Dynamic getters and setters with goal C

Question

I am in a situation where I want to dynamically generate getters and setters for a class at runtime (similar to what NSManagedObject does behind the scenes). In my opinion, this is possible using the resolveInstanceMethod: method in a specific class. At this point you will have to use class_addMethod to dynamically add a selector based method. I understand this on a theoretical level, but I didn’t delve into the obj-c runtime very much, so I was curious if there were any great examples of how to do this. Most of my knowledge is taken from this article:

http://developer.apple.com/library/mac/#documentation/Cocoa/Conceptual/ObjCRuntimeGuide/Articles/ocrtDynamicResolution.html

Any thoughts / examples?

Answer 1

The only nice discussion I know is Mike Ash's blog post. This is actually not so difficult.

I once needed to split a large subclass NSManagedObjectinto two, but decided to keep the fact implementation details so that I did not have to rewrite other parts of my application. So, I needed to synthesize getter and setter, which automatically sends [self foo]to [self.data foo].

To do this, I did the following:

• Prepare a new method already in my class.

  - (id)_getter_
  {
      return objc_msgSend(self.data, _cmd);
  }
  
  - (void)_setter_:(id)value 
  {
      objc_msgSend(self.data, _cmd,value);
  }
  

  , _cmd . , _cmd @selector(_getter_), @selector(_setter_) , _getter_ foo. _cmd @selector(foo) , , self.data foo.

• :

  +(void)synthesizeForwarder:(NSString*)getterName
  {
      NSString*setterName=[NSString stringWithFormat:@"set%@%@:",
            [[getterName substringToIndex:1] uppercaseString],[getterName substringFromIndex:1]];
      Method getter=class_getInstanceMethod(self, @selector(_getter_));
      class_addMethod(self, NSSelectorFromString(getterName), 
                      method_getImplementation(getter), method_getTypeEncoding(getter));
      Method setter=class_getInstanceMethod(self, @selector(_setter_:));
      class_addMethod(self, NSSelectorFromString(setterName), 
                      method_getImplementation(setter), method_getTypeEncoding(setter));
  }
  

  , . , self . , hardcode ( Objective-C runtime, ). , ; , . .

• :

   +(void)load  
   {
       for(NSString*selectorName in [NSArray arrayWithObjects:@"foo", @"bar", @"baz",nil]){
          [self synthesizeForwarder:selectorName];
       }
   }
  

  foo, setFoo:, bar, setBar: baz, setBaz:.

, !

Answer 2

- , , DynamicStorage, :

https://github.com/davedelong/Demos

, , , , NSMutableDictionary ivar. , @property, , /, .. , imp_implementationWithBlock(), ( ).