Delegates in swift?

How does one go about making a delegate, i.e. NSUserNotificationCenterDelegate in swift?

Here's a little help on delegates between two view controllers:

Step 1: Make a protocol in the UIViewController that you will be removing/will be sending the data.

protocol FooTwoViewControllerDelegate:class {
    func myVCDidFinish(_ controller: FooTwoViewController, text: String)
}

Step2: Declare the delegate in the sending class (i.e. UIViewcontroller)

class FooTwoViewController: UIViewController {
    weak var delegate: FooTwoViewControllerDelegate?
    [snip...]
}

Step3: Use the delegate in a class method to send the data to the receiving method, which is any method that adopts the protocol.

@IBAction func saveColor(_ sender: UIBarButtonItem) {
        delegate?.myVCDidFinish(self, text: colorLabel.text) //assuming the delegate is assigned otherwise error
}

Step 4: Adopt the protocol in the receiving class

class ViewController: UIViewController, FooTwoViewControllerDelegate {

Step 5: Implement the delegate method

func myVCDidFinish(_ controller: FooTwoViewController, text: String) {
    colorLabel.text = "The Color is " +  text
    controller.navigationController.popViewController(animated: true)
}

Step 6: Set the delegate in the prepareForSegue:

override func prepare(for segue: UIStoryboardSegue, sender: Any?) {
    if segue.identifier == "mySegue" {
        let vc = segue.destination as! FooTwoViewController
        vc.colorString = colorLabel.text
        vc.delegate = self
    }
}
    

And that should work. This is of course just code fragments, but should give you the idea. For a long explanation of this code you can go over to my blog entry here:

segues and delegates

If you are interested in what's going on under the hood with a delegate I did write on that here:

under the hood with delegates

Delegates always confused me until I realized that a delegate is just a class that does some work for another class. It's like having someone else there to do all the dirty work for you that you don't want to do yourself.

I wrote a little story to illustrate this. Read it in a Playground if you like.

Once upon a time...

// MARK: Background to the story

// A protocol is like a list of rules that need to be followed.
protocol OlderSiblingDelegate: class {
    // The following command (ie, method) must be obeyed by any 
    // underling (ie, delegate) of the older sibling.
    func getYourNiceOlderSiblingAGlassOfWater()
}

// MARK: Characters in the story

class BossyBigBrother {
    
    // I can make whichever little sibling is around at 
    // the time be my delegate (ie, slave)
    weak var delegate: OlderSiblingDelegate?
    
    func tellSomebodyToGetMeSomeWater() {
        // The delegate is optional because even though 
        // I'm thirsty, there might not be anyone nearby 
        // that I can boss around.
        delegate?.getYourNiceOlderSiblingAGlassOfWater()
    }
}

// Poor little sisters have to follow (or at least acknowledge) 
// their older sibling's rules (ie, protocol)
class PoorLittleSister: OlderSiblingDelegate {

    func getYourNiceOlderSiblingAGlassOfWater() {
        // Little sis follows the letter of the law (ie, protocol),
        // but no one said exactly how she had to respond.
        print("Go get it yourself!")
    }
}

// MARK: The Story

// Big bro is laying on the couch watching basketball on TV.
let bigBro = BossyBigBrother()

// He has a little sister named Sally.
let sally = PoorLittleSister()

// Sally walks into the room. How convenient! Now big bro 
// has someone there to boss around.
bigBro.delegate = sally

// So he tells her to get him some water.
bigBro.tellSomebodyToGetMeSomeWater()

// Unfortunately no one lived happily ever after...

// The end.

In review, there are three key parts to making and using the delegate pattern.

1. the protocol that defines what the worker needs to do
2. the boss class that has a delegate variable, which it uses to tell the worker class what to do
3. the worker class that adopts the protocol and does what is required

Real life

In comparison to our Bossy Big Brother story above, delegates are often used for the following practical applications:

1. Communication: one class needs to send some information to another class.
   • Code example 1: sending data from one view controller to another
   • Code example 2: sending text input from a custom keyboard to a text field
2. Customization: one class wants to allow another class to customize it.

The great part is that these classes don't need to know anything about each other beforehand except that the delegate class conforms to the required protocol.

I highly recommend reading the following two articles. They helped me understand delegates even better than the documentation did.

• What is Delegation? – A Swift Developer’s Guide
• How Delegation Works – A Swift Developer’s Guide

One more note

Delegates that reference other classes that they do not own should use the weak keyword to avoid strong reference cycles. See this answer for more details.

It is not that different from obj-c. First, you have to specify the protocol in your class declaration, like following:

class MyClass: NSUserNotificationCenterDelegate

The implementation will look like following:

// NSUserNotificationCenterDelegate implementation
func userNotificationCenter(center: NSUserNotificationCenter, didDeliverNotification notification: NSUserNotification) {
    //implementation
}

func userNotificationCenter(center: NSUserNotificationCenter, didActivateNotification notification: NSUserNotification) {
    //implementation
}

func userNotificationCenter(center: NSUserNotificationCenter, shouldPresentNotification notification: NSUserNotification) -> Bool {
    //implementation
    return true
}

Of course, you have to set the delegate. For example:

NSUserNotificationCenter.defaultUserNotificationCenter().delegate = self;

I got few corrections to post of @MakeAppPie

First at all when you are creating delegate protocol it should conform to Class protocol. Like in example below.

protocol ProtocolDelegate: class {
    func myMethod(controller:ViewController, text:String)
}

Second, your delegate should be weak to avoid retain cycle.

class ViewController: UIViewController {
    weak var delegate: ProtocolDelegate?
}

Last, you're safe because your protocol is an optional value. That means its "nil" message will be not send to this property. It's similar to conditional statement with respondToselector in objC but here you have everything in one line:

if ([self.delegate respondsToSelector:@selector(myMethod:text:)]) {
    [self.delegate myMethod:self text:@"you Text"];
}

Above you have an obj-C example and below you have Swift example of how it looks.

delegate?.myMethod(self, text:"your Text")

Here's a gist I put together. I was wondering the same and this helped improve my understanding. Open this up in an Xcode Playground to see what's going on.

protocol YelpRequestDelegate {
    func getYelpData() -> AnyObject
    func processYelpData(data: NSData) -> NSData
}
 
class YelpAPI {
    var delegate: YelpRequestDelegate?
    
    func getData() {
        println("data being retrieved...")
        let data: AnyObject? = delegate?.getYelpData()
    }
    
    func processYelpData(data: NSData) {
        println("data being processed...")
        let data = delegate?.processYelpData(data)
    }
}
 
class Controller: YelpRequestDelegate {
    init() {
        var yelpAPI = YelpAPI()
        yelpAPI.delegate = self
        yelpAPI.getData()
    }
    func getYelpData() -> AnyObject {
        println("getYelpData called")
        return NSData()
    }
    func processYelpData(data: NSData) -> NSData {
        println("processYelpData called")
        return NSData()
    }
}
 
var controller = Controller()

DELEGATES IN SWIFT 2

I am explaining with example of Delegate with two viewControllers.In this case, SecondVC Object is sending data back to first View Controller.

Class with Protocol Declaration

protocol  getDataDelegate  {
    func getDataFromAnotherVC(temp: String)
}


import UIKit
class SecondVC: UIViewController {

    var delegateCustom : getDataDelegate?
    override func viewDidLoad() {
        super.viewDidLoad()
     }

    override func didReceiveMemoryWarning() {
        super.didReceiveMemoryWarning()
        // Dispose of any resources that can be recreated.
    }
    @IBAction func backToMainVC(sender: AnyObject) {
      //calling method defined in first View Controller with Object  
      self.delegateCustom?.getDataFromAnotherVC(temp: "I am sending data from second controller to first view controller.Its my first delegate example. I am done with custom delegates.")
        self.navigationController?.popViewControllerAnimated(true)
    }
    
}

In First ViewController Protocol conforming is done here:

class ViewController: UIViewController, getDataDelegate

Protocol method definition in First View Controller(ViewController)

func getDataFromAnotherVC(temp : String)
{
  // dataString from SecondVC
   lblForData.text = dataString
}

During push the SecondVC from First View Controller (ViewController)

let objectPush = SecondVC()
objectPush.delegateCustom = self
self.navigationController.pushViewController(objectPush, animated: true)

First class:

protocol NetworkServiceDelegate: class {
        
    func didCompleteRequest(result: String)
}


class NetworkService: NSObject {

    weak var delegate: NetworkServiceDelegate?
    
    func fetchDataFromURL(url : String) {
        delegate?.didCompleteRequest(result: url)
    }
}

Second class:

class ViewController: UIViewController, NetworkServiceDelegate {
    
    let network = NetworkService()
    
    override func viewDidLoad() {
        super.viewDidLoad()
        network.delegate = self
        network.fetchDataFromURL(url: "Success!")
    }

 
    
    func didCompleteRequest(result: String) {
        print(result)
    }


}

Very easy step by step (100% working and tested)

step1: Create method on first view controller

 func updateProcessStatus(isCompleted : Bool){
    if isCompleted{
        self.labelStatus.text = "Process is completed"
    }else{
        self.labelStatus.text = "Process is in progress"
    }
}

step2: Set delegate while push to second view controller

@IBAction func buttonAction(_ sender: Any) {
    
    let secondViewController = self.storyboard?.instantiateViewController(withIdentifier: "secondViewController") as! secondViewController
    secondViewController.delegate = self
    self.navigationController?.pushViewController(secondViewController, animated: true)
}

step3: set delegate like

class ViewController: UIViewController,ProcessStatusDelegate {

step4: Create protocol

protocol ProcessStatusDelegate:NSObjectProtocol{
func updateProcessStatus(isCompleted : Bool)
}

step5: take a variable

var delegate:ProcessStatusDelegate?

step6: While go back to previous view controller call delegate method so first view controller notify with data

@IBAction func buttonActionBack(_ sender: Any) {
    delegate?.updateProcessStatus(isCompleted: true)
    self.navigationController?.popViewController(animated: true)
}

@IBAction func buttonProgress(_ sender: Any) {
    delegate?.updateProcessStatus(isCompleted: false)
    self.navigationController?.popViewController(animated: true)
    
}

Simple Example:

protocol Work: class {
	func doSomething()
}

class Manager {
	weak var delegate: Work?
	func passAlong() {
		delegate?.doSomething()
	}
}

class Employee: Work {
	func doSomething() {
		print("Working on it")
	}
}

let manager = Manager()
let developer = Employee()
manager.delegate = developer
manager.passAlong() // PRINTS: Working on it

Delegates are a design pattern that allows one object to send messages to another object when a specific event happens. Imagine an object A calls an object B to perform an action. Once the action is complete, object A should know that B has completed the task and take necessary action, this can be achieved with the help of delegates! Here is a tutorial implementing delegates step by step in swift 3

Tutorial Link

Here is real life delegate scenario
Lets make our own UITextField and UITextFieldDelegate

// THE MYSTERIOUS UITEXTFIELD

protocol UITextFieldDelegate {
    func textFieldDidChange(_ textField: UITextField) -> Void
}

class UITextField {
    var delegate: UITextFieldDelegate?
    private var mText: String?
    var text: String? {
        get {
            return mText
        }
    }
    
    init(text: String) {
        
    }
    
    init() {
        
    }
    
    func setText(_ text: String) {
        mText = text
        delegate?.textFieldDidChange(self)
    }

}

// HERE IS MY APP

class Main {
    
    let textfield = UITextField()
    
    func viewDidLoad() {
        print("viewDidLoad")
        textfield.delegate = self
        textfield.setText("Hello")
    }
    
}

extension Main: UITextFieldDelegate {
    
    func textFieldDidChange(_ textField: UITextField) {
        print(textField.text ?? "No string")
    }
    
}

let main = Main()
main.viewDidLoad()

Here Simple Code Example of Delegate:

//MARK: - Protocol ShowResult
protocol ShowResult: AnyObject {
       func show(value: Int) 
  }

//MARK: - MyOperation Class
class MyOperation {
weak var delegate: ShowResult?

func sum(fNumber: Int, sNumber: Int) {
    delegate?.show(value: fNumber + sNumber)
  }
}

//MARK: - ViewController Class
class ViewController: UIViewController,ShowResult {
var myOperation: MyOperation?

override func viewDidLoad() {
    super.viewDidLoad()
    loadMyOperation()
    myOperation?.delegate = self
    myOperation?.sum(fNumber: 100, sNumber: 20)
 }

private func loadMyOperation() {
    if myOperation == nil {
        myOperation = MyOperation()
     }
 }

func show(value: Int) {
    print("value: \(value)")
   }
}

The solutions above seemed a little coupled and at the same time avoid reuse the same protocol in other controllers, that's why I've come with the solution that is more strong typed using generic type-erasure.

@noreturn public func notImplemented(){
    fatalError("not implemented yet")
}


public protocol DataChangedProtocol: class{
    typealias DataType
    
    func onChange(t:DataType)
}

class AbstractDataChangedWrapper<DataType> : DataChangedProtocol{
    
    func onChange(t: DataType) {
        notImplemented()
    }
}


class AnyDataChangedWrapper<T: DataChangedProtocol> : AbstractDataChangedWrapper<T.DataType>{
    
    var base: T
    
    init(_ base: T ){
        self.base = base
    }
    
    override func onChange(t: T.DataType) {
        base.onChange(t)
    }
}


class AnyDataChangedProtocol<DataType> : DataChangedProtocol{
    
    var base: AbstractDataChangedWrapper<DataType>
    
    init<S: DataChangedProtocol where S.DataType == DataType>(_ s: S){
        self.base = AnyDataChangedWrapper(s)
    }
    
    func onChange(t: DataType) {
        base.onChange(t)
    }
}



class Source : DataChangedProtocol {
    func onChange(data: String) {
        print( "got new value \(data)" )
    }
}


class Target {
    var delegate: AnyDataChangedProtocol<String>?
    
    func reportChange(data:String ){
        delegate?.onChange(data)
    }
}


var source = Source()
var target = Target()

target.delegate = AnyDataChangedProtocol(source)
target.reportChange("newValue")    

output: got new value newValue

In swift 4.0

Create a delegate on class that need to send some data or provide some functionality to other classes

Like

protocol GetGameStatus {
    var score: score { get }
    func getPlayerDetails()
}

After that in the class that going to confirm to this delegate

class SnakesAndLadders: GetGameStatus {
    func getPlayerDetails() {

 }
}

In swift 5

I am a beginner, I think this is easiest way to understand in practical scenario

Note:Any improvisations are most appreciated

protocol APIService {
   func onSuccessResponse() -> AnyObject
   func onFailureResponse() -> AnyObject
}

class APIHelper{

var delegate : APIService?

func postUsersDataAPI() {
    //assuming API communication is success
    if(success){
       let _: AnyObject? = delegate?.onSuccessResponse()
    }else if(failure){
       let _: AnyObject? = delegate?.onFailureResponse()
  }
 }

func getAllUsersAPI() {
    //assuming API communication is success
   if(success){
       let _: AnyObject? = delegate?.onSuccessResponse()
    }else if(failure){
       let _: AnyObject? = delegate?.onFailureResponse()
  }
 }
}


class ViewController:UIViewController,APIService {

func onSuccessResponse() -> AnyObject {
    print("onSuccessResponse") as AnyObject
}

func onFailureResponse() -> AnyObject {
    print("onFailureResponse") as AnyObject
}

@IBAction func clickBtnToPostUserData(_ sender: Any) { 
    let apiHelper = APIHelper()
    apiHelper.delegate = self
    apiHelper.postAPI()
}