Re: [Pyobjc-dev] NSUndoManager.prepareWithInvocationTarget_

[Ronald O. <ous...@ci...>]

On Monday, Dec 30, 2002, at 21:52 Europe/Amsterdam, bb...@ma... wrote:

> On Monday, Dec 30, 2002, at 15:06 US/Eastern, Ronald Oussoren wrote:
>> I expected that this would work, but it clearly doesn't. This seems 
>> to be caused by a misunderstanding on my part of the semantics of 
>> 'NSObject.respondsToSelector:'. I expected that this returns true 
>> whenever 'NSObject.methodSignatureForSelector:' returns a non-nil 
>> value, but it doesn't.
>
> This is frequently not the case in the context of forwarded 
> invocations and other proxy-like behavior.  A class may return true to 
> respondsToSelector: but not actually provide a signature for the 
> selector.

I sure hope that will never happen because if 
methodSignatureForSelector returns nil PyObjC cannot construct the 
correct NSInvocation. Note that NSUndoManager does return non-nil from 
methodSignatureForSelector but sometimes returns FALSE from 
respondsToSelector for methods that it actually responds to. With the 
patch I just checked in this is no longer a problem.

>
> NSUndoManager is just a big proxy that keeps invocations in a stack--

I noticed that, and I have to say that this a very neat way to 
implement an undo mechanism.

Ronald