How to implement Delegate using Protocol in swift

More than often you, as an iOS app developer, are required to implement protocols then defining you own protocols and use them. However, defining your own protocols, delegates is an elegant way passing data from one class to another. This technic is especially useful in the context of view Controllers. I am giving a simple example below; you can test it in a playground. Once you understand the theory behind it, you can apply it anyway you want.

This example is created in the context of a smaller company who retrieves packages for its clients from specified locations and stores them in its warehouse waiting for client to pick up.  As part of the business operation, there are dispatchers and retrievers. Dispatchers tells retrievers where to pick up the packages, the retrievers retrieve the package and pass them back to dispatcher for to stock up. A important point to note is that the retrievers do not know the location of the warehouse, dispatchers do.

Let’s get it started

I start with defining a protocol as following:

protocol DispatcherDelegate {

    func stockUp (package : String)
    var  package : String { get set}

}

This is the simplest protocol you can get, only one method and one property. Yon can add more to it. E.g. add some properties, and/or add more methods.  But do remember that the implementers of the protocol need to implement all members it defines, though you can make some of them optional.

Then I process with defining Retriever class. It uses the protocol as its delegate and a method simulating the retrieving logic

class Retriever  {

     var dispatcherDelegate: DispatcherDelegate?

    func retrieve (location : String) {

        // go to the location to retrievbe the package

        switch location {

        switch location {

        case "location-1":

            dispatcherDelegate?.stockUp("Dell PC")

            dispatcherDelegate!.package = "Dell PC"

        case "location-2":

            dispatcherDelegate?.stockUp("Apple MacBook")

             dispatcherDelegate!.package = "Apple MacBook"

        default:

            dispatcherDelegate?.stockUp("something else")

             dispatcherDelegate!.package = "something else"

        }

    }

}

As the last step if the setting up, I define dispatcher class as following, the important part is it implements DispatcherDelegate protocol by proving the implementation of stockUp method.

class Dispatcher : DispatcherDelegate {

  

    func stockUp (package : String) {

       println("\(package) is stcoked in")

   

    }

    func dispatch (location : String) {

       var retriever = Retriever()

       retriever.dispatcherDelegate = self

       retriever.retrieve(location)

    }

}

This is class, I have a method call dispatch, which instantiates retriever class and set itself as the retriever’s dispatcherDelegate, then it calls the retrieve’s retrieve method.

Finally, let’s put them all in test by putting the following line of code in playground:

var testDispatcher = Dispatcher()

testDispatcher.dispatch("location-1")

testDispatcher.package

testDispatcher.dispatch("location-2")

testDispatcher.package

testDispatcher.dispatch("somewhere else")

testDispatcher.package

if you put this code block in playground you will see the result at the right panel, showing

{package "Dell PC"}
"Dell PC"
{package "Apple MacBook"}
"Apple MacBook"
{package "Something else"}
"Something else"

Bingo, they all work as expected for me, Hope they do the same for you.