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 Delegator {
    public generic delegate = none
 
    public function operation {
        if (typeof(delegate) == "none") {
            return "default implementation"
        }
        return delegate()
    }
}
 
function thing {
    return "delegate implementation"
}
 
// default, no delegate
var d = new Delegator()
println (d.operation())
 
// delegate
var d1 = new Delegator()
d1.delegate = thing
println (d1.operation())
 
 