selector in Swift

Question

I m trying to create an NSTimer in Swift but I m having some trouble   NSTimer timeInterval  1  target  self  selector  test    userInfo  nil  repeats  true    test   is a function in the same class     I get an error in the editor       Could not find an overload for  init  that accepts the supplied   arguments   When I change selector  test   to selector  nil the error disappears   I ve tried    selector  test   selector  test selector  Selector test      But nothing works and I can t find a solution in the references

User · Answer

Selectors are an internal representation of a method name in Objective-C. In Objective-C "@selector(methodName)" would convert a source-code method into a data type of SEL. Since you can't use the @selector syntax in Swift (rickster is on point there), you have to manually specify the method name as a String object directly, or by passing a String object to the Selector type. Here is an example:

var rightBarButton = UIBarButtonItem(
    title: "Logout", 
    style: UIBarButtonItemStyle.Plain, 
    target: self, 
    action:"logout"
)

or

var rightBarButton = UIBarButtonItem(
    title: "Logout", 
    style: UIBarButtonItemStyle.Plain, 
    target: self, 
    action:Selector("logout")
)

User · Answer

For Swift 3     Sample code to create timer   Timer scheduledTimer timeInterval  1  target  self  selector    selector updateTimer    userInfo  nil  repeats  true   WHERE timeInterval - Interval in which timer should fire like 1s  10s  100s etc   Its value is in secs  target - function which pointed to class  So here I am pointing to current class  selector - function that will execute when timer fires   func updateTimer          Implemetation      repeats - true false specifies that timer should call again n again

User · Answer

Here s a quick example on how to use the Selector class on Swift   override func viewDidLoad         super viewDidLoad        var rightButton   UIBarButtonItem title   Title   style  UIBarButtonItemStyle Plain  target  self  action  Selector  method        self navigationItem rightBarButtonItem   rightButton    func method            Something cool here        Note that if the method passed as a string doesn t work  it will fail at runtime  not compile time  and crash your app  Be careful

User · Answer

Swift 2 2  and Swift 3 Update  Use the new  selector expression  which eliminates the need to use string literals making usage less error-prone  For reference   Selector  keyboardDidHide      becomes   selector keyboardDidHide        See also  Swift Evolution Proposal  Note  Swift 4 0     If using  selectoryou would need to mark the function as  objc  Example    objc func something   sender  UIButton

User · Answer

It may be useful to note where you setup the control that triggers the action matters   For example  I have found that when setting up a UIBarButtonItem  I had to create the button within viewDidLoad or else I would get an unrecognized selector exception   override func viewDidLoad         super viewDidLoad            add button     let addButton   UIBarButtonItem image  UIImage named   746-plus-circle png    style  UIBarButtonItemStyle Plain  target  self  action  Selector  addAction         self navigationItem rightBarButtonItem   addButton    func addAction send  AnyObject              NSLog  addAction

User · Answer

When using performSelector      addtarget   NStimer scheduledTimerWithInterval   methods your method  matching the selector  should be marked as    objc For Swift 2 0                                self performSelector    performMethod     withObject  nil   afterDelay  0 5                                btnHome addTarget self  action     buttonPressed    forControlEvents  UIControlEvents TouchUpInside                            NSTimer scheduledTimerWithTimeInterval 0 5  target  self  selector      timerMethod     userInfo  nil  repeats  false                 objc private func performMethod            objc private func buttonPressed sender UIButton           objc private func timerMethod               For Swift 2 2  you need to write   selector    instead of string and selector name so the possibilities of spelling error and crash due to that will not be there anymore  Below is example  self performSelector  selector MyClass performMethod   withObject  nil   afterDelay  0 5

User · Answer

Also  if your  Swift  class does not descend from an Objective-C class  then you must have a colon at the end of the target method name string and you must use the  objc property with your target method e g   var rightButton   UIBarButtonItem title   Title   style  UIBarButtonItemStyle Plain  target  self  action  Selector  method      objc func method            Something cool here         otherwise you will get a  Unrecognised Selector  error at runtime

User · Answer

Selector in Swift 4   button addTarget self  action   selector buttonTapped sender     for  UIControlEvents touchUpInside

User · Answer

selector is a word from Objective-C world and you are able to use it from Swift to have a possibility to call Objective-C from Swift  It allows you to execute some code at runtime  Before Swift 2 2 the syntax is   Selector  foo      Since a function name is passed into Selector as a String parameter  foo   it is not possible to check a name in compile time  As a result you can get a runtime error   unrecognized selector sent to instance   After Swift 2 2  the syntax is    selector foo        Xcode s autocomplete help you to call a right method

User · Answer

Using  selector will check your code at compile time to make sure the method you want to call actually exists  Even better  if the method doesn   t exist  you   ll get a compile error  Xcode will refuse to build your app  thus banishing to oblivion another possible source of bugs   override func viewDidLoad             super viewDidLoad            navigationItem rightBarButtonItem               UIBarButtonItem barButtonSystemItem   Add  target  self                              action   selector addNewFireflyRefernce              func addNewFireflyReference             gratuitousReferences append  Curse your sudden but inevitable betrayal

User · Answer

If you want to pass a parameter to the function from the NSTimer then here is your solution   var somethingToPass    It worked   let timer   NSTimer scheduledTimerWithTimeInterval 0 01  target  self  selector   tester    userInfo  somethingToPass  repeats  false   func tester timer  NSTimer        let theStringToPrint   timer userInfo as String     println theStringToPrint      Include the colon in the selector text  tester    and your parameter s  go in userInfo   Your function should take NSTimer as a parameter   Then just extract userInfo to get the parameter that passed

User · Answer

Since Swift 3 0 is published  it is even a little bit more subtle to declare a targetAction appropriate  class MyCustomView   UIView        func addTapGestureRecognizer                 the     is important         let tapGestureRecognizer   UITapGestureRecognizer target  self  action   selector MyCustomView handleTapGesture               tapGestureRecognizer numberOfTapsRequired   1         addGestureRecognizer tapGestureRecognizer                since Swift 3 0 this     in the method implementation is very important to         let the selector understand the targetAction     func handleTapGesture   tapGesture   UITapGestureRecognizer             if tapGesture state     ended               print  TapGesture detected

User · Answer

Swift 4 0  you create the Selector like below   1 add the event to a button like    button addTarget self  action   selector clickedButton sender     for  UIControlEvents touchUpInside    and the function will be like below     objc func clickedButton sender  AnyObject

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Just in case somebody else have the same problem I had with NSTimer where none of the other answers fixed the issue  is really important to mention that  if you are using a class that do not inherits from NSObject either directly or deep in the hierarchy e g  manually created swift files   none of the other answers will work even when is specified as follows   let timer   NSTimer timeInterval  1  target  self  selector   test                        userInfo  nil  repeats  false  func test         Without changing anything else other than just making the class inherit from NSObject I stopped getting the  Unrecognized selector  Error and got my logic working as expected

User · Answer

For swift 3  let timer   Timer scheduledTimer timeInterval  0 01  target  self  selector   selector self test   userInfo  nil  repeats  true    Function declaration In same class    objc func test            my function

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for swift 2 2    version 1 buttton addTarget self  action   selector ViewController tappedButton   forControlEvents   TouchUpInside  buttton addTarget self  action   selector ViewController tappedButton2       forControlEvents   TouchUpInside      version 2 buttton addTarget self  action   selector self tappedButton   forControlEvents   TouchUpInside  buttton addTarget self  action   selector self tappedButton2       forControlEvents   TouchUpInside      version 3 buttton addTarget self  action   selector tappedButton   forControlEvents   TouchUpInside  buttton addTarget self  action   selector tappedButton2       forControlEvents   TouchUpInside   func tappedButton       print  tapped      func tappedButton2 sender  UIButton      print  tapped 2         swift 3 x button addTarget self  action   selector tappedButton       for   touchUpInside   func tappedButton   sender  UIButton         tapped    button addTarget self  action   selector tappedButton         for   touchUpInside   func tappedButton   sender  UIButton    event  UIEvent         tapped

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For future readers  I found that I experienced a problem and was getting an unrecognised selector sent to instance error that was caused by marking the target func as private   The func MUST be publicly visible to be called by an object with a reference to a selector

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I found many of these answers to be helpful but it wasn t clear how to do this with something that wasn t a button  I was adding a gesture recognizer to a UILabel in swift and struggled so here s what I found worked for me after reading everything above   let tapRecognizer   UITapGestureRecognizer              target  self              action   labelTapped      Where the  Selector  was declared as   func labelTapped sender  UILabel        Note that it is public and that I am not using the Selector   syntax but it is possible to do this as well   let tapRecognizer   UITapGestureRecognizer              target  self              action  Selector  labelTapped

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Create Refresh control using Selector method         var refreshCntrl   UIRefreshControl      refreshCntrl   UIRefreshControl       refreshCntrl tintColor   UIColor whiteColor       refreshCntrl attributedTitle   NSAttributedString string   Please Wait          refreshCntrl addTarget self  action  refreshControlValueChanged   forControlEvents  UIControlEvents ValueChanged      atableView addSubview refreshCntrl      Refresh Control Method  func refreshControlValueChanged        atableView reloadData       refreshCntrl endRefreshing

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Swift itself doesn t use selectors     several design patterns that in Objective-C make use of selectors work differently in Swift   For example  use optional chaining on protocol types or is as tests instead of respondsToSelector   and use closures wherever you can instead of performSelector  for better type memory safety   But there are still a number of important ObjC-based APIs that use selectors  including timers and the target action pattern  Swift provides the Selector type for working with these   Swift automatically uses this in place of ObjC s SEL type   In Swift 2 2  Xcode 7 3  and later  including Swift 3   Xcode 8 and Swift 4   Xcode 9   You can construct a Selector from a Swift function type using the  selector expression  let timer   Timer timeInterval  1  target  object                    selector   selector MyClass test                     userInfo  nil  repeats  false  button addTarget object  action   selector MyClass buttonTapped                    for   touchUpInside  view perform  selector UIView insertSubview   aboveSubview                  with  button  with  otherButton   The great thing about this approach  A function reference is checked by the Swift compiler  so you can use the  selector expression only with class method pairs that actually exist and are eligible for use as selectors  see  quot Selector availability quot  below   You re also free to make your function reference only as specific as you need  as per the Swift 2 2  rules for function-type naming   This is actually an improvement over ObjC s  selector   directive  because the compiler s -Wundeclared-selector check verifies only that the named selector exists  The Swift function reference you pass to  selector checks existence  membership in a class  and type signature   There are a couple of extra caveats for the function references you pass to the  selector expression   Multiple functions with the same base name can be differentiated by their parameter labels using the aforementioned syntax for function references  e g  insertSubview   at   vs insertSubview   aboveSubview     But if a function has no parameters  the only way to disambiguate it is to use an as cast with the function s type signature  e g  foo as    - gt     vs foo       There s a special syntax for property getter setter pairs in Swift 3 0   For example  given a var foo  Int  you can use  selector getter  MyClass foo  or  selector setter  MyClass foo    General notes  Cases where  selector doesn t work  and naming  Sometimes you don t have a function reference to make a selector with  for example  with methods dynamically registered in the ObjC runtime   In that case  you can construct a Selector from a string  e g  Selector  quot dynamicMethod  quot       though you lose the compiler s validity checking  When you do that  you need to follow ObjC naming rules  including colons     for each parameter  Selector availability  The method referenced by the selector must be exposed to the ObjC runtime  In Swift 4  every method exposed to ObjC must have its declaration prefaced with the  objc attribute   In previous versions you got that attribute for free in some cases  but now you have to explicitly declare it   Remember that private symbols aren t exposed to the runtime  too     your method needs to have at least internal visibility  Key paths  These are related to but not quite the same as selectors  There s a special syntax for these in Swift 3  too  e g  chris valueForKeyPath  keyPath Person friends firstName    See SE-0062 for details  And even more KeyPath stuff in Swift 4  so make sure you re using the right KeyPath-based API instead of selectors if appropriate  You can read more about selectors under Interacting with Objective-C APIs in Using Swift with Cocoa and Objective-C  Note  Before Swift 2 2  Selector conformed to StringLiteralConvertible  so you might find old code where bare strings are passed to APIs that take selectors  You ll want to run  quot Convert to Current Swift Syntax quot  in Xcode to get those using  selector

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you create the Selector like below  1   UIBarButtonItem      title   Some Title       style  UIBarButtonItemStyle Done      target  self      action   flatButtonPressed      2   flatButton addTarget self  action   flatButtonPressed    forControlEvents  UIControlEvents TouchUpInside    Take note that the  selector syntax is gone and replaced with a simple String naming the method to call  There   s one area where we can all agree the verbosity got in the way  Of course  if we declared that there is a target method called flatButtonPressed  we better write one   func flatButtonPressed sender  AnyObject      NSLog  flatButtonPressed       set the timer       var timer   NSTimer scheduledTimerWithTimeInterval 1 0                       target  self                       selector  Selector  flatButtonPressed                         userInfo  userInfo                       repeats  true      let mainLoop   NSRunLoop mainRunLoop      1     mainLoop addTimer timer  forMode  NSDefaultRunLoopMode    2 this two line is optinal   In order to be complete  here   s the flatButtonPressed    func flatButtonPressed timer  NSTimer

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Swift 4 1  With sample of tap gesture  let gestureRecognizer   UITapGestureRecognizer   self view addGestureRecognizer gestureRecognizer  gestureRecognizer addTarget self  action   selector self dismiss completion         Use destination  Class Name  directly  if you selector  function  is not in same class    gestureRecognizer addTarget self  action   selector DestinationClass dismiss completion        objc func dismiss completion      - gt  Void            self dismiss animated  true  completion  completion      See Apple s document for more details about  Selector Expression

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Change as a simple string naming in the method calling for selector syntax  var timer1   NSTimer    nil timer1  NSTimer timeInterval  0 1  target  self  selector  Selector  test    userInfo  nil  repeats  true    After that  type func test

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