How do I do a basic implementation of Delegation in Swift?

Question

I am trying to get my head around delegation and am stripping it down to a basic implementation. I've came up with this, but the delegate function never gets called. Could anyone shed some light?

protocol MyDelegate{
    func delegatedFunction (a:String)
}

class DelegatorClass {
    var delegate: MyDelegate?
    func callDelegate() {
        delegate?.delegatedFunction("hello")
    }
}

class DelegateClass: MyDelegate {
    func delegatedFunction (a:String){
        print(a)
    }
}

Answer 1

let myDelegatorObj = DelegatorClass()
myDelegatorObj.delegate = DelegateClass()
myDelegatorObj.callDelegate()

Point is before you call callDelegate() you need to assign the delegate. To check your delegation working, you can initialize the DelegatorClass with the delegate.

class DelegatorClass {
    var delegate: MyDelegate? = DelegateClass()
    func callDelegate() {
        delegate?.delegatedFunction("hello")
    }
}

Answer 2

protocol MyDelegate{
    func delegatedFunction (a:String)
}

class DelegatorClass {
    var delegate: MyDelegate?
    func callDelegate() {
        delegate?.delegatedFunction("hello")
    }
}

class DelegateClass: MyDelegate {
    func delegatedFunction (a:String){
        print(a)
    }
}
let delegator = DelegatorClass()
delegator.callDelegate() // print nothing, because delegate is nil by default

// set your delegate !!!!!!!
delegator.delegate = DelegateClass()
delegator.callDelegate() // print "hello"

It is nothing wrong with your approach, just use it the right way. The point is set delegate variable to some instance of type T conforming to protocol MyDelegate. In your case this is DelegateClass instance. Generally, T could be almost everything conforming to MyDelegate protocol.

struct S:MyDelegate {
    func delegatedFunction(a: String) {
        print("Hello from struct conforming to MyDelegate protocol")
    }
}

delegator.delegate = S()
delegator.callDelegate() // print "Hello from struct conforming to MyDelegate protocol"

delegator.delegate = nil
delegator.callDelegate() // print nothing again :-)