What is delegate?

Ans : Delegate is means of communication between objects of iOS Applications. Delegate allows one object to send message to another object when an event occurs.

i.e

UIAlertView *alert = [[UIAlertView alloc] initWithTitle:@”title” message:@”message” delegate:self cancelButtonTitle:@”Ok” otherButtonTitle:nil];

Here delegate is self. So now self is responsible for handling  all event fired by this instance of UIAlertView class.

Which button of UIAlertView is clicked, for that, event is clickedButtonAtIndex is called in or by Self or currentViewController.

Create PreDefined delegate :

1. There are two ViewController NameVC and SurNameVC.
2.  In NameVC, there are 2 textfield named as Name and FullName and 1 button as Submit.
3. If I write in name and click on submit, it went to SurNameVC to take Surname parameter.
4. On SurNameVC, after write Surname, on clicking of Submit, It call delegate method and went back to NameVC and Print Full Name in FullName textfield.

Implement above delegate and protocol in Objective-C : 

I have made protocol on SurNameVC like

    @protocol SurNameVCDelegate

      -(void)setSurName:(NSString *) strSurName;

    @end

    

    @property (nonatomic, retain) id delegate;

Now on NameVC submit button click, choose delegate of  SurNameVC object as self.

   objSurNameVC.delegate = self

and create method -(void)setSurName:(NSString *) strSurName;

on NameVC and it is called from surNameVC submit button. So setSurName is delegate method. We can print fullname by concatenating Name and Surname in FullName textfield.

So we delegate just pass message from one view controller to another view controller by delegate method.

Implement delegate and protocol in Swift : 

I have made custom UISlider. I want to send some value from custom UISlider value changed to view controller in which it is used. So for that, I have used delegate - protocol method.

customSlider.swift  Custom Slider file

import UIKit

protocol SliderDelegate: class {

    func sliderValueChanged(_ sender : UISlider)

}

class mpgpsSlider: UIView {

     

      weak var delegate : SliderDelegate?

      required init?(coder aDecoder: NSCoder) {

        super.init(coder: aDecoder)

        

        let bundle = Bundle.init(for: type(of: self))

        let nib = UINib(nibName: "Slider", bundle: bundle)

        let view = nib.instantiate(withOwner: self, options: nil)[0] as! UIView

        view.frame = bounds

        view.autoresizingMask = [.flexibleWidth,.flexibleHeight]

        addSubview(view)

        

        slider.addTarget(self, action: #selector(sliderValueChanged(_:)), for:                  .valueChanged)

    }

     @objc func sliderValueChanged(_ sender : UISlider)  {

        delegate?.sliderValueChanged(sender)

      }

}

ViewController.swift ViewController in which custom slider is used.

import UIKit

class VehicleProfileVC: BaseViewController,SliderDelegate{

    

    override func viewDidLoad() {

        super.viewDidLoad()

        slider.delegate = self

    }

    

    func sliderValueChanged(_ sender: UISlider) {

        label.text = String(sender.value)

    }

}