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Swift delegate design sample – The.Swift.Dev.

Implementing delegation in Swift

You will want a delegate protocol, a delegator who really 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 reviews a bug. The mission supervisor creates a problem 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 typical interface (difficulty describing the issue for each get together) to do obtain one thing (repair the 🐛).

To display how delegation works in actual life I made a reasonably easy instance. I’ll use an analogous method (as a result of Xcode playgrounds are nonetheless freezing each 1-5 minutes) like I did for the command sample, however the goal of this one goes to be nearly fully totally 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) {
        swap enter {
        case "hello":
            print("🌎 Hey 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 personal delegate sample in Swift. You possibly can think about that Apple is doing the identical factor below the hood, with UICollectionViewDataSource, UICollectionViewDelegate and so forth. You solely should implement the delegate, they will present the protocol and the delegator. 🤔

Weak properties, delegates and courses

Reminiscence administration is an important factor so it is price to say that each one the category delegates must 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 vary your protocol somewhat bit and the property contained in the delegator. All the time use weak delegate variables if you’re going to assign a category as a delegate. ⚠️

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



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