Previously we learn how to add methods from outside the class : Objective-C : Categories


Objective-C permits a class to wholly replace another class within a program. The replacing class is said to "pose as" the target class.

Note: Class posing was declared deprecated with Mac OS X v10.5, and is unavailable in the 64-bit runtime.

For the versions still supporting posing, all messages sent to the target class are instead received by the posing class.

There are several restrictions 

• A class may only pose as one of its direct or indirect superclasses.

• The posing class must not define any new instance variables that are absent from the target class (though it may define or override methods).

• The target class may not have received any messages prior to the posing.

• Posing, similarly with categories, allows global augmentation of existing classes. Posing permits two features absent from categories:

• A posing class can call overridden methods through super, thus incorporating the implementation of the target class.

• A posing class can override methods defined in categories


For example,

@interface CustomNSApplication : NSApplication



@implementation CustomNSApplication

- (void) setMainMenu: (NSMenu*) menu


     // do something with menu




class_poseAs ([CustomNSApplication class], [NSApplication class]);

This intercepts every invocation of setMainMenu to NSApplication. 

  Modified On Mar-13-2018 12:26:20 AM

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