What is the purpose of willSet and didSet in Swift

Question

Swift has a property declaration syntax very similar to C  s   var foo  Int       get   return getFoo         set   setFoo newValue        However  it also has willSet and didSet actions  These are called before and after the setter is called  respectively  What is their purpose  considering that you could just have the same code inside the setter

User · Accepted Answer

The point seems to be that sometimes  you need a property that has automatic storage and some behavior  for instance to notify other objects that the property just changed  When all you have is get set  you need another field to hold the value  With willSet and didSet  you can take action when the value is modified without needing another field  For instance  in that example   class Foo       var myProperty  Int   0           didSet               print  The value of myProperty changed from   oldValue  to   myProperty                        myProperty prints its old and new value every time it is modified  With just getters and setters  I would need this instead   class Foo       var myPropertyValue  Int   0     var myProperty  Int           get   return myPropertyValue           set               print  The value of myProperty changed from   myPropertyValue  to   newValue                myPropertyValue   newValue                     So willSet and didSet represent an economy of a couple of lines  and less noise in the field list

User · Answer

These are called Property Observers      Property observers observe and respond to changes in a property   s   value  Property observers are called every time a property   s value is   set  even if the new value is the same as the property   s current   value    Excerpt From  Apple Inc     The Swift Programming Language     iBooks  https   itun es ca jEUH0 l  I suspect it s to allow for things we would traditionally do with KVO such as data binding with UI elements  or triggering side effects of changing a property  triggering a sync process  background processing  etc  etc

User · Answer

In your own  base  class  willSet and didSet are quite reduntant   as you could instead define a calculated property  i e get- and set- methods  that access a  propertyVariable and does the desired pre- and post- prosessing   If  however   you override a class where the property is already defined  then the willSet and didSet are useful and not redundant

User · Answer

The many well-written existing answers cover the question well  but I ll mention  in some detail  an addition that I believe is worth covering     The willSet and didSet property observers can be used to call delegates  e g   for class properties that are only ever updated by user interaction  but where you want to avoid calling the delegate at object initialization   I ll cite Klaas up-voted comment to the accepted answer      willSet and didSet observers are not called when a property is first   initialized  They are only called when the property   s value is set   outside of an initialization context    This is a quite neat as it means e g  the didSet property is a good choice of launch point for delegate callbacks  amp  functions  for your own custom classes    As an example  consider some custom user control object  with some key property value  e g  position in rating control   implemented as a subclass of UIView      CustomUserControl swift protocol CustomUserControlDelegate       func didChangeValue value  Int         func didChangeValue newValue  Int  oldValue  Int         func didChangeValue customUserControl  CustomUserControl             other more sophisticated delegate functions    class CustomUserControl  UIView           Properties                private var value   0           didSet                  Possibly do something                     Call delegate              delegate  didChangeValue value                 delegate  didChangeValue value  oldValue  oldValue                 delegate  didChangeValue self                       var delegate  CustomUserControlDelegate          Initialization     required init                     Initialise something                 E g   value   1  would not call didSet at this point                   some methods actions associated with your user control      After which your delegate functions can be used in  say  some view controller to observe key changes in the model for CustomViewController  much like you d use the inherent delegate functions of the UITextFieldDelegate for UITextField objects  e g  textFieldDidEndEditing          For this simple example  use a delegate callback from the didSet of the class property value to tell a view controller that one of it s outlets have had associated model update      ViewController swift Import UIKit         class ViewController  UIViewController  CustomUserControlDelegate           Properties                 IBOutlet weak var customUserControl  CustomUserControl       override func viewDidLoad             super viewDidLoad                              Custom user control  handle through delegate callbacks          customUserControl   self                           CustomUserControlDelegate     func didChangeValue value  Int               do some stuff with  value                    func didChangeValue newValue  Int  oldValue  Int               do some stuff with new as well as old  value                 custom transitions                    func didChangeValue customUserControl  CustomUserControl                 Do more advanced stuff                 Here  the value property has been encapsulated  but generally  in situations like these  be careful not to update the value property of the customUserControl object in the scope of the associated delegate function  here  didChangeValue    in the view controller  or you ll end up with infinite recursion

User · Answer

I do not know C   but with a little guesswork I think I understand what   foo   int       get   return getFoo          set   setFoo newValue         does  It looks very similar to what you have in Swift  but it s not the same  in Swift you do not have the getFoo and setFoo  That is not a little difference  it means you do not have any underlying storage for your value   Swift has stored and computed properties   A computed property has get and may have set  if it s writable   But the code in the getter and setter  if they need to actually store some data  must do it in other properties  There is no backing storage   A stored property  on the other hand  does have backing storage  But it does not have get and set  Instead it has willSet and didSet which you can use to observe variable changes and  eventually  trigger side effects and or modify the stored value  You do not have willSet and didSet for computed properties  and you do not need them because for computed properties you can use the code in set to control changes

User · Answer

My understanding is that set and get are for computed properties  no backing from stored properties   if you are coming from an Objective-C bare in mind that the naming conventions have changed  In Swift an iVar or instance variable is named stored property  Example 1  read only property  - with warning   var test   Int       get           return test           This will result in a warning because this results in a recursive function call  the getter calls itself  The warning in this case is  Attempting to modify  test  within its own getter     Example 2  Conditional read write - with warning  var test   Int       get           return test           set  aNewValue              I ve contrived some condition on which this property can be set            prevents same value being set          if  aNewValue    test                test   aNewValue                     Similar problem - you cannot do this as it s recursively calling the setter  Also  note this code will not complain about no initialisers as there is no stored property to initialise   Example 3  read write computed property - with backing store  Here is a pattern that allows conditional setting of an actual stored property    True model data var  test   Int   0  var test   Int       get           return  test           set  aNewValue              I ve contrived some condition on which this property can be set         if  aNewValue    test                 test   aNewValue                     Note The actual data is called  test  although it could be any data or combination of data  Note also the need to provide an initial value  alternatively you need to use an init method  because  test is actually an instance variable  Example 4  Using will and did set    True model data var  test   Int   0          First this     willSet           println  Old value is    test   new value is   newValue                 value is set        Finaly this     didSet           println  Old value is   oldValue   new value is    test             var test   Int       get           return  test           set  aNewValue              I ve contrived some condition on which this property can be set         if  aNewValue    test                 test   aNewValue                     Here we see willSet and didSet intercepting a change in an actual stored property  This is useful for sending notifications  synchronisation etc     see example below   Example 5  Concrete Example - ViewController Container    Underlying instance variable  would ideally be private  var  childVC   UIViewController        willSet             REMOVE OLD VC         println  Property will set           if   childVC    nil                 childVC  willMoveToParentViewController nil              self setOverrideTraitCollection nil  forChildViewController   childVC               childVC  view removeFromSuperview                childVC  removeFromParentViewController                     if  newValue                self addChildViewController newValue                          I can t see a way to  stop  the value being set to the same controller - hence the computed property      didSet             ADD NEW VC         println  Property did set           if   childVC                      var views    NSDictionaryOfVariableBindings self view        NOT YET SUPPORTED  NSDictionary bridging not yet available                 Add subviews   constraints              childVC  view setTranslatesAutoresizingMaskIntoConstraints false          For now - until I add my own constraints             self view addSubview  childVC  view              let views     view     childVC  view  as NSMutableDictionary             let layoutOpts   NSLayoutFormatOptions 0              let lc1   AnyObject     NSLayoutConstraint constraintsWithVisualFormat    view      options  layoutOpts  metrics  NSDictionary    views  views              let lc2   AnyObject     NSLayoutConstraint constraintsWithVisualFormat  V   view     options  layoutOpts  metrics  NSDictionary    views  views              self view addConstraints lc1              self view addConstraints lc2                 Forward messages to child              childVC  didMoveToParentViewController self                        Computed property - this is the property that must be used to prevent setting the same value twice   unless there is another way of doing this  var childVC   UIViewController        get           return  childVC           set suggestedVC            if  suggestedVC     childVC                 childVC   suggestedVC                     Note the use of BOTH computed and stored properties  I ve used a computed property to prevent setting the same value twice  to avoid bad things happening    I ve used willSet and didSet to forward notifications to viewControllers  see UIViewController documentation and info on viewController containers   I hope this helps  and please someone shout if I ve made a mistake anywhere here

User · Answer

Getter and setter are sometimes too heavy to implement just to observe proper value changes  Usually this needs extra temporary variable handling and extra checks  and you will want to avoid even those tiny labour  if you write hundreds of getters and setters  These stuffs are for the situation

User · Answer

One thing where didSet is really handy is when you use outlets to add additional configuration    IBOutlet weak var loginOrSignupButton  UIButton      didSet           let title   NSLocalizedString  signup required button           loginOrSignupButton setTitle title  for   normal          loginOrSignupButton setTitle title  for   highlighted

User · Answer

You can also use the didSet to set the variable to a different value  This does not cause the observer to be called again as stated in Properties guide  For example  it is useful when you want to limit the value as below   let minValue   1  var value   1       didSet           if value  lt  minValue               value   minValue                    value   -10    value is minValue now

User · Answer

NOTE willSet and didSet observers are not called when a property is set in an initializer before delegation takes place

User · Answer

The willSet and didSet observers for the properties whenever the property is assigned a new value  This is true even if the new value is the same as the current value    And note that willSet needs a parameter name to work around  on the other hand  didSet does not      The didSet observer is called after the value of property is updated  It compares against the old value  If the total number of steps has increased  a message is printed to indicate how many new steps have been taken  The didSet observer does not provide a custom parameter name for the old value  and the default name of oldValue is used instead

[swift] What is the purpose of willSet and didSet in Swift?

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