Re: swizzling a class to a subclass inorder to add ivars (& methods) to a class
Re: swizzling a class to a subclass inorder to add ivars (& methods) to a class
- Subject: Re: swizzling a class to a subclass inorder to add ivars (& methods) to a class
- From: Mike Abdullah <email@hidden>
- Date: Mon, 30 Oct 2006 08:54:52 +0000
Hi Scott, perhaps I'm being stupid here, but how about using
poseAsClass: ?
Mike.
On 30 Oct 2006, at 04:08, Scott Morrison wrote:
The mantra for categories and posed class is that you cannot add
ivars to a category or a posed class -- leaving you to fiddle with
mutable dictionarys and tablemaps to implement extra variables. (ugh!)
However, you can add ivars to a subclass -- so I got to thinking --
can you redirect an allocation for a class to a subclass so that
for all intents and purposes you can add ivars.
In playing around, I found that by swizzling the classMethod: +
[NSObject alloc], you can redirect the allocation of a class to a
subclass as follows:
//--------------------------------------------------------------------
--------------------------------------------------------------------
@interface SubClass: AnyClass
{
NSString * extraIvar;
}
@end
static IMP oldAlloc = NULL;
@implementation NSObject (myCategory)
+(id) MYalloc{
id result;
if(self == [AnyClass class]){
self = objc_getClass("SubClass"); //restate the class as the
subclass
result = NSAllocateObject(self,0,nil); // allocate the subclass
}
else
result = oldAlloc(self,_cmd);
return result;
}
+ (void) load {
oldAlloc = replaceClassMethod(@selector(alloc),self,@selector
(MYalloc),self); // standard class method swizzle the alloc method
of NSObject
}
//--------------------------------------------------------------------
--------------------------------------------------------------------
One catch would be that any factory methods and any instance init
methods may have to be reproduced in the subclass if you want to
init your subclass ivars
eg.
(id) init___:(id) anObject{
if ((self = [super init___:anObject])){
extraIvar = [[NSString alloc] initWithString:@"test iVar"];
}
return self;
}
Another catch would be that any test directly for an instance's
class type would fail (but tests for isKindOfClass: would be ok)
In anycase -- technically this is possible -- I have it working on
some test code
However -- IS IT SAFE ?? -- if no -- why not ---
if "sorta" -- what are the gotchas and pitfalls?
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