Question

I’m doing research about Objective-C, and I’d like to know if it has the concept of reflection. None of the documents I’ve found so far describes reflection.

Answer 1

There are runtime functions described in Runtime Reference which allows not only querying for features of a class or an instance, but also adding a method, or even creating a new class at runtime. I say this is a very dynamic kind of reflection, which was not usually available to C-based languages. Mike Ash’s wrappers is an Objective-C wrapper around that. Again, it can even add methods! The base class of Cocoa, NSObject, also provide wrappers for many of the runtime functions, see NSObject protocol reference. For example

     [foo respondsToSelector:@selector(bar:)];

     if([foo isKindOfClass:[NSString class]]){ ... } 

does what the method names say. You can even add a method on the fly. For example,

   #import <Foundation/Foundation.h>
   #import <objc/runtime.h>

   @interface Foo:NSObject
   {
   }
   @end
   @implementation Foo
   -(void)sayHi
   {
        NSLog(@"Hi! from %@",NSStringFromSelector(_cmd));
   }
   +(BOOL)resolveInstanceMethod:(SEL)sel
   {
        Method method=class_getInstanceMethod(self,@selector(sayHi));
        class_addMethod(self,sel,method_getImplementation(method),method_getTypeEncoding(method));
        return YES;
   }
   @end

   int main(){
    NSAutoreleasePool*pool=[[NSAutoreleasePool alloc] init];
        Foo* foo=[[Foo alloc] init];
        [foo aeiou];
        [foo bark];
        [foo mew];
        [pool drain];
        return 0;
  }

This produces the output

  Hi! from aeiou
  Hi! from bark
  Hi! from mew

What it does is as follows:

1. SEL is the variable which represents the sent message (or method call, in the other terminology.)
2. Objective-C runtime calls resolveInstanceMethod: of a class if a message sent to an instance is not implemented in the class
3. So, in this case, I just copy an implementation of a predefined method called sayHi to that method’s implementation.
4. From the method, you can use _cmd to see what was the selector used in calling the method. So, even from a single sayHi implementation, we can get different output.

Some of the standard Cocoa’s implementation (Key-Value-Coding, Key-Value-Observing and Core Data in particular) uses the runtime to dynamically modify the class.