[Qt-interest] QObject::moveToThread: Widgets cannot be moved to a new thread

[Konrad Rosenbaum]

Hi,

On Friday 31 July 2009, Arnold Krille wrote:
> The technical reason is (as explained at various places in the docs and
> explained regularly here on the list) that all the gui-interacting things
> have to be done in the main thread (where main-thread is not necessarily
> the one where main() is executed but the one where QApplication is
> instantiated). Because at least some of the Qt-supported graphics systems
> need all the communication to be done by one thread (X11 for example).

To expand a little bit:

For X11 it is easy to explain: it uses a socket connection between client 
(Qt program) and server (X11 graphics system) to transport GUI commands 
(like "allocate window", "show window", "draw rectangle"). This socket must 
be used synchronously or the bytes of different commands might get mixed up 
and X11 would terminate your GUI. In theory it would be possible to protect 
that connection with a Mutex, but that would make Qt much slower.

Other GUI environments require that graphics contexts are used in one thread 
exclusively because they have similar synchronicity requirements (eg. 
access to GUI memory must be synchronous or GUI functions are not reentrant 
or thread safe).

As for the GUI-thread-check, it is there, I just found this in the source of 
QWidget:
#ifndef QT_NO_THREAD
    if (!q->parent()) {
        Q_ASSERT_X(q->thread() == qApp->thread(), "QWidget",
                   "Widgets must be created in the GUI thread.");
    }
#endif

I'm not entirely sure why the test is not executed if the widget has a 
parent on creation.


	Konrad
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