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".

import Foundation
 
protocol Thingable { // prior to Swift 1.2, needs to be declared @objc
  func thing() -> String
}
 
class Delegator {
  weak var delegate: AnyObject?
  func operation() -> String {
    if let f = self.delegate?.thing {
      return f()
    } else {
      return "default implementation"
    }
  }
}
 
class Delegate {
  dynamic func thing() -> String { return "delegate implementation" }
}
 
// Without a delegate:
let a = Delegator()
println(a.operation())    // prints "default implementation"
 
// With a delegate that does not implement thing:
a.delegate = "A delegate may be any object"
println(a.operation())    // prints "default implementation"
 
// With a delegate that implements "thing":
let d = Delegate()
a.delegate = d
println(a.operation())    // prints "delegate implementation"