On 24 Jan 2009, at 18:09, Bill Bumgarner wrote:
Or, more specifically, why do you want to make some bit of framework
code which currently allocates an instance of A allocate an instance
of SubA instead?
Because of, what I assume to be, a bug in PDFClerkThumbnailView which
when one drags multiple pages from or within it, puts the pages in a
PDFDocument instead of in the subclass of PDFDocument, which is where
the pages live. Dragging a single page works as expected and the page
is of the correct subclass. Dragging multiple pages leads to errors. I
filed a bug report for this a while back, but I really need a safe
workaround until the issue is fixed. (Assuming Apple considers it a
bug too. I have never received feedback on it.)
This is an excellent demonstration of why category based overrides
are rife with danger. The above code is making a boatload of
assumptions.
I know. That's why I wrote that I understand it is not the way to go,
but yet that it _appears_ to work.
I would use method_exchangeImplementations(), if absolutely
positively necessary. It is about the most innocuous of an
otherwise noxious encapsulation breaking implementation pattern.
Thanks. I found that a few hours ago and was able to create the
exchange. However, writing a correct replacement method is another
thing altogether. The following doesn't work:
Early in application startup:
Method originalMethod = class_getClassMethod([PDFDocument class],
@selector(alloc));
Method replacedMethod = class_getClassMethod([PDFDocument class],
@selector(allocReplacement));
method_exchangeImplementations(originalMethod, replacedMethod);
Addition to PDFDocument:
+ (id)allocReplacement
{
if ([[self class] isEqual:[PDFDocument class]]) {
return [ANPDFDocument alloc];
} else {
return [super alloc];
}
}
I get the impression it replaces alloc for everything, not just for
the PDFClerkDocument class. So the app crashes immediately after the
implementation exchange. Which doesn't surprise me really. But: Is
there a way to make this work using method_exchangeImplementations, or
maybe for the particular problem at hand, is there a better way of
making the PDFThumbnailView use the correct PDFDocument (sub)class
when it deals with multiple selections.
Just to make this clear: I would really rather not create this
workaround, but I can't really afford not to let users drag multiple
pages in the PDFClerkThumbnailView either.
-António
----------------------------------------------------
There is nothing as strong as real gentleness, and
there is nothing as gentle as real strength.
----------------------------------------------------
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