Help with QWidget::grab

[d_stranz]

It did however run as written and printed correctly.

It did? The same code you posted above after fixing it? I didn't think it should have, but maybe Qt queued up all those events until the event loop was running, and then started executing them. That does make sense, actually - the myWidget.show() call occurs before the loop is running, so the event that it triggers must just sit there in the queue waiting until it can be processed. Learned something here.

[anda_skoa]

QWidget::grab() might just render the widget into a pixmap buffer instead of the usual backing store buffer, thus be independent of event processing.
Also the show() and respective show event will have likely have been processed by the dialog's nested event loop.

Cheers,
_

[d_stranz]

Also the show() and respective show event will have likely have been processed by the dialog's nested event loop.

Very good point. Forgot that QDialog runs its own event loop. So it must be independent of the QApplication event loop (which implies not nested, and sharing the same queue) and also not require that the QApplication event loop be running. This is more interesting than I thought - a lot of side-effects happening which just happen to cause the example to work.

[wysota]

I think a lot depends on the content of the widget being grabbed, as if it has a layout, then the layout is (by default) executed when the widget is first shown. So either grab() would have to force the layout to be processed or you'd need to do it yourself for all this to work without the event loop.

As for the event loop itself, both QCoreApplication::exec() and QDialog::exec() are merely interfaces to QAbstractEventDispatcher which is a kind-of singleton (or actually a per-thread-instance) that does event processing. So to be precise, a dialog does not have its own event loop but rather its own event loop entry point (triggering the same event loop).