Dynamic method over* from a Javascript standpoint

In Javascript, you can easily override or overload any method of any object : MyObject.prototype.someFunction = newFunction will set a new class or instance method for all MyObject instances, MyObject.someFunction = newFunction sets a new method just for that instance. While the latter is not possible in ObjC, the former is thanks to the runtime functions, namely
  • class_addMethod to add a C function as a method to a class — we'll need to provide a signature that defines the function parameters and return values. For example, the signature of drawRect: is v24@0:4{_NSRect={_NSPoint=ff}{_NSSize=ff}}8 (get it with method_getTypeEncoding)
  • class_getInstanceMethod and class_getClassMethod to get pointers (called implementations) to class and instance methods
  • once we have the implementations of our new and old methods, use method_exchangeImplementations to exchange them

To overload a class method, we add a new method to that class and exchange their implementations. Then when our new method is called, we do our job and call the original one. To add a method, we use either class_addMethod or a class category :

@implementation ExistingClass (MyOverrideMethod)
- (void)doStuff2
 // do extra stuff
 // call original method (this is not a recursive call as implementations have been swapped)
 [self doStuff2]


Be careful before overloading a method. If you overload NSButton's drawRect with a new method drawRect2:, and have a slider in your interface, you'll crash in [NSSlider drawRect:]no such method drawRect2. Why ? NSButton does not have its own drawRect method : its drawing is done by its parent class, NSControl. method_exchangeImplementations will happily swap a parent class method (NSControl's drawRect) with a derived class method (NSButton's drawRect2), guaranteeing a crash in sibling classes (NSSlider, NSTextField). To overload NSButton's drawRect, no need to jump through hoops : adding a drawRect: method to NSButton with a category and calling [super drawRect:…] will work just fine ! :)

So before overloading a method, compare its implementation with the superclass implementation (get the superclass with class_getSuperclass, then compare the results of class_getInstanceMethod). If they differ, use the method_exchangeImplementations technique. If not, overload with a category method.

Objective-C 2.0 Runtime Reference
Dynamic overloading : sample code

Objective-C from Javascript

2008 04 28

wouldn't it be simpler to just use MethodSwizzling for that? Jonathan Rentzsch did a nice project to have a clean implementation of it. Michael Tsai bloged about it here:


Patrick Geiller
2008 04 28

Hi Karsten,

Ah yes, jrswizzle copies the parent implementation at the derived class level with class_addMethod. So it is simpler ! Thanks for the link.

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