c++ - 为什么我不能在信号上启动QThread？

Question

我想在另一个启动时启动QThread，但是它不起作用。
main.cpp片段

Worker stat_worker;
stat_worker.moveToThread(stat_worker.stat_thread);

Worker some;
some.moveToThread(some.somethread);
QObject::connect(stat_worker.stat_thread, SIGNAL(started()), some.somethread, SLOT(start()));
QObject::connect(some.somethread, SIGNAL(started()), &some, SLOT(print_some()));
stat_worker.stat_thread->start();

worker

class Worker : public QObject
{
    Q_OBJECT
public:
    explicit Worker();

    QThread *stat_thread = new QThread;
    QThread *somethread = new QThread;
signals:
//some signals
    void start_thread();
public slots:
//some slots
    void print_some();
    void somethread_starter();
};

#endif // WORKER_H

worker.cpp相关功能

void Worker::print_some()
{
    qInfo() << "-somethread started() signal arrived!";
}

当我尝试通过单击按钮启动线程时，也没有用。
甚至创建一个启动线程的插槽:

QObject::connect(stat_worker.stat_thread, &QThread::started, &some, &Worker::somethread_starter);

void Worker::somethread_starter()
{
    qInfo() << "-I got started by another thread!";
    somethread->start();
}

或在启动另一个线程时发出的信号:

void Worker::wip_status(){
    emit start_thread();
}

QObject::connect(stat_worker.stat_thread, &QThread::started, &stat_worker, &Worker::wip_status);
QObject::connect(&stat_worker, &Worker::start_thread, &some, &Worker::somethread_starter);

工作。
在此先感谢您回复我的帖子。

Answer 1

我试图用自己的MCVE(只是短一点)重现OP的问题。

#include <QtWidgets>

struct Worker: QObject {
  QString name;
  QThread qThread;

  Worker(const QString &name): name(name)
  {
    moveToThread(&qThread);
    connect(&qThread, &QThread::finished, this, &Worker::reportFinished);
  }

  void start()
  {
    qDebug() << "Start" << name;
    qThread.start();
  }

  void reportFinished()
  {
    qDebug() << "Exit" << name;
  }
};

// main application
int main(int argc, char **argv)
{
  qDebug() << "Qt Version:" << QT_VERSION_STR;
  QCoreApplication app(argc, argv);
  Worker worker1("worker 1");
  Worker worker2("worker 2");
  // install signal handlers
  QObject::connect(&worker1.qThread, &QThread::started, &worker2, &Worker::start);
  worker1.start();
  // runtime loop
  return app.exec();
}

输出:

Qt Version: 5.13.0
Start "worker 1"

这是OP观察到的。所以呢？

worker1.qThread.started信号连接到worker2.start插槽

worker1已启动

worker2似乎没有开始。

是什么让我感到怀疑: moveToThread()。
目的是将 Worker对象与其成员 QThread相关联。
我不确定的是:在启动 QThread之前有可能吗？
为了检查这一点，我评论了 moveToThread():

  Worker(const QString &name): name(name)
  {
    //moveToThread(&qThread);
    connect(&qThread, &QThread::finished, this, &Worker::reportFinished);
  }

输出:

Qt Version: 5.13.0
Start "worker 1"
Start "worker 2"

我之所以评论 moveToThread()的原因: qThread::start()的调用应在主应用程序(线程)的上下文中进行。
因此，将 worker2移至其 QThread意味着该信号已发送到 worker2.qThread的事件循环-实际上尚未启动。
因此，该事件无法处理。 moveToThread()应该稍后执行-例如在 started()信号的 react 中:

#include <QtWidgets>

struct Worker: QObject {
  QString name;
  QThread qThread;

  Worker(const QString &name): name(name)
  {
    connect(&qThread, &QThread::started, this, &Worker::moveThisToThread);
    connect(&qThread, &QThread::finished, this, &Worker::reportFinished);
  }

  void start()
  {
    qDebug() << "Start" << name;
    qThread.start();
  }

  void moveThisToThread()
  {
    moveToThread(&qThread);
    qDebug() << name << "associated to its thread, from now.";
  }

  void reportFinished()
  {
    qDebug() << "Exit" << name;
  }
};

// main application
int main(int argc, char **argv)
{
  qDebug() << "Qt Version:" << QT_VERSION_STR;
  QCoreApplication app(argc, argv);
  Worker worker1("worker 1");
  Worker worker2("worker 2");
  // install signal handlers
  QObject::connect(&worker1.qThread, &QThread::started, &worker2, &Worker::start);
  worker1.start();
  // runtime loop
  return app.exec();
}

输出:

Qt Version: 5.13.0
Start "worker 1"
"worker 1" associated to its thread, from now.
Start "worker 2"
"worker 2" associated to its thread, from now.

奖励问题:

So does that mean "QThread::start" is useless as receiver of a signal?

不，这不对。即使不存在带有该签名的信号(我知道)，应用程序开发人员也可以“发明”一个信号。
但是，请记住，Qt5实际上并不需要显式标记的 SLOT来将它们用于信号，过去可能会找到一个更明显的答案:
对于Qt4信号，可以将 QThread::start插槽直接连接到 QThread::started信号。 (然后 QThread::start中唯一的一个参数的默认值生效。)
由于我没有使用Qt4信号的经验(我从Qt5开始)，因此我修改了示例代码以证明自己是正确的:

  QObject::connect(&worker1.qThread, SIGNAL(started()), &worker2.qThread, SLOT(start()));

输出:

Qt Version: 5.13.0
Start "worker 1"
"worker 1" associated to its thread, from now.
"worker 2" associated to its thread, from now.

由于 Start "worker 2"现在直接调用 worker1.started()，因此不再发出 worker2.qThread.start()。
因此，使用Qt4信号可能已经运行了OP的原始代码。
引起问题的不是信号和插槽的不兼容(有人猜到了)，但上述 moveToThread()问题(也可能)并没有使它令人满意地工作。