PyQT: pausing an application for the next event

[Phil Thompson]

On Monday 13 October 2003 10:35 am, Srinath Avadhanula wrote:
> Hello,
>
> [Python 2.3 + QT 2.3.0 + PyQT 3.8.0-nc]
>
> I am wondering if QT has something like QWaitForNextEvent() function.
> This function would block execution of the application till another key
> was pressed and then return the event which occured.
>
> Would like to utlize this in a small application I am building:
> I am trying to create vi key-bindings for a simple QMultiLineEdit
> control. I created a class which overloads the QMultiLineEdit class
> and then re-defined its keyPressEvent() function. If something like
> QWaitForNextEvent() were available, I would have done something like the
> following:
>
> def MySimpleVi(QMultiLineEdit):
>     def keyPressEvent(self, event):
>         if event.text() == 'd':
>             self.deleteLine(event)
>
>     def deleteLine(self, event):
>         count = 0
>         movement = None
>
>         nextevent = QWaitForNextEvent()
>         while iscount(nextevent):
>             count = 10*count + int(nextevent).text
>             nextevent = QWaitForNextEvent()
>
>         if ismovement(nextevent):
>             movement = nextevent.text()
>
>         range = calculateRange(self.getCursorPosition(), movement, count)
>         self.deleteRange(range)
>
> Without a function like QWaitForNextEvent(), I will have to make
> deleteLine() somehow remember its state between successive calls. I will
> also have to make the keyPressEvent() function remember state between
> calls so that it can keep passing events to deleteLine() till
> deleteLine() is "done". Such things will get even more complex if I plan
> to implement vi commands like "3d3j", which take an optional count
> before the command description and then execute the command so many
> times.
>
> Any advice/suggestions would be greatly appreciated.
>
> If PyQT does not have something like this, is there some completeley
> different way to accomplish something similar? Basically have a function
> pause between successive calls and resume from last time but with one of
> the local variables corresponding to the function arguments changed?
> This sounds a little like generator functions in Python, but I cannot
> think of a way to use them here.
>
> I searched the QT-interest mailing list and found this to be of
> interest:
>
> http://lists.trolltech.com/qt-interest/2000-10/thread00270-0.html
>
> However, as Mr. Peter Becker says after offering a solution:
>
> 	But my last experiences using such techniques taught me: don't use
> 	such stuff if you can avoid it. You can run in different kinds of
> 	deadlocks and livelocks -- I even got a reproducable Zombie this way
> 	(NT showed a running process without task). Plus: to much calling of
> 	QApplication::processEvents() seems to result in big CPU load.

The (now obsolete) QApplication.processOneEvent() is probably the nearest - 
but it processes the event and does not return it for you to process the way 
you want to. Also, as the documentation says "Using this function in new 
applications may be an indication of design problems."

Your code seems to make assumptions about what event is expected next. Events 
are asynchronous and used for all sorts or things.

What's wrong with keeping state information? Handling an optional prefix 
hardly complicates things much.

Phil