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 NonDelegate { }
 
class Delegate {
    method thing {
        return "delegate implementation"
    }
}
 
class Delegator (delegate = null) {
    method operation {
 
        if (delegate.respond_to(:thing)) {
            return delegate.thing
        }
 
        return "default implementation"
    }
}
 
var d = Delegator()
say "empty: #{d.operation}"
d.delegate = NonDelegate()
say "NonDelegate: #{d.operation}"
d.delegate = Delegate()
say "Delegate: #{d.operation}"