Re: How Do Classes Respond to performSelector?

FROM : Peter Ammon
DATE : Wed Jan 30 22:36:09 2008

On Jan 30, 2008, at 1:14 PM, Francisco Tolmasky wrote:

> This is just a question out of curiosity.  In the developer docs, 
> only - (id)performSelector: is defined.
> So I was going to write my own + (id)perfromSelector, but first 
> figured I'd check whether there was
> one already by writing up a quick test in XCode - which it turns 
> out there is (saved me the work).


In Objective-C, a class object gets all the instance methods of the 
root class for its hierarchy.  This means that every class object 
that descends from NSObject gets all of NSObject's instance methods - 
including performSelector:.

This isn't a magic property of NSObject.  Your own root classes do 
this too:

#import <objc/objc.h>

@interface MyRootClass @end

@implementation MyRootClass
- (void)sayHi { puts("Hi!"); }
+ (void)initialize { /* Must be implemented */ }
@end

@interface MySubclass : MyRootClass @end
@implementation MySubclass @end

int main(void) {
    [MySubclass sayHi];
    return 0;
}

Calling sayHi on the class object invokes the instance method of the 
root class.

Hope that's clear,
-Peter