环_thread事件循环。

请注意，QObject本身没有自己的事件循环。事件循环由对话框、线程和应用程序创建。

实际上，在您的简单情况下，我建议只使用QtConcurent::run，因为您不会在新线程中执行实际的事件处理，而只是运行单个函数。

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