A delegate is a helper object used by another object. The delegator may send the delegate certain messages, and provide a default implementation when there is no delegate or the delegate does not respond to a message. This pattern is heavily used in Cocoa framework on Mac OS X. See also wp:Delegation pattern.

Objects responsibilities:

Delegator:

 Keep an optional delegate instance.
 Implement "operation" method, returning the delegate "thing" if the delegate respond to "thing", or the string "default implementation".
Delegate:

 Implement "thing" and return the string "delegate implementation"
Show how objects are created and used. First, without a delegate, then with a delegate that does not implement "thing", and last with a delegate that implements "thing".

coclass 'delegator'
  operation=:3 :'thing__delegate ::thing y'
  thing=: 'default implementation'"_
  setDelegate=:3 :'delegate=:y'  NB. result is the reference to our new delegate
  delegate=:<'delegator'
 
coclass 'delegatee1'
 
coclass 'delegatee2'
  thing=: 'delegate implementation'"_
 
NB. set context in case this script was used interactively, instead of being loaded
cocurrent 'base'