How can I make a weak protocol reference in pure Swift without objc

Question

weak references don t seem to work in Swift unless a protocol is declared as  objc  which I don t want in a pure Swift app   This code gives a compile error  weak cannot be applied to non-class type MyClassDelegate    class MyClass     weak var delegate  MyClassDelegate     protocol MyClassDelegate       I need to prefix the protocol with  objc  then it works   Question  What is the  pure  Swift way to accomplish a weak delegate

User · Answer

Update   It looks like the manual has been updated and the example I was referring to has been removed  See the edit to  flainez s answer above   Original  Using  objc is the right way to do it even if you re not interoperating with Obj-C  It ensures that your protocol is being applied to a class and not an enum or struct  See  Checking for Protocol Conformance  in the manual

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AnyObject is the official way to use a weak reference in Swift   class MyClass       weak var delegate  MyClassDelegate     protocol MyClassDelegate  AnyObject       From Apple      To prevent strong reference cycles  delegates should be declared as   weak references  For more information about weak references  see   Strong Reference Cycles Between Class Instances  Marking the protocol   as class-only will later allow you to declare that the delegate must   use a weak reference  You mark a protocol as being class-only by   inheriting from AnyObject  as discussed in Class-Only Protocols    https   developer apple com library content documentation Swift Conceptual Swift Programming Language Protocols html   apple ref doc uid TP40014097-CH25-ID276

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Supplemental Answer  I was always confused about whether delegates should be weak or not  Recently I ve learned more about delegates and when to use weak references  so let me add some supplemental points here for the sake of future viewers    The purpose of using the weak keyword is to avoid strong reference cycles  retain cycles   Strong reference cycles happen when two class instances have strong references to each other  Their reference counts never go to zero so they never get deallocated   You only need to use weak if the delegate is a class  Swift structs and enums are value types  their values are copied when a new instance is made   not reference types  so they don t make strong reference cycles  weak references are always optional  otherwise you would used unowned  and always use var  not let  so that the optional can be set to nil when it is deallocated  A parent class should naturally have a strong reference to its child classes and thus not use the weak keyword  When a child wants a reference to its parent  though  it should make it a weak reference by using the weak keyword  weak should be used when you want a reference to a class that you don t own  not just for a child referencing its parent  When two non-hierarchical classes need to reference each other  choose one to be weak  The one you choose depends on the situation  See the answers to this question for more on this  As a general rule  delegates should be marked as weak because most delegates are referencing classes that they do not own  This is definitely true when a child is using a delegate to communicate with a parent  Using a weak reference for the delegate is what the documentation recommends   But see this  too   Protocols can be used for both reference types  classes  and value types  structs  enums   So in the likely case that you need to make a delegate weak  you have to make it an object-only protocol  The way to do that is to add AnyObject to the protocol s inheritance list   In the past you did this using the class keyword  but AnyObject is preferred now    protocol MyClassDelegate  AnyObject                 class SomeClass       weak var delegate  MyClassDelegate       Further Study  Reading the following articles is what helped me to understand this much better  They also discuss related issues like the unowned keyword and the strong reference cycles that happen with closures    Delegate documentation Swift documentation  Automatic Reference Counting  Weak  Strong  Unowned  Oh My   - A Guide to References in Swift Strong  Weak  and Unowned     Sorting out ARC and Swift   Related   How to make delegates in Swift iOS  How To Make Weak Delegates In Swift Swift delegation - when to use weak pointer on delegate

User · Answer

protocol must be subClass of AnyObject  class example given below     protocol NameOfProtocol  class         member of protocol          class ClassName  UIViewController         weak var delegate  NameOfProtocol

User · Answer

You need to declare the type of the protocol as AnyObject   protocol ProtocolNameDelegate  AnyObject          Protocol stuff goes here    class SomeClass       weak var delegate  ProtocolNameDelegate      Using AnyObject you say that only classes can conform to this protocol  whereas structs or enums can t

User · Answer

Apple uses  NSObjectProtocol  instead of  class     public protocol UIScrollViewDelegate   NSObjectProtocol              This also works for me and removed the errors I was seeing when trying to implement my own delegate pattern

[swift] How can I make a weak protocol reference in 'pure' Swift (without @objc)

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