Delegates in swift?

Question

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

Answer 1

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;

Answer 2

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

Answer 3

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

Answer 4

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)

}

Answer 5

First class:

protocol NetworkServiceDelegate: class {

    func didCompleteRequest(result: String)
}


class NetworkService: NSObject {

    weak var delegate: NetworkServiceDelegate?

    func fetchDataFromURL(url : String) {
        delegate?.didCompleteRequest(url)
    }
}

Second class:

class ViewController: UIViewController, NetworkServiceDelegate {

    let network = NetworkService()

    override func viewDidLoad() {
        super.viewDidLoad()
        network.delegate = self
        network.fetchDataFromURL("Success!")
    }



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


}

Answer 6

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