Store a closure as a variable in Swift

Question

In Objective-C  you can define a block s input and output  store one of those blocks that s passed in to a method  then use that block later      in  h      typedef void   APLCalibrationProgressHandler  float percentComplete       typedef void   APLCalibrationCompletionHandler  NSInteger measuredPower  NSError  error           in  m       property  strong  APLCalibrationProgressHandler progressHandler       property  strong  APLCalibrationCompletionHandler completionHandler       -  id initWithRegion  CLBeaconRegion   region completionHandler  APLCalibrationCompletionHandler handler               self    super init           if self                                         completionHandler    handler copy                                     return self     -  void performCalibrationWithProgressHandler  APLCalibrationProgressHandler handler                        self progressHandler    handler copy                         dispatch async dispatch get main queue                        completionHandler 0  error                                So I m trying to do the equivilant in Swift   var completionHandler  Float - gt Void      init         locationManager   CLLocationManager       region   CLBeaconRegion       timer   NSTimer      convenience init region  CLBeaconRegion  handler   Float - gt Void         self init       locationManager delegate   self     self region   region     completionHandler   handler     rangedBeacons   NSMutableArray       The compiler doesn t like that declaration of completionHandler  Not that I blame it  but  how do I define a closure that can be set and used later in Swift

User · Answer

Objective-C   interface PopupView   UIView  property  nonatomic  copy  void   onHideComplete      end   interface PopupView          -  IBAction hideButtonDidTouch  id sender           Do something                Callback     if  onHideComplete  onHideComplete         end  PopupView   popupView     PopupView alloc  init  popupView onHideComplete                     Swift  class PopupView  UIView       var onHideComplete      - gt  Void         IBAction func hideButtonDidTouch sender  AnyObject               Do something                         Callback         if let callback   self onHideComplete               callback                       var popupView   PopupView    popupView onHideComplete            - gt  Void in

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In Swift 4 and 5  I created a closure variable containing two parameter dictionary and bool    var completionHandler   String Any   Bool - gt Void     dict  success  in     if success         print dict              Calling the closure variable   self completionHandler   name   Gurjinder singh   true

User · Answer

For me following was working   var completionHandler   Float - gt Void

User · Answer

Closures can be declared as typealias as below  typealias Completion    Bool  Any  Error  - gt  Void   If you want to use in your function anywhere in code  you can write like normal variable  func xyz with param1  String  completion  Completion

User · Answer

I ve provide an example not sure if this is what you re after   var completionHandler     value  Float  - gt      func printFloat value  Float        print value     completionHandler   printFloat  completionHandler 5    It simply prints 5 using the completionHandler variable declared

User · Answer

The compiler complains on  var completionHandler   Float - gt Void        because the right-hand side is not a closure of the appropriate signature  i e  a closure taking a float argument  The following would assign a  do nothing  closure to the completion handler   var completionHandler   Float - gt Void          arg  Float  - gt  Void in     and this can be shortened to  var completionHandler   Float - gt Void     arg in     due to the automatic type inference   But what you probably want is that the completion handler is initialized to nil in the same way that an Objective-C instance variable is inititialized to nil  In Swift this can be realized with an optional   var completionHandler    Float - gt Void     Now the property is automatically initialized to nil   no value    In Swift you would use optional binding to check of a the completion handler has a value  if let handler   completionHandler       handler result      or optional chaining   completionHandler  result

User · Answer

Depends on your needs there is an addition to accepted answer  You may also implement it like this  var parseCompletion      - gt Void    and later in some func assign to it func someHavyFunc completion   escaping    - gt  Void       self parseCompletion   completion    and in some second function use it func someSecondFunc        if let completion   self parseCompletion           completion            note that  escaping parameter is a mandatory here

User · Answer

This works too   var exeBlk            - gt  Void in   exeBlk           do something     instead of nil  exeBlk

[swift] Store a closure as a variable in Swift

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