Re: IB instantiating objects
Re: IB instantiating objects
- Subject: Re: IB instantiating objects
- From: Ashley Clark <email@hidden>
- Date: Mon, 23 Mar 2009 23:22:18 -0500
On Mar 23, 2009, at 10:31 PM, Gmail wrote:
On Mar 24, 2009, at 10:01 AM, Ashley Clark wrote:
On Mar 23, 2009, at 9:30 PM, Gmail wrote:
Thanks! I think that document explains everything I need to know
to take control over IB.
This is more Objective-C related by maybe you have a quick tip. My
first tests suggest that my method for overriding is not correct
because overriding "init" is getting invoked from all sorts of
other classes (like NSFileManager to name a few) when the NIB is
loading. I use class_getInstanceMethod (with the instance of the
custom class I registered with the objective-c runtime) to get the
method then replace the implementation with my function pointer.
This method worked before for overriding drawRect: in NSView so
I'm not sure what is different now. Any ideas?
class_getInstanceMethod searches the entire class hierarchy. So, if
the class you're working with didn't override that implementation
you'll be replacing the superclass' implementation.
If you call class_addMethod on self with the result of
class_getMethodImplemenation you'll be adding it to the subclass if
it's not already overridden. After that calling
method_exchangeImplementations will only affect that class.
Hmmm, I overrode the superclasses implementation, i.e. NSObject,
which sounds about right considering the results.
So, I need to be using class_addMethod for "init" because my new
class has no implementation for that method. I think the previous
attempt to override NSView worked because drawRect: DID have an
implementation for that selector already added to the runtime, which
is not the case with my new class. Thank you Ashley I think you are
correct.
Is there anyway way to determine if a class has an implementation
for a method so I can decide to add a new method or override an
existing? As it stands I don't know in code when I should add or
exchange implementations.
You could call class_copyMethodList and iterate through that to find
only methods defined directly on a class, but there's no harm in just
calling class_addMethod. It will add an override of a superclass'
implementation but will not replace an existing implementation if one
was already defined on the class.
So, there's no need to decide between add or exchange. Do both!
eg. (typed in Mail, YMMV)
Class klass = ...
SEL selector = ...
Method originalMethod = class_getInstanceMethod(klass, selector);
class_addMethod(klass, selector, class_getMethodImplementation(klass,
selector), method_getTypeEncoding(originalMethod));
class_replaceMethod(...);
You'll get the right behavior every time, assuming "right" means you
only want to replace the method on the class and not its' superclasses.
Ashley
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