How to define optional methods in Swift protocol

Question

Is it possible in Swift  If not then is there a workaround to do it

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Here s a very simple example for swift Classes ONLY  and not for structures or enumerations  Note that the protocol method being optional  has two levels of optional chaining at play   Also the class adopting the protocol needs the  objc attribute in its declaration    objc protocol CollectionOfDataDelegate     optional func indexDidChange index  Int       objc class RootView  CollectionOfDataDelegate       var data   CollectionOfData       init          data delegate   self       data indexIsNow           func indexDidChange index  Int          println  The index is currently    index            class CollectionOfData      var index   Int      weak var delegate   CollectionOfDataDelegate      func indexIsNow          index   23       delegate  indexDidChange  index

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I think that before asking how you can implement an optional protocol method  you should be asking why you should implement one    If we think of swift protocols as an Interface in classic object oriented programming  optional methods do not make much sense  and perhaps a better solution would be to create default implementation  or separate the protocol into a set of protocols  perhaps with some inheritance relations between them  to represent the possible combination of methods in the protocol   For further reading  see https   useyourloaf com blog swift-optional-protocol-methods   which gives an excellent overview on this matter

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A pure Swift approach with protocol inheritance     Required methods protocol MyProtocol       func foo        Optional methods protocol MyExtendedProtocol  MyProtocol       func bar      class MyClass       var delegate  MyProtocol     func myMethod              delegate as  MyExtendedProtocol  bar

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In Swift 3 0   objc protocol CounterDataSource        objc optional func increment forCount count  Int  - gt  Int      objc optional var fixedIncrement  Int   get       It will save your time

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In Swift 2 and onwards it s possible to add default implementations of a protocol  This creates a new way of optional methods in protocols    protocol MyProtocol       func doSomethingNonOptionalMethod       func doSomethingOptionalMethod      extension MyProtocol       func doSomethingOptionalMethod                leaving this empty            It s not a really nice way in creating optional protocol methods  but gives you the possibility to use structs in in protocol callbacks   I wrote a small summary here  https   www avanderlee com swift-2-0 optional-protocol-methods

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There are two ways you can create optional method in swift protocol   1 - The first option is to mark your protocol using the  objc attribute  While this means it can be adopted only by classes  it does mean you mark individual methods as being optional like this    objc protocol MyProtocol        objc optional func optionalMethod       2 - A swiftier way  This option is better  Write default implementations of the optional methods that do nothing  like this   protocol MyProtocol       func optionalMethod       func notOptionalMethod      extension MyProtocol        func optionalMethod               this is a empty implementation to allow this method to be optional           Swift has a feature called extension that allow us to provide a default implementation for those methods that we want to be optional

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One option is to store them as optional function variables   struct MyAwesomeStruct       var myWonderfulFunction   Optional lt  Int  - gt  Int gt    nil    let squareCalculator       MyAwesomeStruct myWonderfulFunction    input in return input   input    let thisShouldBeFour   squareCalculator myWonderfulFunction  2

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1  Using default implementations  preferred    protocol MyProtocol       func doSomething      extension MyProtocol       func doSomething                return a default value or just leave empty             struct MyStruct  MyProtocol          no compile error        Advantages   No Objective-C runtime is involved  well  no explicitly at least   This means you can conform structs  enums and non-NSObject classes to it  Also  this means you can take advantage of powerful generics system  You can always be sure that all requirements are met when encountering types that conform to such protocol  It s always either concrete implementation or default one  This is how  interfaces  or  contracts  behave in other languages    Disadvantages   For non-Void requirements  you need to have a reasonable default value  which is not always possible  However  when you encounter this problem  it means that either such requirement should really have no default implementation  or that your you made a mistake during API design  You can t distinguish between a default implementation and no implementation at all  at least without addressing that problem with special return values  Consider the following example   protocol SomeParserDelegate       func validate value  Any  - gt  Bool     If you provide a default implementation which just returns true     it s fine at the first glance  Now  consider the following pseudo code   final class SomeParser       func parse data  Data  - gt   Any            if    delegate validate value   is not implemented                     parse very fast without validating              else                  parse and validate every value                        There s no way to implement such an optimization     you can t know if your delegate implements a method or not   Although there s a number of different ways to overcome this problem  using optional closures  different delegate objects for different operations to name a few   that example presents the problem clearly      2  Using  objc optional    objc protocol MyProtocol        objc optional func doSomething      class MyClass  NSObject  MyProtocol          no compile error        Advantages   No default implementation is needed  You just declare an optional method or a variable and you re ready to go    Disadvantages   It severely limits your protocol s capabilities by requiring all conforming types to be Objective-C compatible  This means  only classes that inherit from NSObject can conform to such protocol  No structs  no enums  no associated types  You must always check if an optional method is implemented by either optionally calling or checking if the conforming type implements it  This might introduce a lot of boilerplate if you re calling optional methods often

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Define function in protocol and create extension for that protocol  then create empty implementation for function which you want to use as optional

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if you want to do it in pure swift the best way is to provide a default implementation particullary if you return a Swift type like for example  struct with Swift types  example    struct magicDatas       var damagePoints   Int      var manaPoints   Int     protocol magicCastDelegate       func castFire   - gt  magicDatas     func castIce   - gt  magicDatas    extension magicCastDelegate       func castFire   - gt  magicDatas           return magicDatas              func castIce   - gt  magicDatas           return magicDatas             then you can implement protocol without defines every func

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Since there are some answers about how to use optional modifier and  objc attribute to define optional requirement protocol  I will give a sample about how to use protocol extensions define optional protocol   Below code is Swift 3         Protocol has empty default implementation of the following methods making them optional to implement       cancel    protocol Cancelable            default implementation is empty      func cancel      extension Cancelable        func cancel         class Plane  Cancelable       Since cancel   have default implementation  that is optional to class Plane    let plane   Plane   plane cancel      Print out  United Airlines can t cancelable    Please notice protocol extension methods can t invoked by Objective-C code  and worse is Swift team won t fix it  https   bugs swift org browse SR-492

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How to create optional and required delegate methods    objc protocol InterViewDelegate class         objc optional func optfunc          This is optional     func requiredfunc         This is required

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Put the  optional in front of methods or properties

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To illustrate the mechanics of Antoine s answer    protocol SomeProtocol       func aMethod      extension SomeProtocol       func aMethod             print  extensionImplementation            class protocolImplementingObject  SomeProtocol       class protocolImplementingMethodOverridingObject  SomeProtocol       func aMethod             print  classImplementation            let noOverride   protocolImplementingObject   let override   protocolImplementingMethodOverridingObject    noOverride aMethod     prints  extensionImplementation  override aMethod     prints  classImplementation

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You need to add optional keyword prior to each method   Please note  however  that for this to work  your protocol must be marked with the  objc attribute  This further implies that this protocol would be applicable to classes but not structures

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The other answers here involving marking the protocol as   objc  do not work when using swift-specific types    struct Info       var height  Int     var weight  Int      objc protocol Health       func isInfoHealthy info  Info  - gt  Bool      Error  Method cannot be marked  objc because the type of the parameter cannot be represented in Objective-C    In order to declare optional protocols that work well with swift  declare the functions as variables instead of func s    protocol Health       var isInfoHealthy   Info  - gt   Bool     get set       And then implement the protocol as follows  class Human  Health       var isInfoHealthy   Info  - gt   Bool       info in         if info weight  lt  200  amp  amp  info height  gt  72               return true                   return false             Or leave out the implementation and declare it as          var isInfoHealthy   Info  - gt   Bool       You can then use     to check whether or not the function has been implemented  func returnEntity   - gt  Health       return Human      var anEntity  Health   returnEntity    var isHealthy   anEntity isInfoHealthy Info height  75  weight  150        isHealthy  is true

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Here is a concrete example with the delegation pattern   Setup your Protocol    objc protocol MyProtocol class       func requiredMethod       optional func optionalMethod      class MyClass  NSObject       weak var delegate MyProtocol       func callDelegate                 delegate  requiredMethod           delegate  optionalMethod              Set the delegate to a class and implement the Protocol  See that the optional method does not need to be implemented   class AnotherClass  NSObject  MyProtocol       init                 super init            let myInstance   MyClass           myInstance delegate   self            func requiredMethod                   One important thing is that the optional method is optional and needs a     when calling  Mention the second question mark   delegate  optionalMethod

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Slightly off topic from the original question  but it builds off Antoine   s idea and I thought it might help someone    You can also make computed properties optional for structs with protocol extensions   You can make a property on the protocol optional  protocol SomeProtocol       var required  String   get       var optional  String    get       Implement the dummy computed property in the protocol extension  extension SomeProtocol       var optional  String    return nil       And now you can use structs that do or don   t have the optional property implemented  struct ConformsWithoutOptional       let required  String    struct ConformsWithOptional       let required  String     let optional  String      I   ve also written up how to do optional properties in Swift protocols on my blog  which I   ll keep updated in case things change through the Swift 2 releases

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To define Optional Protocol in swift you should use  objc keyword before Protocol declaration and attribute method declaration inside that protocol  Below is a sample of Optional Property of a protocol    objc protocol Protocol       objc optional var name String      class MyClass  Protocol          No error

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