How to have stored properties in Swift the same way I had on Objective-C

Question

I am switching an application from Objective-C to Swift  which I have a couple of categories with stored properties  for example    interface UIView  MyCategory   -  void alignToView  UIView   view           alignment  UIViewRelativeAlignment alignment  -  UIView   clone    property  strong  PFObject  xo   property  nonatomic  BOOL isAnimating    end   As Swift extensions don t accept stored properties like these  I don t know how to maintain the same structure as the Objc code  Stored properties are really important for my app and I believe Apple must have created some solution for doing it in Swift   As said by jou  what I was looking for was actually using associated objects  so I did  in another context    import Foundation import QuartzCore import ObjectiveC  extension CALayer       var shapeLayer  CAShapeLayer            get               return objc getAssociatedObject self   shapeLayer   as  CAShapeLayer                   set newValue                objc setAssociatedObject self   shapeLayer   newValue  UInt OBJC ASSOCIATION RETAIN                        var initialPath  CGPathRef            get               return objc getAssociatedObject self   initialPath   as CGPathRef                   set               objc setAssociatedObject self   initialPath   newValue  UInt OBJC ASSOCIATION RETAIN                       But I get an EXC BAD ACCESS when doing   class UIBubble   UIView       required init coder aDecoder  NSCoder                        self layer shapeLayer   CAShapeLayer                         Any ideas

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Another example with using Objective-C associated objects and computed properties for Swift 3 and Swift 4  import CoreLocation  extension CLLocation        private struct AssociatedKeys           static var originAddress    originAddress          static var destinationAddress    destinationAddress             var originAddress  String            get               return objc getAssociatedObject self   amp AssociatedKeys originAddress  as  String                   set               if let newValue   newValue                   objc setAssociatedObject                      self                       amp AssociatedKeys originAddress                      newValue as NSString                        OBJC ASSOCIATION RETAIN NONATOMIC                                                      var destinationAddress  String            get               return objc getAssociatedObject self   amp AssociatedKeys destinationAddress  as  String                   set               if let newValue   newValue                   objc setAssociatedObject                      self                       amp AssociatedKeys destinationAddress                      newValue as NSString                        OBJC ASSOCIATION RETAIN NONATOMIC

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My  0 02  This code is written in Swift 2 0  extension CALayer       private struct AssociatedKeys           static var shapeLayer CAShapeLayer             var shapeLayer  CAShapeLayer            get               return objc getAssociatedObject self   amp AssociatedKeys shapeLayer  as  CAShapeLayer                   set               if let newValue   newValue                   objc setAssociatedObject self   amp AssociatedKeys shapeLayer  newValue as CAShapeLayer   objc AssociationPolicy OBJC ASSOCIATION RETAIN NONATOMIC                                    I have tried many solutions  and found this is the only way to actually extend a class with extra variable parameters

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You can t define categories  Swift extensions  with new storage  any additional properties must be computed rather than stored  The syntax works for Objective C because  property in a category essentially means  I ll provide the getter and setter   In Swift  you ll need to define these yourself to get a computed property  something like   extension String       public var Foo   String           get                       return  Foo                     set                          What do you want to do here                      Should work fine  Remember  you can t store new values in the setter  only work with the existing available class state

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I prefer doing code in pure Swift and not rely on Objective-C heritage  Because of this I wrote pure Swift solution with two advantages and two disadvantages   Advantages    Pure Swift code Works on classes and completions or more specifically on Any object   Disadvantages    Code should call method willDeinit   to release objects linked to specific class instance to avoid memory leaks You cannot make extension directly to UIView for this exact example because var frame is extension to UIView  not part of class    EDIT   import UIKit  var extensionPropertyStorage   NSObject   String  Any          var didSetFrame     didSetFrame   extension UILabel        override public var frame  CGRect            get               return didSetFrame    CGRectNull                    set               didSetFrame   newValue                      var didSetFrame  CGRect             get               return extensionPropertyStorage self   didSetFrame   as  CGRect                    set               var selfDictionary   extensionPropertyStorage self      String  Any                 selfDictionary didSetFrame     newValue              extensionPropertyStorage self    selfDictionary                      func willDeinit             extensionPropertyStorage self    nil

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The solution pointed out by jou doesn t support value types   this works fine with them as well  Wrappers  import ObjectiveC  final class Lifted lt T gt        let value  T     init   x  T            value   x          private func lift lt T gt  x  T  - gt  Lifted lt T gt         return Lifted x     func setAssociatedObject lt T gt  object  AnyObject  value  T  associativeKey  UnsafePointer lt Void gt   policy  objc AssociationPolicy        if let v  AnyObject   value as  AnyObject           objc setAssociatedObject object  associativeKey  v   policy            else           objc setAssociatedObject object  associativeKey  lift value    policy           func getAssociatedObject lt T gt  object  AnyObject  associativeKey  UnsafePointer lt Void gt   - gt  T        if let v   objc getAssociatedObject object  associativeKey  as  T           return v           else if let v   objc getAssociatedObject object  associativeKey  as  Lifted lt T gt            return v value           else           return nil           A possible Class extension  Example of usage    extension UIView        private struct AssociatedKey           static var viewExtension    viewExtension             var referenceTransform  CGAffineTransform            get               return getAssociatedObject self  associativeKey   amp AssociatedKey viewExtension                     set               if let value   newValue                   setAssociatedObject self  value  value  associativeKey   amp AssociatedKey viewExtension  policy  objc AssociationPolicy OBJC ASSOCIATION RETAIN NONATOMIC                                    This is really such a great solution  I wanted to add another usage example that included structs and values that are not optionals  Also  the AssociatedKey values can be simplified   struct Crate       var name  String    class Box       var name  String      init name  String            self name   name          extension UIViewController        private struct AssociatedKey           static var displayed    UInt8   0         static var box          UInt8   0         static var crate        UInt8   0            var displayed  Bool            get               return getAssociatedObject self  associativeKey   amp AssociatedKey displayed                     set               if let value   newValue                   setAssociatedObject self  value  value  associativeKey   amp AssociatedKey displayed  policy  objc AssociationPolicy OBJC ASSOCIATION RETAIN NONATOMIC                                     var box  Box           get               if let result Box   getAssociatedObject self  associativeKey   amp AssociatedKey box                    return result               else                   let result   Box name                      self box   result                 return result                                  set               setAssociatedObject self  value  newValue  associativeKey   amp AssociatedKey box  policy  objc AssociationPolicy OBJC ASSOCIATION RETAIN NONATOMIC                       var crate  Crate           get               if let result Crate   getAssociatedObject self  associativeKey   amp AssociatedKey crate                    return result               else                   let result   Crate name                      self crate   result                 return result                                  set               setAssociatedObject self  value  newValue  associativeKey   amp AssociatedKey crate  policy  objc AssociationPolicy OBJC ASSOCIATION RETAIN NONATOMIC

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In PURE SWIFT with WEAK reference handling import Foundation import UIKit  extension CustomView               can make private     static let storedProperties   WeakDictionary lt UIView  Properties gt             struct Properties           var url  String    quot  quot          var status   false         var desc  String    quot url    url   status    status  quot                   var properties  Properties           get               return CustomView storedProperties get forKey  self     Properties                     set               CustomView storedProperties set forKey  self  object  newValue                     var view  CustomView    CustomView   print  quot 1 print quot   view  properties desc     quot nil quot   view  properties url    quot abc quot  view  properties status   true print  quot 2 print quot   view  properties desc     quot nil quot   view   nil  WeakDictionary swift import Foundation  private class WeakHolder lt T  AnyObject gt   Hashable       weak var object  T      let hash  Int      init object  T            self object   object         hash   ObjectIdentifier object  hashValue            func hash into hasher  inout Hasher            hasher combine hash             static func    lhs  WeakHolder  rhs  WeakHolder  - gt  Bool           return lhs hash    rhs hash          class WeakDictionary lt T1  AnyObject  T2 gt        private var dictionary    WeakHolder lt T1 gt   T2         func set forKey  T1  object  T2             dictionary WeakHolder object  forKey     object            func get forKey  T1  - gt  T2            let obj   dictionary WeakHolder object  forKey           return obj            func forEach   handler    key  T1  value  T2   - gt  Void            dictionary forEach               if let object    0 key object  let value   dictionary  0 key                    handler  object  value                                          func clean             var removeList    WeakHolder lt T1 gt             dictionary forEach               if  0 key object    nil                   removeList append  0 key                                  removeList forEach               dictionary  0    nil

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Associated objects API is a bit cumbersome to use  You can remove most of the boilerplate with a helper class   public final class ObjectAssociation lt T  AnyObject gt         private let policy  objc AssociationPolicy          - Parameter policy  An association policy that will be used when linking objects      public init policy  objc AssociationPolicy    OBJC ASSOCIATION RETAIN NONATOMIC             self policy   policy                Accesses associated object          - Parameter index  An object whose associated object is to be accessed      public subscript index  AnyObject  - gt  T             get   return objc getAssociatedObject index  Unmanaged passUnretained self  toOpaque    as  T            set   objc setAssociatedObject index  Unmanaged passUnretained self  toOpaque    newValue  policy              Provided that you can  add  a property to objective-c class in a more readable manner   extension SomeType        private static let association   ObjectAssociation lt NSObject gt         var simulatedProperty  NSObject             get   return SomeType association self            set   SomeType association self    newValue             As for the solution   extension CALayer        private static let initialPathAssociation   ObjectAssociation lt CGPath gt        private static let shapeLayerAssociation   ObjectAssociation lt CAShapeLayer gt         var initialPath  CGPath            get   return CALayer initialPathAssociation self            set   CALayer initialPathAssociation self    newValue              var shapeLayer  CAShapeLayer            get   return CALayer shapeLayerAssociation self            set   CALayer shapeLayerAssociation self    newValue

User · Answer

With Obj-c Categories you can only add methods  not instance variables   In you example you have used  property as a shortcut to adding getter and setter method declarations  You still need to implement those methods   Similarly in Swift you can add use extensions to add instance methods  computed properties etc  but not stored properties

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Here is an alternative that works also  public final class Storage   AnyObject        var object Any       public init   object Any            self object   object          extension Date        private static let associationMap   NSMapTable lt NSString  AnyObject gt        private struct Keys           static var Locale NSString    locale             public var locale Locale            get                if let storage   Date associationMap object forKey  Keys Locale                    return  storage as  Storage  object as  Locale                           return nil                   set               if newValue    nil                   Date associationMap setObject Storage newValue   forKey  Keys Locale                                     var date   Date   date locale   Locale identifier   pt BR   print  date locale

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I tried to store properties by using objc getAssociatedObject  objc setAssociatedObject  without any luck  My goal was create extension for UITextField  to validate text input characters length   Following code works fine for me  Hope this will help someone    private var  min  Int  private var  max  Int   extension UITextField            IBInspectable var minLength  Int           get               return  min    0                   set                min   newValue                       IBInspectable var maxLength  Int           get               return  max    1000                   set                max   newValue                      func validation   - gt   valid  Bool  error  String            var valid  Bool   true         var error  String              guard let text   self text else   return  true                 if text characters count  lt  minLength               valid   false             error    Textfield should contain at least   minLength  characters                     if text characters count  gt  maxLength               valid   false             error    Textfield should not contain more then   maxLength  characters                     if  text characters count  lt  minLength   amp  amp   text characters count  gt  maxLength                valid   false             error    Textfield should contain at least   minLength  characters n              error    Textfield should not contain more then   maxLength  characters                     return  valid  error

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Here is simplified and more expressive solution  It works for both value and reference types  The approach of lifting is taken from  HepaKKes answer   Association code   import ObjectiveC  final class Lifted lt T gt        let value  T     init   x  T            value   x          private func lift lt T gt    x  T  - gt  Lifted lt T gt         return Lifted x     func associated lt T gt  to base  AnyObject                  key  UnsafePointer lt UInt8 gt                   policy  objc AssociationPolicy    OBJC ASSOCIATION RETAIN                  initialiser     - gt  T  - gt  T       if let v   objc getAssociatedObject base  key  as  T           return v            if let v   objc getAssociatedObject base  key  as  Lifted lt T gt            return v value            let lifted   Lifted initialiser        objc setAssociatedObject base  key  lifted  policy      return lifted value    func associate lt T gt  to base  AnyObject  key  UnsafePointer lt UInt8 gt   value  T  policy  objc AssociationPolicy    OBJC ASSOCIATION RETAIN        if let v  AnyObject   value as AnyObject            objc setAssociatedObject base  key  v  policy            else           objc setAssociatedObject base  key  lift value   policy            Example of usage   1  Create extension and associate properties to it  Let s use both value and reference type properties   extension UIButton        struct Keys           static fileprivate var color  UInt8   0         static fileprivate var index  UInt8   0            var color  UIColor           get               return associated to  self  key   amp Keys color     green                     set               associate to  self  key   amp Keys color  value  newValue                       var index  Int           get               return associated to  self  key   amp Keys index    -1                     set               associate to  self  key   amp Keys index  value  newValue                       2  Now you can use just as regular properties       let button   UIButton       print button color     UIExtendedSRGBColorSpace 0 1 0 1    green     button color    black     print button color     UIExtendedGrayColorSpace 0 1    black      print button index     -1     button index   3     print button index     3   More details    Lifting is needed for wrapping value types    Default associated object behavior is retain  If you want to learn more about associated objects  I d recommend checking this article

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I also get an EXC BAD ACCESS problem The value in objc getAssociatedObject   and objc setAssociatedObject   should be an Object  And the objc AssociationPolicy should match the Object

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So I think I found a method that works cleaner than the ones above because it doesn t require any global variables  I got it from here  http   nshipster com swift-objc-runtime   The gist is that you use a struct like so   extension UIViewController       private struct AssociatedKeys           static var DescriptiveName    nsh DescriptiveName             var descriptiveName  String            get               return objc getAssociatedObject self   amp AssociatedKeys DescriptiveName  as  String                   set               if let newValue   newValue                   objc setAssociatedObject                      self                       amp AssociatedKeys DescriptiveName                      newValue as NSString                       UInt OBJC ASSOCIATION RETAIN NONATOMIC                                                      UPDATE for Swift 2  private struct AssociatedKeys       static var displayed    displayed       this lets us check to see if the item is supposed to be displayed or not var displayed   Bool       get           guard let number   objc getAssociatedObject self   amp AssociatedKeys displayed  as  NSNumber else               return true                   return number boolValue            set value            objc setAssociatedObject self  amp AssociatedKeys displayed NSNumber bool  value  objc AssociationPolicy OBJC ASSOCIATION RETAIN NONATOMIC

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I tried using objc setAssociatedObject as mentioned in a few of the answers here  but after failing with it a few times I stepped back and realized there is no reason I need that   Borrowing from a few of the ideas here  I came up with this code which simply stores an array of whatever my extra data is  MyClass in this example  indexed by the object I want to associate it with   class MyClass       var a   1     init a  Int                self a   a          extension UIView       static var extraData    UIView  MyClass         var myClassData  MyClass            get               return UIView extraData self                    set value                UIView extraData self    value                       Test Code   Ran in a Swift Playground  var view1   UIView   var view2   UIView    view1 myClassData   MyClass a  1  view2 myClassData   MyClass a  2  print view1 myClassData  a  print view2 myClassData  a

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I found this solution more practical  UPDATED for Swift 3  extension UIColor        static let graySpace   UIColor init red  50 255  green  50 255  blue  50 255  alpha  1 0      static let redBlood   UIColor init red  102 255  green  0 255  blue  0 255  alpha  1 0      static let redOrange   UIColor init red  204 255  green  17 255  blue  0 255  alpha  1 0       func alpha value   CGFloat  - gt  UIColor           var r   CGFloat 0   g   CGFloat 0   b   CGFloat 0   a   CGFloat 0          self getRed  amp r  green   amp g  blue   amp b  alpha   amp a          return UIColor red  r  green  g  blue  b  alpha  value                then in your code  class gameController  UIViewController         IBOutlet var game  gameClass       override func viewDidLoad             self view backgroundColor   UIColor graySpace

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Why relying on objc runtime  I don t get the point  By using something like the following you will achieve almost the identical behaviour of a stored property  by using only a pure Swift approach   extension UIViewController       private static var  myComputedProperty    String Bool         var myComputedProperty Bool           get               let tmpAddress   String format    p   unsafeBitCast self  to  Int self               return UIViewController  myComputedProperty tmpAddress     false                   set newValue                let tmpAddress   String format    p   unsafeBitCast self  to  Int self               UIViewController  myComputedProperty tmpAddress    newValue

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How about storing static map to class that is extending like this    extension UIView        struct Holder           static var  padding  UIView UIEdgeInsets                   var padding   UIEdgeInsets           get  return UIView Holder  padding self      zero          set   UIView Holder  padding self    newValue

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if you are looking to set a custom string attribute to a UIView  this is how I did it on Swift 4  Create a UIView extension  extension UIView        func setStringValue value  String  key  String            layer setValue value  forKey  key             func stringValueFor key  String  - gt  String            return layer value forKey  key  as  String           To use this extension  let key    COLOR   let redView   UIView        To set redView setStringAttribute value   Red   key  key      To read print redView stringValueFor key  key      Optional  Red

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As in Objective-C  you can t add stored property to existing classes  If you re extending an Objective-C class  UIView is definitely one   you can still use Associated Objects to emulate stored properties    for Swift 1  import ObjectiveC  private var xoAssociationKey  UInt8   0  extension UIView       var xo  PFObject            get               return objc getAssociatedObject self   amp xoAssociationKey  as  PFObject                   set newValue                objc setAssociatedObject self   amp xoAssociationKey  newValue  objc AssociationPolicy OBJC ASSOCIATION RETAIN                       The association key is a pointer that should be the unique for each association  For that  we create a private global variable and use it s memory address as the key with the  amp  operator  See the Using Swift with Cocoa and Objective-C  on more details how pointers are handled in Swift     UPDATED for Swift 2 and 3  import ObjectiveC  private var xoAssociationKey  UInt8   0  extension UIView       var xo  PFObject            get               return objc getAssociatedObject self   amp xoAssociationKey  as  PFObject                   set newValue                objc setAssociatedObject self   amp xoAssociationKey  newValue  objc AssociationPolicy OBJC ASSOCIATION RETAIN                      UPDATED for Swift 4  In Swift 4  it s much more simple  The Holder struct will contain the private value that our computed property will expose to the world  giving the illusion of a stored property behaviour instead   Source  extension UIViewController       struct Holder           static var  myComputedProperty Bool   false           var myComputedProperty Bool           get               return Holder  myComputedProperty                   set newValue                Holder  myComputedProperty   newValue

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