Running code in the main loop

Question

I need a way to run an update function of my own in the main thread. I couldn\'t find a signal that would tick me every time the main loop runs.

Am I doing this wr

Answer 1

Yet another way would be to override:

bool QCoreApplication::event(QEvent *e)

register a user QEvent and post the event to QCoreApplicatio::instance(). Obviously, the QTimer approach is superior, but this one will work even if the issuing thread was not created by Qt (QThread).

example:

class MainThreadEvent: public QEvent
{
  std::function f_;

public:
  template 
  explicit MainThreadEvent(F&& f) :
    QEvent(event_type()),
    f_(std::forward(f))
  {
  }

  void invoke()
  {
    f_();
  }

  static auto event_type()
  {
    static int et{-1};

    return QEvent::Type(-1 == et ? et = registerEventType() : et);
  }

  template 
  static void post(F&& f)
  {
    auto const app(QCoreApplication::instance());

    app->postEvent(app, new MainThreadEvent(std::forward(f)));
  }
};

class UserApplication: public QApplication
{
  using QApplication::QApplication;

  bool event(QEvent* const e) final
  {
    if (MainThreadEvent::event_type() == e->type())
    {
      return static_cast(e)->invoke(), true;
    }
    else
    {
      return QApplication::event(e);
    }
  }
};

EDIT: example for an update() from an arbitrary thread:

MainThreadEvent::post(
  [p = QPointer(this)]()
  {
    if (p)
    {
      p->update();
    }
  }
);