What is the Swift equivalent of respondsToSelector

Question

I ve googled but not been able to find out what the swift equivalent to respondsToSelector  is   This is the only thing I could find  Swift alternative to respondsToSelector   but isn t too relevant in my case as its checking the existence of the delegate  I don t have a delegate I just want to check if a new API exists or not when running on the device and if not fall back to a previous version of the api

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As I started to update my old project to Swift 3 2  I just needed to change the method from  respondsToSelector selector    to   responds to  selector

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If the method you are testing for is defined as an optional method in a  objc protocol  which sounds like your case   then use the optional chaining pattern as   if let result   object method  args         method exists  result assigned  use result      else           When the method is declare as returning Void  simply use   if object method  args            See         Calling Methods Through Optional Chaining      Excerpt From  Apple Inc     The Swift Programming Language       iBooks  https   itun es us jEUH0 l

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For swift 3 0  import UIKit   objc protocol ADelegate   NSObjectProtocol         objc optional func hi1        objc optional func hi2 message1 String  message2 String     class SomeObject   NSObject        weak var delegate ADelegate       func run                 single method         if let methodHi1   delegate  hi1               methodHi1             else               print  fail h1                         multiple parameters         if let methodHi2   delegate  hi2               methodHi2  superman    batman             else               print  fail h2                      class ViewController  UIViewController  ADelegate        let someObject   SomeObject        override func viewDidLoad             super viewDidLoad            someObject delegate   self         someObject run                 MARK  ADelegate     func hi1              print  Hi              func hi2 message1  String  message2  String             print  Hi   message1    message2

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Functions are first-class types in Swift  so you can check whether an optional function defined in a protocol has been implemented by comparing it to nil   if  someObject someMethod    nil        someObject someMethod  someArgument    else          do something else

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Update Mar 20  2017 for Swift 3 syntax   If you don t care whether the optional method exists  just call delegate  optionalMethod     Otherwise  using guard is probably the best approach    weak var delegate  SomeDelegateWithOptionals   func someMethod         guard let method   delegate  optionalMethod else              optional not implemented         alternativeMethod           return           method       Original answer   You can use the  if let  approach to test an optional protocol like this   weak var delegate  SomeDelegateWithOptionals   func someMethod       if let delegate   delegate       if let theMethod   delegate theOptionalProtocolMethod          theMethod         return                Reaching here means the delegate doesn t exist or doesn t respond to the optional method   alternativeMethod

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The equivalent is the   operator   var value  NSNumber    myQuestionableObject  importantMethod     importantMethod will only be called if myQuestionableObject exists and implements it

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Currently  Swift 2 1  you can check it using 3 ways    Using respondsToSelector answered by  Erik at Digit Using     answered by  Sulthan And using as  operator   if let delegateMe   self delegate as  YourCustomViewController      delegateMe onSuccess        Basically it depends on what you are trying to achieve    If for example your app logic need to perform some action and the delegate isn t set or the pointed delegate didn t implement the onSuccess   method  protocol method  so option 1 and 3 are the best choice  though I d use option 3 which is Swift way  If you don t want to do anything when delegate is nil or method isn t implemented then use option 2

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There is no real Swift replacement   You can check in the following way   someObject someMethod      This calls the method someMethod only if it s defined  on object someObject but you can use it only for  objc protocols which have declared the method as optional   Swift is inherently a safe language so everytime you call a method Swift has to know the method is there  No runtime checking is possible  You can t just call random methods on random objects   Even in Obj-C you should avoid such things when possible because it doesn t play well with ARC  ARC then triggers warnings for performSelector     However  when checking for available APIs  you can still use respondsToSelector   even if Swift  if you are dealing with NSObject instances    interface TestA   NSObject  -  void someMethod    end   implementation TestA    this triggers a warning   end        var a   TestA    if a respondsToSelector  someMethod        a someMethod

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another possible syntax by swift     if let delegate   self delegate  method   delegate somemethod          method

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I just implement this myself in a project  see code below  As mentions by  Christopher Pickslay it is important to remember that functions are first class citizens and can therefore be treated like optional variables    objc protocol ContactDetailsDelegate  class        optional func deleteContact contact  Contact  - gt  NSError          weak var delegate ContactDetailsDelegate   if let deleteContact   delegate deleteContact       deleteContact contact

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For swift3  If you just want to call the method  run the code below   self delegate  method

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In Swift 2 Apple introduced a new feature called API availability checking  which might be a replacement for respondsToSelector  method The following code snippet comparison is copied from the WWDC2015 Session 106 What s New in Swift which I thought might help you please check it out if you need to know more      The Old Approach     IBOutlet var dropButton  NSButton  override func awakeFromNib         if dropButton respondsToSelector  setSpringLoaded              dropButton springLoaded   true              The Better Approach     IBOutlet var dropButton  NSButton  override func awakeFromNib         if  available OSX 10 10 3               dropButton springLoaded   true

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I use guard let else  so that can do some default stuff if the delegate func is not implemented    objc protocol ViewController2Delegate  NSObjectProtocol        optional func viewController2 controller  ViewController2  didSomethingWithStringAndReturnVoid string  String       optional func viewController2 controller  ViewController2  didSomethingWithStringAndReturnString string  String  - gt  String    class ViewController2  UIViewController        weak var delegate  ViewController2Delegate                IBAction func onVoidButtonClicked sender  AnyObject            if  delegate    nil  amp  amp  delegate  respondsToSelector Selector  viewController2 didSomethingWithStringAndReturnVoid                    NSLog  ReturnVoid is implemented                delegate  viewController2  self  didSomethingWithStringAndReturnVoid   dummy                     else              NSLog  ReturnVoid is not implemented                  Do something by default                       IBAction func onStringButtonClicked sender  AnyObject            guard let result   delegate  viewController2  self  didSomethingWithStringAndReturnString   dummy   else               NSLog  ReturnString is not implemented                  Do something by default             return                    NSLog  ReturnString is implemented with result    result

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It seems you need to define your protocol as as subprotocol of NSObjectProtocol     then you ll get respondsToSelector method   objc protocol YourDelegate   NSObjectProtocol       func yourDelegateMethod passObject  SomeObject      note that only specifying  objc was not enough  You should be also careful that the actual delegate is a subclass of NSObject - which in Swift might not be

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Swift 3   protocol   objc protocol SomeDelegate        objc optional func method       Object  class SomeObject   NSObject    weak var delegate SomeObject   func delegateMethod           if let delegateMethod   delegate  method           delegateMethod         else             Failed

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I guess you want to make a default implementation for delegate  You can do this  let defaultHandler       delegate  method    defaultHandler

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As mentioned  in Swift most of the time you can achieve what you need with the   optional unwrapper operator  This allows you to call a method on an object if and only if the object exists  not nil  and the method is implemented   In the case where you still need respondsToSelector   it is still there as part of the NSObject protocol   If you are calling respondsToSelector  on an Obj-C type in Swift  then it works the same as you would expect  If you are using it on your own Swift class  you will need to ensure your class derives from NSObject   Here s an example of a Swift class that you can check if it responds to a selector   class Worker   NSObject       func work           func eat food  AnyObject          func sleep hours  Int  minutes  Int         let worker   Worker    let canWork   worker respondsToSelector Selector  work         true let canEat   worker respondsToSelector Selector  eat           true let canSleep   worker respondsToSelector Selector  sleep minutes           true let canQuit   worker respondsToSelector Selector  quit         false   It is important that you do not leave out the parameter names  In this example  Selector  sleep     is not the same as Selector  sleep minutes

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