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".

 
class DelegateA 'not implmenting thing()
'==============
'
string message
 
end class
 
class DelegateB 'implementing thing()
'==============
'
string message
 
method thing() as string
return message
end method
'
end class
 
 
Class Delegator
'==============
'
has DelegateA dgA
has DelegateB dgB
'
method operation() as DelegateB
dgB.message="Delegate Implementation"
return @dgB
end method
 
method thing() as string
return "not using Delegate"
end method
'
end class
 
'====
'TEST
'====
 
Delegator dgr
let dg=dgr.operation
print dgr.thing 'result "not using Delegate"
print dg.thing  'result "Delegate Implementation"
 