Re: [Pyobjc-dev] Headache over hacking an XML editor

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

On Monday, Jan 13, 2003, at 21:27 Europe/Amsterdam, Just van Rossum 
wrote:

> I wrote:
>
>> I've attached a little Python Object browser using an NSOutlineView I
>> recently hacked up. The attached is slightly newer than what I posted
>> here before.
>
> Btw. there's in interesting thing going on in this app: if the
> PythonItem class is a subclass of NSObject everything works fine, but 
> if
> it isn't (a pure Python class), the app crashes as soon as you click
> inside the tree widget. Maybe there's some refcount problem for Python
> objects passed to objc?

This is a feature(*) of NSOutlineView. The outline view does not call 
retain
on the objects you return from methods like  outlineView:child:ofItem:, 
but it
does keep around references to those objects. This is documented in
'Using an Outline View Data Source' in Apple's developer documents.

The reason your code is crashing when PythonItem is not subclassed from
NSObject and runs correctly when it is twofold.

First of all 'normal' python objects are brigded to Objective-C using 
autoreleased
proxy objects. Because the outline-view doesn't call retain these will 
be released
'soon'. And because the outline-view does actually use the pointer 
later your
program crashes...

If you subclass from NSObject your object is bridged as itself (more or 
less). And
if you keep around a reference to the objects you return these objects 
won't
go away before the outline-view uses the pointer it stashed away. Which 
brings me
to your second problem: You code is leaking memory.

PythonItem.__new__ should return cls.alloc().init().release(). The call 
to release
is necessary to adjust the reference count. The problem is that alloc 
is one of
a few methods that returns a object that is owned by the caller, almost 
all other
methods that return an object require that you call retain if you want 
to keep the
object around. The bridge contains code to deal with this, but I didn't 
manage
to convince Bill that automaticly adjusting the reference is a good 
idea. The
mailinglist archive should contain more information. I still think that 
automaticly
adjusting the references would be a good idea, and will get back to 
this later...

Ronald

(*) Actually I think this is not a feature but a bug. I've filed a bug 
for this
at Apple, but did not receive a response for this yet. I hope Apple 
changes this,
especially because adding the proper calls to retain and release should 
be
backward compatible (although I have no idea what effect this would 
have on the
performance of NSOutlineView)