Re: [Pyobjc-dev] Controller Layer

[b.bum <bb...@ma...>]

On Dec 23, 2003, at 5:00 PM, Ronald Oussoren wrote:
> For some reason the instance of 'Converter' is an instance of 
> 'NSNotifying_Converter' during the call to 'getAmount', and 
> 'getAmount' is not recognized as a Python method which is causing 
> infinite recursion. The class of the object is probably changed to 
> implement some feature of KVO.

That is exactly what is happening.   As others have discovered, 
automatic KVO works leveraging posing type behavior to override the 
setters and getters such that change notification can be automated.

Somewhat evil, yes, but it is efficient and transparent in the case 
where you aren't playing intense games with the runtime (as does 
PyObjC).

> This is fixable by using libffi more agressively, although this may 
> have other negative effects. E.g., we should use libffi to find the 
> exact Python function to the method stub, instead of looking up the 
> Python method in the class when it is called.

This would defeat the dynamic dispatch inherit to both objc and python, 
correct?   I.e. if I write...

x.doSomething()
y.editTheRuntime()
x.doSomething()

... and editTheRuntime() does exactly that, then the second invocation 
could lead to a completely different implementation of doSomething().

KVO is somewhat misnamed -- it isn't limited to key/value coding.    
That is the whole point behind doing some relatively low level edits to 
the runtime to make KVO work the way it does.   By doing so, it breaks 
the dependency on the developer always changing the value through the 
NSKeyValueCoding methods, thus making change observation transparent to 
the usage of the class (as long as the developer uses the setters and 
getters, directly or indirectly, the changes will be observed).

Could the existing functionality be modified such that the presence of 
the notifying posed class does not break the dispatch into Python?

b.bum