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Swift delegate design sample – The.Swift.Dev.

Implementing delegation in Swift

You will want a delegate protocol, a delegator who truly delegates out the duties and a delegate object that implements the delegate protocol and does the precise work that was requested by the “boss”. Let’s translate this into human.

The shopper studies a bug. The mission supervisor creates a difficulty and tells one of many builders to repair the issue asap.

See? That is delegation. Sooner or later an occasion occurred, so the delegator (supervisor) utilized an exterior useful resource (a developer) utilizing a standard interface (subject describing the issue for each social gathering) to do obtain one thing (repair the 🐛).

To reveal how delegation works in actual life I made a fairly easy instance. I will use an identical method (as a result of Xcode playgrounds are nonetheless freezing each 1-5 minutes) like I did for the command sample, however the objective of this one goes to be virtually completely completely different, as a result of we’re speaking about delegation. 😅

#!/usr/bin/env swift

import Basis

protocol InputDelegate {

    var shouldContinueListening: Bool { get }

    func didStartListening()
    func didReceive(enter: String)

class InputHandler {

    var delegate: InputDelegate?

    func pay attention() {

        repeat {
            guard let enter = readLine() else {
            self.delegate?.didReceive(enter: enter)
        whereas self.delegate?.shouldContinueListening ?? false

struct InputReceiver: InputDelegate {

    var shouldContinueListening: Bool {
        return true

    func didStartListening() {
        print("👻 Please be good and say "hello", if you wish to go away simply inform me "bye":")

    func didReceive(enter: String) {
        change enter {
        case "hello":
            print("🌎 Hi there world!")
        case "bye":
            print("👋 Bye!")
            print("🔍 Command not discovered! Please attempt once more:")

let inputHandler = InputHandler()
let inputReceiver = InputReceiver()
inputHandler.delegate = inputReceiver attention()

That is how one can create your individual delegate sample in Swift. You possibly can think about that Apple is doing the identical factor below the hood, with UICollectionViewDataSource, UICollectionViewDelegate and many others. You solely need to implement the delegate, they will present the protocol and the delegator. 🤔

Weak properties, delegates and lessons

Reminiscence administration is a vital factor so it is price to say that every one the category delegates ought to be weak properies, otherwise you’ll create a extremely dangerous reatin cycle. 😱

protocol InputDelegate: class {  }

class InputHandler {

    weak var delegate: InputDelegate?


class InputReceiver: InputDelegate {

Right here is the altered Swift code snippet, however now utilizing a category because the delegate. You simply have to alter your protocol just a little bit and the property contained in the delegator. All the time use weak delegate variables if you will assign a category as a delegate. ⚠️

As you’ll be able to see delegation is fairly straightforward, however it may be harmful. It helps decoupling by offering a standard interface that can be utilized by anybody who implements the delegate (generally knowledge supply) protocol. There are actually wonderful articles about delegates, if you would like to know extra about this sample, it’s best to verify them out.



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