[PyQt] update qt labels from another thread

[Roberto Alsina]

On Wed 11 Apr 2007 09:13:33 Pradnyesh Sawant wrote:
> Hello,
> I have a Qt4 form, which contains some labels, and 2 buttons "start"
> and "stop". The start button, when clicked, creates an instance of a
> python class which does some communication over sockets in another
> independent thread of its own. The stop button kills the instance
> created above, and kills the thread.
> What i want to do is to update the text of my form labels with the
> data that i receive over the socket communication. For this reason, i
> sent the "form" object to the backend class object, and tried updating
> the form labels from within the thread doing the socket communication.
> However, i get an error saying that Qt objects cannot be modified from
> other threads :(
>
> How do i achieve updates of form labels? In a previous mail on this
> list itself, i read something about signals/slots/callbacks. I don't
> see how signals/slots will be helpful here (however, plz note that i
> am relatively new to PyQt4 and this might be incomplete knowledge on
> my part). As far as callbacks are concerned, i don't know what they
> mean. Any pointers/tutorials for me would be greatly helpful...

If the answer is "use signals and slots"...

Keep in mind that the code below is written without any testing, and I am not 
even sure if signals/slots actually work cross-threads in Qt4 ;-)

Suppose the form has a "setLabelText" function like this:

def setLabelText(self,newtext):
	self.ui.theLabel.setText(newtext)

And the "object in another thread" wants to update it. You need to make that 
object inherit QtCore.QObject (or use a QSignal object... those still exist 
in Qt4, right?)

Then the object can do this:

emit (QtCore.SIGNAL("newData"),"data")

If you previously connected that object's newData signal to the form's 
setLabelText slot... you are golden, regardless of on what thread the signal 
is emitted, I suppose.
   
-- 
 ("\''/").__..-''"`-. .         Roberto Alsina
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