Optional Protocol methods in swift without using @objc

Question

Using @objc in swift, we can create an optional methods inside protocol like

@objc protocol MyProtocol {
  @objc optional func anOptionalMethod()
}

But how to create an optional protocol methods without using @objc ?

You cannot, because they are specific to ObjC. The easiest way is to simply provide empty default implementations in an extension and then override those as needed. — BallpointBen, Feb 17 '18 at 04:55

score 22 · Accepted Answer · answered Feb 17 '18 at 06:29

22

You can define default func implementation by:

protocol Opt {
    func requiredFunc()
    func optionalFunc()
}

extension Opt {
    func optionalFunc() {}
}

With this you don't have to implement optionalFunc() in classes conforming to Opt, because they already have their default implementation.

answered Feb 17 '18 at 06:29

tereks

1,256
9
15

With a very limited amount of testing, I don't think this works anymore! – paul_f Mar 02 '22 at 21:26
This still works. @paul_f Make sure that your extended methods are an exact match to the delegate protocol's methods. – Alexander Sandberg Nov 15 '22 at 09:09

Ashis Laha · Answer 2 · 2018-02-17T04:48:30.357

Let's create an optional protocol methods using extension of that protocol and use it:

   //: Playground - noun: a place where people can play

   import UIKit

   protocol someProtocol {
     func a()
     func b()
   }

  extension someProtocol {
    func c() {
        print("This is optional protocol method")
     }

    var someProperty: String { 
        return "some optional computed property"
    }
  }

 class A: someProtocol {
   func a() {
       print("a")
    }

   func b() {
      print("b")
   }

   // It's upto you to implement c() functionality, 
   // this is an optional protocol method

   func c() {
      print("I am overriding C functionality")
   }
  }

 let obj = A()
 obj.a()
 obj.b()
 obj.c()
 print(obj.someProperty)

Output:

 a
 b
 I am overriding C functionality
 some optional computed property

This is not an optional protocol method. Whether or not it's overriden, there's always an implementation done, so it's not optional. — Jake, Feb 17 '18 at 05:17

Jake · Answer 3 · 2018-02-17T05:31:22.590

1

Swift protocols do not support optional methods.

Apple Docs:

Optional Protocol Requirements

You can define optional requirements for protocols, These requirements don’t have to be implemented by types that conform to the protocol. Optional requirements are prefixed by the optional modifier as part of the protocol’s definition. Optional requirements are available so that you can write code that interoperates with Objective-C. Both the protocol and the optional requirement must be marked with the @objc attribute. Note that @objc protocols can be adopted only by classes that inherit from Objective-C classes or other @objc classes. They can’t be adopted by structures or enumerations.

Apple Protocol Documentation

edited Feb 17 '18 at 05:31

answered Feb 17 '18 at 05:26

Jake

13,097
9
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Apple uses the keyword "optional" in many of their protocols, written in Swift, without using @objc. (Example: UITextFieldDelegate) Any idea how? – Marcy Mar 05 '20 at 19:33
I believe UITextFieldDelegate is actually defined as an ObjC interface – Jake Mar 05 '20 at 19:36
I think you're probably right. I'm just not seeing @interface or Objective-C code but rather Swift and the protocol keyword. So, I'm a bit confused as to the mechanism that's making that work. Thank-you for your response. – Marcy Mar 06 '20 at 00:35

score 1 · Answer 4 · answered Feb 17 '18 at 06:51

One common use for optional methods in protocols is to define optional callbacks a delegate can listen for. This scenario is best achieved using the protocol + extension approach (to provide do-nothing default implementations for tall of the optional methods).

For some other cases, it may be more appropriate to use multiple protocols:

protocol Storage {
    func read() -> Data
}

protocol MutableStorage: Storage {
    func write(data: Data
}

The standard library uses this approach for Sequence, Collection, and friends.

Optional Protocol methods in swift without using @objc

4 Answers4

Output: