How do I create delegates in Objective-C

Question

I know how delegates work  and I know how I can use them   But how do I create them

User · Answer

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  delegate  changeBackgroundColor tapGesture view  backgroundColor    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

User · Answer

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   - gt  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   - gt  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   - gt  String           return  quot Go get it yourself  quot                   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      quot Go get it yourself  quot     In actual practice  delegates are often used in the following situations  When a class needs to communicate some information to another class 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   What is Delegation      A Swift Developer   s Guide How Delegation Works     A Swift Developer   s Guide

User · Answer

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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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

User · Answer

Answer is actually answered  but I would like to give you a  cheat sheet  for creating a delegate   DELEGATE SCRIPT  CLASS A - Where delegate is calling function   protocol  lt  Protocol Name  gt   lt NSObject gt   - void delegateMethod    end   interface  lt  Some ViewController  gt     lt  UIViewController  gt     property  nonatomic  assign  id  lt  lt  Protocol Name  gt  gt  delegate    end    implementation  lt  Some ViewController  gt    - void someMethod        self delegate methodName       end     CLASS B - Where delegate is called    interface  lt  Other ViewController  gt    lt  Delegate Name  gt       end   implementation  lt  Other ViewController  gt    - void otherMethod       CLASSA  classA     CLASSA alloc  init         classA setDelegate self      -delegateMethod          end

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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  lt NSObject gt           end    amp  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  lt NSObject gt                           Declaration for Methods that  must  be implemented                       optional                  Declaration for Methods that  need not necessarily  be implemented by the class conforming to your delegate          end   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

User · Answer

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    DDSTATE   DDCITY  DropDownType    protocol DropDownListDelegate  lt NSObject gt   required -  void dropDownDidSelectItemWithString  NSString  itemString     DropDownType  DropDownType dropDownType   end  interface DropDownViewController   UIViewController    BOOL isFiltered     property  nonatomic  assign  DropDownType dropDownType   property  weak  id  lt DropDownListDelegate gt  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 delegate           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  gt  0                 Here i am printing the selected row              self dropDownBtn1 setTitle itemString forState UIControlStateNormal                           break      case DDSTATE              Here i am printing the selected row          self dropDownBtn2 setTitle itemString forState UIControlStateNormal              default          break

User · Answer

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    FirstViewController implement delegate  which provides data   SecondViewController declare the delegate  which will receive data

User · Answer

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

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1    Custom delegate   protocol TB RemovedUserCellTag  lt NSObject gt   - void didRemoveCellWithTag  NSInteger tag    end    2    Create a weak reference in a class where you declared the delegate  property weak nonatomic id  lt TB RemovedUserCellTag gt  removedCellTagDelegate     3      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    lt TB RemovedUserCellTag gt    end     5  Implement the method in the class  m - void didRemoveCellWithTag  NSInteger tag      NSLog   Tag  d  tag

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ViewController h   protocol NameDelegate  lt NSObject gt   - void delegateMEthod   ArgType  arg    end   property id  lt NameDelegate gt  delegate    ViewController m   self delegate delegateMEthod  argument     MainViewController m  ViewController viewController    ViewController new   viewController delegate   self    Method   - void delegateMEthod   ArgType  arg

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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  lt BlaClass BlaClass h gt    class MyClass                define class  so protocol can see MyClass  protocol MyClassDelegate  lt NSObject gt      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  lt MyClassDelegate gt  delegate    define MyClassDelegate as delegate   end   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         end   To use your delegate in another class  UIViewController called MyVC in this case  MyVC h    import  MyClass h   interface MyVC UIViewController  lt MyClassDelegate gt      make it a delegate for MyClassDelegate     MyVC m   myClass delegate   self             set its delegate to self somewhere   Implement delegate method  -  void  myClassDelegateMethod   MyClass    sender       NSLog   Delegates are great

User · Answer

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  amp  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  amp  different teams can be like multiple views   Here is code consider ShippingView as Shipping team  amp  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  amp  tracking info class DeliveryView  UIView ShippingDelegate       func productShipped productID   String                   update status on view  amp  perform delivery            Main page on app which has both views  amp  shows updated info on product whole status class ProductViewController   UIViewController       var shippingView   ShippingView     var deliveryView   DeliveryView      override func viewDidLoad             super 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  amp  update UI          shippingView delegate   deliveryView

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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  lt NSObject gt   - void myClass  MyClass  myObject requiredEventHandlerWithParameter  ParamType  param    optional - void myClass  MyClass  myObject optionalEventHandlerWithParameter  ParamType  param    end    interface MyClass   NSObject   property nonatomic weak id lt  MyClassDelegate gt  delegate    end   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 lt  MyClassDelegate gt   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

User · Answer

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                                                      object self                                                   userInfo  NSDictionary dictionaryWithObject selectedLocation   myClassData   forKey   myClassData        end   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

User · Answer

An Objective-C delegate is an object that has been assigned to the delegate property another object  To create one  you 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 a UIWebView  If you d like to implement its delegate s webViewDidStartLoad  method  you could create a class like this    interface MyClass lt UIWebViewDelegate gt          end   implementation MyClass -  void webViewDidStartLoad  UIWebView   webView                   end   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  lt NSObject gt   optional -  void webViewDidStartLoad  UIWebView   webView         other methods here  end   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  lt UIWebViewDelegate gt          end   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    Naming  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  lt NSObject gt   optional -  void something  id something didFinishLoadingItem  id item  -  void something  id something didFailWithError  NSError   error   end   interface Something   NSObject  property  nonatomic  weak  id  lt SomethingDelegate gt  delegate   end   implementation Something     struct       unsigned int didFinishLoadingItem 1      unsigned int didFailWithError 1      delegateRespondsTo     synthesize delegate   -  void setDelegate  id  lt SomethingDelegate gt  aDelegate     if  delegate    aDelegate        delegate   aDelegate       delegateRespondsTo didFinishLoadingItem    delegate respondsToSelector  selector something didFinishLoadingItem         delegateRespondsTo didFailWithError    delegate respondsToSelector  selector something didFailWithError            end   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  end   This 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 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

User · Answer

Delegate  - Create   protocol addToCartDelegate  lt NSObject gt   - void addToCartAction  ItemsModel   itemsModel isAdded  BOOL added    end   Send and please assign delegate to view you are sending data   self delegate addToCartAction itemsModel isAdded YES

User · Answer

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  lt NSObject gt   -  void  serviceDidFailWithRequestType  NSString  error  -  void  serviceDidFinishedSucessfully  NSString  success    end   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  lt ServiceResponceDelegate gt   delegate     -  void  setDelegate  id delegate  -  void  someTask    end   implementation ServiceClass  -  void  setDelegate  id delegate    delegate   delegate     -  void  someTask          perform task     if   success      delegate serviceDidFailWithRequestType     task failed        else     delegate serviceDidFinishedSucessfully     task success           end   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  lt ServiceResponceDelegate gt    ServiceClass   service     -  void  go    end   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

User · Answer

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  lt  As the protocol I am going to use is UIViewController class gt    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  lt UIKit UIkit h gt   class YourViewController    protocol YourViewController Delegate  lt NSObject gt     optional - void defineDelegateMethodName   YourViewController    controller    required - BOOL delegateMethodReturningBool   YourViewController    controller      end    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 lt  YourViewController Delegate gt  delegate    end   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          lt YourViewControllerDelegate gt      end      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            end     test whether the method has been defined before you call it    -  void  someMethodToCallDelegate        if    self delegate  respondsToSelector  selector defineDelegateMethodName                   self delegate delegateMethodName self

[ios] How do I create delegates in Objective-C?

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