By Andy Jacobs

2009-03-09 16:06:14 8 Comments

I know how delegates work, and I know how I can use them.

But how do I create them?


@Jesse Rusak 2009-03-09 16:16:24

An Objective-C delegate is an object that has been assigned to the delegate property another object. To create one, you simply define a class that implements the delegate methods you're interested in, and mark that class as implementing the delegate protocol.

For example, suppose you have an UIWebView. If you'd like to implement its delegate's webViewDidStartLoad: method, you could create a class like this:

@interface MyClass<UIWebViewDelegate>
// ...

@implementation MyClass
- (void)webViewDidStartLoad:(UIWebView *)webView { 
    // ... 

Then you could create an instance of MyClass and assign it as the web view's delegate:

MyClass *instanceOfMyClass = [[MyClass alloc] init];
myWebView.delegate = instanceOfMyClass;

On the UIWebView side, it probably has code similar to this to see if the delegate responds to the webViewDidStartLoad: message using respondsToSelector: and send it if appropriate.

if([self.delegate respondsToSelector:@selector(webViewDidStartLoad:)]) {
    [self.delegate webViewDidStartLoad:self];

The delegate property itself is typically declared weak (in ARC) or assign (pre-ARC) to avoid retain loops, since the delegate of an object often holds a strong reference to that object. (For example, a view controller is often the delegate of a view it contains.)

Making Delegates for Your Classes

To define your own delegates, you'll have to declare their methods somewhere, as discussed in the Apple Docs on protocols. You usually declare a formal protocol. The declaration, paraphrased from UIWebView.h, would look like this:

@protocol UIWebViewDelegate <NSObject>
- (void)webViewDidStartLoad:(UIWebView *)webView;
// ... other methods here

This is analogous to an interface or abstract base class, as it creates a special type for your delegate, UIWebViewDelegate in this case. Delegate implementors would have to adopt this protocol:

@interface MyClass <UIWebViewDelegate>
// ...

And then implement the methods in the protocol. For methods declared in the protocol as @optional (like most delegate methods), you need to check with -respondsToSelector: before calling a particular method on it.


Delegate methods are typically named starting with the delegating class name, and take the delegating object as the first parameter. They also often use a will-, should-, or did- form. So, webViewDidStartLoad: (first parameter is the web view) rather than loadStarted (taking no parameters) for example.

Speed Optimizations

Instead of checking whether a delegate responds to a selector every time we want to message it, you can cache that information when delegates are set. One very clean way to do this is to use a bitfield, as follows:

@protocol SomethingDelegate <NSObject>
- (void)something:(id)something didFinishLoadingItem:(id)item;
- (void)something:(id)something didFailWithError:(NSError *)error;

@interface Something : NSObject
@property (nonatomic, weak) id <SomethingDelegate> delegate;

@implementation Something {
  struct {
    unsigned int didFinishLoadingItem:1;
    unsigned int didFailWithError:1;
  } delegateRespondsTo;
@synthesize delegate;

- (void)setDelegate:(id <SomethingDelegate>)aDelegate {
  if (delegate != aDelegate) {
    delegate = aDelegate;

    delegateRespondsTo.didFinishLoadingItem = [delegate respondsToSelector:@selector(something:didFinishLoadingItem:)];
    delegateRespondsTo.didFailWithError = [delegate respondsToSelector:@selector(something:didFailWithError:)];

Then, in the body, we can check that our delegate handles messages by accessing our delegateRespondsTo struct, rather than by sending -respondsToSelector: over and over again.

Informal Delegates

Before protocols existed, it was common to use a category on NSObject to declare the methods a delegate could implement. For example, CALayer still does this:

@interface NSObject(CALayerDelegate)
- (void)displayLayer:(CALayer *)layer;
// ... other methods here

This essentially tells the compiler that any object might implement displayLayer:.

You would then use the same -respondsToSelector: approach as described above to call this method. Delegates simply implement this method and assign the delegate property, and that's it (there's no declaring you conform to a protocol). This method is common in Apple's libraries, but new code should use the more modern protocol approach above, since this approach pollutes NSObject (which makes autocomplete less useful) and makes it hard for the compiler to warn you about typos and similar errors.

@Roland 2013-11-25 08:34:55

I think you need to cast the unsigned int type to BOOL as the return value of delegate respondsToSelector is of type BOOL.

@user4657588 2015-04-23 15:27:51

Can delegate be used for Polymorphism like in C++?

@Jesse Rusak 2015-04-23 20:09:22

@Dan Yes, sure. Protocols in general are used for polymorphism.

@Marin 2016-12-17 20:06:07

Hi Jesse, quick question, how can I set delegate = self, if I cannot instantiate an object because it is a generic class which I don't have access to in the other class, yet I want the generics class to call a function in the other class, hence the need for delegate?

@Jesse Rusak 2016-12-17 20:10:57

@Marin you should probably ask a question about that with more details rather than using a comment.

@Marin 2016-12-17 20:15:22

Hi Jesse, actually I was referring to my question here:… Thanks

@Hans Knöchel 2017-01-23 16:07:17

@JesseRusak I think "JSSomethingDelegate" should be "SomethingDelegate" for consistency :)

@Miras Maratuly 2018-03-16 04:40:48

Answer is actually answered, but I would like to give you a "cheat sheet" for creating a delegate:


CLASS A - Where delegate is calling function

@protocol <#Protocol Name#> <NSObject>



@interface <#Some ViewController#> : <#UIViewController#> 

@property (nonatomic, assign) id <<#Protocol Name#>> delegate;


@implementation <#Some ViewController#> 

-(void)someMethod {
    [self.delegate methodName];


CLASS B - Where delegate is called 

@interface <#Other ViewController#> (<#Delegate Name#>) {}

@implementation <#Other ViewController#> 

-(void)otherMethod {
    CLASSA *classA = [[CLASSA alloc] init];

    [classA setDelegate:self];

-delegateMethod() {



@James Rochabrun 2017-04-26 05:27:43

Disclaimer: this is the Swift version of how to create a delegate.

So, what are delegates? …in software development, there are general reusable solution architectures that help to solve commonly occurring problems within a given context, these “templates”, so to speak, are best known as design patterns. 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!

For a better explanation, I am going to show you how to create a custom delegate that passes data between classes, with Swift in a simple application,start by downloading or cloning this starter project and run it!

You can see an app with two classes, ViewController A and ViewController B. B has two views that on tap changes the background color of the ViewController, nothing too complicated right? well now let’s think in an easy way to also change the background color of class A when the views on class B are tapped.

The problem is that this views are part of class B and have no idea about class A, so we need to find a way to communicate between this two classes, and that’s where delegation shines. I divided the implementation into 6 steps so you can use this as a cheat sheet when you need it.

step 1: Look for the pragma mark step 1 in ClassBVC file and add this

//MARK: step 1 Add Protocol here.
protocol ClassBVCDelegate: class {
func changeBackgroundColor(_ color: UIColor?)

The first step is to create a protocol, in this case, we will create the protocol in class B, inside the protocol you can create as many functions that you want based on the requirements of your implementation. In this case, we just have one simple function that accepts an optional UIColor as an argument. Is a good practice to name your protocols adding the word delegate at the end of the class name, in this case, ClassBVCDelegate.

step 2: Look for the pragma mark step 2 in ClassVBC and add this

//MARK: step 2 Create a delegate property here.
weak var delegate: ClassBVCDelegate?

Here we just create a delegate property for the class, this property must adopt the protocol type, and it should be optional. Also, you should add the weak keyword before the property to avoid retain cycles and potential memory leaks, if you don’t know what that means don’t worry for now, just remember to add this keyword.

step 3: Look for the pragma mark step 3 inside the handleTap method in ClassBVC and add this

//MARK: step 3 Add the delegate method call here.

One thing that you should know, run the app and tap on any view, you won’t see any new behavior and that’s correct but the thing that I want to point out is that the app it’s not crashing when the delegate is called, and it’s because we create it as an optional value and that’s why it won’t crash even the delegated doesn’t exist yet. Let’s now go to ClassAVC file and make it, the delegated.

step 4: Look for the pragma mark step 4 inside the handleTap method in ClassAVC and add this next to your class type like this.

//MARK: step 4 conform the protocol here.
class ClassAVC: UIViewController, ClassBVCDelegate {

Now ClassAVC adopted the ClassBVCDelegate protocol, you can see that your compiler is giving you an error that says “Type ‘ClassAVC does not conform to protocol ‘ClassBVCDelegate’ and this only means that you didn’t use the methods of the protocol yet, imagine that when class A adopts the protocol is like signing a contract with class B and this contract says “Any class adopting me MUST use my functions!”

Quick note: If you come from an Objective-C background you are probably thinking that you can also shut up that error making that method optional, but for my surprise, and probably yours, Swift language does not support optional protocols, if you want to do it you can create an extension for your protocol or use the @objc keyword in your protocol implementation.

Personally, If I have to create a protocol with different optional methods I would prefer to break it into different protocols, that way I will follow the concept of giving one single responsibility to my objects, but it can vary based on the specific implementation.

here is a good article about optional methods.

step 5: Look for the pragma mark step 5 inside the prepare for segue method and add this

//MARK: step 5 create a reference of Class B and bind them through the `prepareforsegue` method.
if let nav = segue.destination as? UINavigationController, let classBVC = nav.topViewController as? ClassBVC {
classBVC.delegate = self

Here we are just creating an instance of ClassBVC and assign its delegate to self, but what is self here? well, self is the ClassAVC which has been delegated!

step 6: Finally, look for the pragma step 6 in ClassAVC and let’s use the functions of the protocol, start typing func changeBackgroundColor and you will see that it’s auto-completing it for you. You can add any implementation inside it, in this example, we will just change the background color, add this.

//MARK: step 6 finally use the method of the contract
func changeBackgroundColor(_ color: UIColor?) {
view.backgroundColor = color

Now run the app!

Delegates are everywhere and you probably use them without even notice, if you create a tableview in the past you used delegation, many classes of UIKIT works around them and many other frameworks too, they solve these main problems.

  • Avoid tight coupling of objects.
  • Modify behavior and appearance without the need to subclass objects.
  • Allow tasks to be handled off to any arbitrary object.

Congratulations, you just implement a custom delegate, I know that you are probably thinking, so much trouble just for this? well, delegation is a very important design pattern to understand if you want to become an iOS developer, and always keep in mind that they have one to one relationship between objects.

You can see the original tutorial here

@Mahesh 2015-08-03 04:47:04

To create your own delegate, first you need to create a protocol and declare the necessary methods, without implementing. And then implement this protocol into your header class where you want to implement the delegate or delegate methods.

A protocol must be declared as below:

@protocol ServiceResponceDelegate <NSObject>

- (void) serviceDidFailWithRequestType:(NSString*)error;
- (void) serviceDidFinishedSucessfully:(NSString*)success;


This is the service class where some task should be done. It shows how to define delegate and how to set the delegate. In the implementation class after the task is completed the delegate's the methods are called.

@interface ServiceClass : NSObject
id <ServiceResponceDelegate> _delegate;

- (void) setDelegate:(id)delegate;
- (void) someTask;


@implementation ServiceClass

- (void) setDelegate:(id)delegate
_delegate = delegate;

- (void) someTask

   perform task

if (!success)
[_delegate serviceDidFailWithRequestType:@”task failed”];
[_delegate serviceDidFinishedSucessfully:@”task success”];

This is the main view class from where the service class is called by setting the delegate to itself. And also the protocol is implemented in the header class.

@interface viewController: UIViewController <ServiceResponceDelegate>
ServiceClass* _service;

- (void) go;


@implementation viewController

//some methods

- (void) go
_service = [[ServiceClass alloc] init];
[_service setDelegate:self];
[_service someTask];

That's it, and by implementing delegate methods in this class, control will come back once the operation/task is done.

@Ellen 2017-07-14 10:14:49

Let's start with an example , if we buy a product online ,it goes through process like shipping/delivery handled by different teams.So if shipping gets completed ,shipping team should notify delivery team & it should be one to one communication as broadcasting this information would be overhead for other people / vendor might want to pass this information only to required people.

So if we think in terms of our app, an event can be an online order & different teams can be like multiple views.

Here is code consider ShippingView as Shipping team & DeliveryView as delivery team :

//Declare the protocol with functions having info which needs to be communicated
protocol ShippingDelegate : class {
    func productShipped(productID : String)
//shippingView which shows shipping status of products
class ShippingView : UIView

    weak var delegate:ShippingDelegate?
    var productID : String

    @IBAction func checkShippingStatus(sender: UIButton)
        // if product is shipped
        delegate?.productShipped(productID: productID)
//Delivery view which shows delivery status & tracking info
class DeliveryView: UIView,ShippingDelegate
    func productShipped(productID : String)
        // update status on view & perform delivery

//Main page on app which has both views & shows updated info on product whole status
class ProductViewController : UIViewController
    var shippingView : ShippingView
    var deliveryView : DeliveryView

    override func viewDidLoad() {
        // as we want to update shipping info on delivery view, so assign delegate to delivery object
        // whenever shipping status gets updated it will call productShipped method in DeliveryView & update UI.
        shippingView.delegate = deliveryView

@User558 2017-02-08 09:30:59

In my point of view create separate class for that delegate method and you can use where you want.

in my Custom DropDownClass.h

typedef enum

@protocol DropDownListDelegate <NSObject>
- (void)dropDownDidSelectItemWithString:(NSString*)itemString     DropDownType:(DropDownType)dropDownType;
@interface DropDownViewController : UIViewController
 BOOL isFiltered;
@property (nonatomic, assign) DropDownType dropDownType;
@property (weak) id <DropDownListDelegate> delegate;
@property (strong, nonatomic) NSMutableArray *array1DropDown;
@property (strong, nonatomic) NSMutableArray *array2DropDown;

after that in.m file create array with objects,

 - (CGFloat)tableView:(UITableView *)tableView heightForRowAtIndexPath:(NSIndexPath *)indexPath{
CGFloat rowHeight = 44.0f;
return rowHeight;

-(NSInteger)numberOfSectionsInTableView:(UITableView *)tableView {
return 1;
- (NSInteger)tableView:(UITableView *)tableView numberOfRowsInSection:(NSInteger)section{
return isFiltered?[self.array1DropDown count]:[self.array2DropDown count];
- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath {
static NSString *simpleTableIdentifier = @"TableCell";
UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:simpleTableIdentifier];
if (cell == nil) {
    cell = [[UITableViewCell alloc] initWithStyle:UITableViewCellStyleDefault reuseIdentifier:simpleTableIdentifier];

if (self.delegate) {
    if (self.dropDownType == DDCITY) {
        cell.textLabel.text = [self.array1DropDown objectAtIndex:indexPath.row];
    else if (self.dropDownType == DDSTATE) {
        cell.textLabel.text = [self.array2DropDown objectAtIndex:indexPath.row];
return cell;

- (void)tableView:(UITableView *)tableView didSelectRowAtIndexPath:(NSIndexPath *)indexPath
 [self dismissViewControllerAnimated:YES completion:^{
        if(self.dropDownType == DDCITY){
            [self.delegate dropDownDidSelectItemWithString:[self.array1DropDown objectAtIndex:indexPath.row] DropDownType:self.dropDownType];
        else if (self.dropDownType == DDSTATE) {
            [self.delegate dropDownDidSelectItemWithString:[self.array2DropDown objectAtIndex:indexPath.row] DropDownType:self.dropDownType];

Here all are set for Custom delegate class.after that you can use this delegate method where you want.for example...

in my another viewcontroller import after that

create action for calling delegate method like this

- (IBAction)dropDownBtn1Action:(id)sender {
DropDownViewController *vehicleModelDropView = [[DropDownViewController alloc]init];
vehicleModelDropView.dropDownType = DDCITY;
vehicleModelDropView.delegate = self;
[self presentViewController:vehicleModelDropView animated:YES completion:nil];

after that call delegate method like this

- (void)dropDownDidSelectItemWithString:(NSString *)itemString DropDownType:(DropDownType)dropDownType {
switch (dropDownType) {
    case DDCITY:{
        if(itemString.length > 0){
            //Here i am printing the selected row
            [self.dropDownBtn1 setTitle:itemString forState:UIControlStateNormal];
    case DDSTATE: {
        //Here i am printing the selected row
        [self.dropDownBtn2 setTitle:itemString forState:UIControlStateNormal];


@Rohit Kashyap 2017-01-02 16:31:44

//Custom delegate 
@protocol TB_RemovedUserCellTag <NSObject>



//Create a weak reference in a class where you declared the delegate
@property(weak,nonatomic)id <TB_RemovedUserCellTag> removedCellTagDelegate;

// use it in the class
  [self.removedCellTagDelegate didRemoveCellWithTag:self.tag];

//4. import the header file in the class where you want to conform to the protocol
@interface MyClassUsesDelegate ()<TB_RemovedUserCellTag>


//5. Implement the method in the class .m -(void)didRemoveCellWithTag:(NSInteger)tag { [email protected]("Tag %d",tag);


@Jean 2013-08-04 08:39:59

As a good practice recommended by Apple, it's good for the delegate (which is a protocol, by definition), to conform to NSObject protocol.

@protocol MyDelegate <NSObject>

& to create optional methods within your delegate (i.e. methods which need not necessarily be implemented), you can use the @optional annotation like this :

@protocol MyDelegate <NSObject>
      // Declaration for Methods that 'must' be implemented'
      // Declaration for Methods that 'need not necessarily' be implemented by the class conforming to your delegate

So when using methods that you have specified as optional, you need to (in your class) check with respondsToSelector if the view (that is conforming to your delegate) has actually implemented your optional method(s) or not.

@Lal Krishna 2016-12-19 04:14:58


@protocol NameDelegate <NSObject>

-(void)delegateMEthod: (ArgType) arg;


@property id <NameDelegate> delegate;


[self.delegate delegateMEthod: argument];


ViewController viewController = [ViewController new];
viewController.delegate = self;


-(void)delegateMEthod: (ArgType) arg{

@Vaibhav Gaikwad 2016-12-08 10:50:58

Delegate :- Create

@protocol addToCartDelegate <NSObject>

-(void)addToCartAction:(ItemsModel *)itemsModel isAdded:(BOOL)added;


Send and please assign delegate to view you are sending data

[self.delegate addToCartAction:itemsModel isAdded:YES];

@m.eldehairy 2015-04-08 13:24:20

lets say you have a class that you developed and want to declare a delegate property to be able to notify it when some event happens :

@class myClass;

@protocol myClassDelegate <NSObject>

-(void)myClass:(MyClass*)myObject requiredEventHandlerWithParameter:(ParamType*)param;

-(void)myClass:(MyClass*)myObject optionalEventHandlerWithParameter:(ParamType*)param;


@interface MyClass : NSObject

@property(nonatomic,weak)id< MyClassDelegate> delegate;


so you declare a protocol in MyClass header file (or a separate header file) , and declare the required/optional event handlers that your delegate must/should implement , then declare a property in MyClass of type (id< MyClassDelegate>) which means any objective c class that conforms to the protocol MyClassDelegate , you'll notice that the delegate property is declared as weak , this is very important to prevent retain cycle (most often the delegate retains the MyClass instance so if you declared the delegate as retain, both of them will retain each other and neither of them will ever be released).

you will notice also that the protocol methods passes the MyClass instance to the delegate as parameter , this is best practice in case the delegate want to call some methods on MyClass instance and also helps when the delegate declares itself as MyClassDelegate to multiple MyClass instances , like when you have multiple UITableView's instances in your ViewController and declares itself as a UITableViewDelegate to all of them.

and inside your MyClass you notify the delegate with declared events as follows :

if([_delegate respondsToSelector:@selector(myClass: requiredEventHandlerWithParameter:)])
     [_delegate myClass:self requiredEventHandlerWithParameter:(ParamType*)param];

you first check if your delegate responds to the protocol method that you are about to call in case the delegate doesn't implement it and the app will crash then (even if the protocol method is required).

@Suragch 2015-11-05 17:11:08

Swift version

A delegate is just a class that does some work for another class. Read the following code for a somewhat silly (but hopefully enlightening) Playground example that shows how this is done in Swift.

// A protocol is just a list of methods (and/or properties) that must
// be used by any class that adopts the protocol.
protocol OlderSiblingDelegate: class {
    // This protocol only defines one required method
    func getYourNiceOlderSiblingAGlassOfWater() -> String

class BossyBigBrother {

    // The delegate is the BossyBigBrother's slave. This position can 
    // be assigned later to whoever is available (and conforms to the 
    // protocol).
    weak var delegate: OlderSiblingDelegate?

    func tellSomebodyToGetMeSomeWater() -> String? {
        // The delegate is optional because there might not be anyone
        // nearby to boss around.
        return delegate?.getYourNiceOlderSiblingAGlassOfWater()

// PoorLittleSister conforms to the OlderSiblingDelegate protocol
class PoorLittleSister: OlderSiblingDelegate {

    // This method is repquired by the protocol, but the protocol said
    // nothing about how it needs to be implemented.
    func getYourNiceOlderSiblingAGlassOfWater() -> String {
        return "Go get it yourself!"


// initialize the classes
let bigBro = BossyBigBrother()
let lilSis = PoorLittleSister()

// Set the delegate 
// bigBro could boss around anyone who conforms to the 
// OlderSiblingDelegate protocol, but since lilSis is here, 
// she is the unlucky choice.
bigBro.delegate = lilSis

// Because the delegate is set, there is a class to do bigBro's work for him.
// bigBro tells lilSis to get him some water.
if let replyFromLilSis = bigBro.tellSomebodyToGetMeSomeWater() {
    print(replyFromLilSis) // "Go get it yourself!"

In actual practice, delegates are often used in the following situations

  1. When a class needs to communicate some information to another class
  2. When a class wants to allow another class to customize it

The 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.

@Sujania 2015-09-29 09:23:09

Here is a simple method to create delegates

Create Protocol in .h file. Make sure that is defined before the protocol using @class followed by the name of the UIViewController < As the protocol I am going to use is UIViewController class>.

Step : 1 : Create a new class Protocol named "YourViewController" which will be the subclass of UIViewController class and assign this class to the second ViewController.

Step : 2 : Go to the "YourViewController" file and modify it as below:

#import <UIKit/UIkit.h>
@class YourViewController;

@protocol YourViewController Delegate <NSObject>

-(void)defineDelegateMethodName: (YourViewController *) controller;

-(BOOL)delegateMethodReturningBool: (YourViewController *) controller;

  @interface YourViewController : UIViewController

  //Since the property for the protocol could be of any class, then it will be marked as a type of id.

  @property (nonatomic, weak) id< YourViewController Delegate> delegate;


The methods defined in the protocol behavior can be controlled with @optional and @required as part of the protocol definition.

Step : 3 : Implementation of Delegate

    #import "delegate.h"

   @interface YourDelegateUser ()

   @implementation YourDelegateUser

   - (void) variousFoo {
      YourViewController *controller = [[YourViewController alloc] init];
      controller.delegate = self;

   -(void)defineDelegateMethodName: (YourViewController *) controller {
      // handle the delegate being called here

   -(BOOL)delegateMethodReturningBool: (YourViewController *) controller {
      // handle the delegate being called here
      return YES;


//test whether the method has been defined before you call it

 - (void) someMethodToCallDelegate {
     if ([[self delegate] respondsToSelector:@selector(defineDelegateMethodName:)]) {
           [self.delegate delegateMethodName:self]; 

@Tibidabo 2013-07-17 04:58:08

Ok, this is not really an answer to the question, but if you are looking up how to make your own delegate maybe something far simpler could be a better answer for you.

I hardly implement my delegates because I rarely need. I can have ONLY ONE delegate for a delegate object. So if you want your delegate for one way communication/passing data than you are much better of with notifications.

NSNotification can pass objects to more than one recipients and it is very easy to use. It works like this:

MyClass.m file should look like this

#import "MyClass.h"
@implementation MyClass 

- (void) myMethodToDoStuff {
//this will post a notification with myClassData (NSArray in this case)  in its userInfo dict and self as an object
[[NSNotificationCenter defaultCenter] postNotificationName:@"myClassUpdatedData"
                                                  userInfo:[NSDictionary dictionaryWithObject:selectedLocation[@"myClassData"] forKey:@"myClassData"]];

To use your notification in another classes: Add class as an observer:

[[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(otherClassUpdatedItsData:) name:@"myClassUpdatedData" object:nil];

Implement the selector:

- (void) otherClassUpdatedItsData:(NSNotification *)note {
    NSLog(@"*** Other class updated its data ***");
    MyClass *otherClass = [note object];  //the object itself, you can call back any selector if you want
    NSArray *otherClassData = [note userInfo][@"myClassData"]; //get myClass data object and do whatever you want with it

Don't forget to remove your class as an observer if

- (void)dealloc
    [[NSNotificationCenter defaultCenter] removeObserver:self];

@Tibidabo 2012-09-30 10:25:40

The approved answer is great, but if you're looking for a 1 minute answer try this:

MyClass.h file should look like this (add delegate lines with comments!)

#import <BlaClass/BlaClass.h>

@class MyClass;             //define class, so protocol can see MyClass
@protocol MyClassDelegate <NSObject>   //define delegate protocol
    - (void) myClassDelegateMethod: (MyClass *) sender;  //define delegate method to be implemented within another class
@end //end protocol

@interface MyClass : NSObject {
@property (nonatomic, weak) id <MyClassDelegate> delegate; //define MyClassDelegate as delegate


MyClass.m file should look like this

#import "MyClass.h"
@implementation MyClass 
@synthesize delegate; //synthesise  MyClassDelegate delegate

- (void) myMethodToDoStuff {
    [self.delegate myClassDelegateMethod:self]; //this will call the method implemented in your other class    


To use your delegate in another class (UIViewController called MyVC in this case) MyVC.h:

#import "MyClass.h"
@interface MyVC:UIViewController <MyClassDelegate> { //make it a delegate for MyClassDelegate


myClass.delegate = self;          //set its delegate to self somewhere

Implement delegate method

- (void) myClassDelegateMethod: (MyClass *) sender {
    NSLog(@"Delegates are great!");

@Arno van der Meer 2013-04-25 07:16:23

Great to use this answer as a quick reference. But why is the delegate property in your MyClass.h marked as 'IBOutlet'?

@Tibidabo 2013-04-28 13:05:39

@ArnovanderMeer Good catch! I can't remember why. I need it in my project but not in this example, I removed it. thx

@svmrajesh 2014-01-28 10:52:19

@Tibidabo : Its really awesome.

@Jason McDermott 2014-02-19 01:13:37

Very useful, thanks.

@Tom Kincaid 2014-02-21 00:38:37

This is the much better way.

@Fattie 2014-03-04 23:20:52

Beautiful and perfect answer.

@sudo 2014-03-13 01:14:47

Thanks. As nice and thorough as the accepted answer is, I learn best from some compact sample code. It's good to have two answers.

@wz366 2014-04-20 07:26:12

Why you synthesize MyClassDelegate delegate?

@CalZone 2014-06-07 15:47:55

More precise answer than the marked one. Thanks a bunch!

@Joze 2015-04-07 08:14:39

myClass.delegate on MyVC doesn't find the property delegate. I synthesized on myClass the delegate property but it can't find it on MyVC. What could I be doing wrong?.?

@Charles Robertson 2016-11-26 18:23:50

@Tibidabo Totally outstanding. I really wish everyone could explain programming concepts like this. I have seen hundreds of explanations, about 'delegates', over the years and have never really grasped this theory until now! Thank you so much...

@Lane Rettig 2017-02-10 02:15:42

Where is myClass instantiated inside MyVC.m?

@DrBug 2013-06-19 10:34:42

I think all these answers make a lot of sense once you understand delegates. Personally I came from the land of C/C++ and before that procedural languages like Fortran etc so here is my 2 min take on finding similar analogues in C++ paradigm.

If I were to explain delegates to a C++/Java programmer I would say

What are delegates ? These are static pointers to classes within another class. Once you assign a pointer, you can call functions/methods in that class. Hence some functions of your class are "delegated" (In C++ world - pointer to by a class object pointer) to another class.

What are protocols ? Conceptually it serves as similar purpose as to the header file of the class you are assigning as a delegate class. A protocol is a explicit way of defining what methods needs to be implemented in the class who's pointer was set as a delegate within a class.

How can I do something similar in C++? If you tried to do this in C++, you would by defining pointers to classes (objects) in the class definition and then wiring them up to other classes that will provide additional functions as delegates to your base class. But this wiring needs to be maitained within the code and will be clumsy and error prone. Objective C just assumes that programmers are not best at maintaining this decipline and provides compiler restrictions to enforce a clean implementation.

@DrBug 2014-12-23 10:34:21

What you are talking about is semantics while I was talking about the intuition. What you are talking about is virtual function - but just to get used to the new terminology can be challenging. The answer serves beginers who want to think of a paralell in C++/C

@DrBug 2014-12-29 15:53:35

What you are saying isnt really clear to me. Why dont you write a fresh response and lets see if more people find it useful, they will vote it up ?

@swiftBoy 2013-02-27 12:21:24

Please! check below simple step by step tutorial to understand how Delegates works in iOS.

Delegate in iOS

I have created two ViewControllers (for sending data from one to another)

  1. FirstViewController implement delegate (which provides data).
  2. SecondViewController declare the delegate (which will receive data).

@Tom Andersen 2009-03-10 00:18:37

Maybe this is more along the lines of what you are missing:

If you are coming from a C++ like viewpoint, delegates takes a little getting used to - but basically 'they just work'.

The way it works is that you set some object that you wrote as the delegate to NSWindow, but your object only has implementations (methods) for one or a few of the many possible delegate methods. So something happens, and NSWindow wants to call your object - it just uses Objective-c's respondsToSelector method to determine if your object wants that method called, and then calls it. This is how objective-c works - methods are looked up on demand.

It is totally trivial to do this with your own objects, there is nothing special going on, you could for instance have an NSArray of 27 objects, all different kinds of objects, only 18 some of them having the method -(void)setToBue; The other 9 don't. So to call setToBlue on all of 18 that need it done, something like this:

for (id anObject in myArray)
  if ([anObject respondsToSelector:@selector(@"setToBlue")])
     [anObject setToBlue]; 

The other thing about delegates is that they are not retained, so you always have to set the delegate to nil in your MyClass dealloc method.

@umop 2010-05-04 20:42:59

When using the formal protocol method for creating delegate support, I've found that you can ensure proper type checking (albeit, runtime, not compile time) by adding something like:

if (![delegate conformsToProtocol:@protocol(MyDelegate)]) {
    [NSException raise:@"MyDelegate Exception"
                format:@"Parameter does not conform to MyDelegate protocol at line %d", (int)__LINE__];

in your delegate accessor (setDelegate) code. This helps minimize mistakes.

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