Objective-C Language Low-level Runtime Environment Attach object to another existing object (association)
Example
It's possible to attach an object to an existing object as if there was a new property. This is called association and allows one to extend existing objects. It can be used to provide storage when adding a property via a class extension or otherwise add additional information to an existing object.
The associated object is automatically released by the runtime once the target object is deallocated.
#import <objc/runtime.h>
// "Key" for association. Its value is never used and doesn't
// matter. The only purpose of this global static variable is to
// provide a guaranteed unique value at runtime: no two distinct
// global variables can share the same address.
static char key;
id target = ...;
id payload = ...;
objc_setAssociateObject(target, &key, payload, OBJC_ASSOCIATION_RETAIN);
// Other useful values are OBJC_ASSOCIATION_COPY
// and OBJ_ASSOCIATION_ASSIGN
id queryPayload = objc_getAssociatedObject(target, &key);
