Re: [Pyobjc-dev] Trouble with respondsToSelector and performSelector

[Bob I. <bo...@re...>]

On Mar 31, 2005, at 9:41 PM, Scott Johnston wrote:

> I have a controller which receives mouse activity from a TableView.
> The controller sets a mouse "target" and mouse "actions" in the 
> tableview.
>
> This code, added to my variation of DNDTableView, works fine, but is 
> "pythonic":
>
>   def mouseDown_(self, event):
>     if self._mousetarget is not None and 
> hasattr(self._mousetarget,self._mousedownaction):
>       getattr(self._mousetarget,self._mousedownaction)(event)
>     else:
>       super(DNDTableView, self).mouseDown_(event)
>
> When I try to translate the code into "Cocoa", the app segfaults 
> shortly after execution:
>
>   def mouseDown_(self, event):
>     if self._mousetarget is not None and 
> self._mousetarget.respondsToSelector_(self._mousedownaction):
>       
> self._mousetarget.performSelector_withObject_(self._mousedownaction, 
> event)
>     else:
>       super(DNDTableView, self).mouseDown_(event)
>
> As my code works, this is not an emergency, but I'd like to know just 
> how deep my misunderstanding of Cocoa is. (Code for Dragged and Up is 
> similar to the above).
>
> Running pyobjc1.3b1 under Panther.

performSelector:withObject: only works for selectors that have a 
certain signature, this one probably doesn't.  The answer is: don't do 
that.  It's hard to say without knowing what the value of 
"_mousedownaction" is, and what the implementation of that selector 
looks like.

respondsToSelector: should work fine, though.

There is no real benefit to "ObjC-izing" a PyObjC app..  Personally, I 
would use the respondsToSelector:, but not attempt to use any of the 
performSelector_blah methods unless it was to defer execution or run 
something on the main thread.

It's generally not a good idea to use ivars that start with just an 
underscore because those are reserved by Apple and can interfere with 
KVC if you have a similarly named accessor.

-bob