Swift - Protocol extensions - Property default values

Question

Let's say that I have the following protocol:

protocol Identifiable {     var id: Int {get}     var name: String {get} } 

And that I have the following structs:

struct A: Identifiable {     var id: Int     var name: String }  struct B: Identifiable {     var id: Int     var name: String } 

As you can see, I had to 'conform' to the Identifiable protocol in struct A and struct B. But imagine if I had N more structs that needs to conform to this protocol... I don't want to 'copy/paste' the conformance (var id: Int, var name: String)

So I create a protocol extension:

extension Identifiable {     var id: Int {         return 0     }      var name: String {         return "default"     } } 

With this extension now I can create a struct that conforms to the Identifiable protocol without having to implement both properties:

struct C: Identifiable {  } 

Now the problem is that I can't set a value to the id property or the name property:

var c: C = C() c.id = 12 // Cannot assign to property: 'id' is a get-only property 

This happens because in the Identifiable protocol, id and name are only gettable. Now if I change the id and name properties to {get set} I get the following error:

Type 'C' does not conform to protocol 'Identifiable'

This error happens because I haven't implemented a setter in the protocol extension... So I change the protocol extension:

extension Identifiable {     var id: Int {         get {             return 0         }          set {          }     }      var name: String {         get {             return "default"         }          set {          }     } } 

Now the error goes away but if I set a new value to id or name, it gets the default value (getter). Of course, the setter is empty.

My question is: What piece of code do I have to put inside the setter? Because if I add self.id = newValue it crashes (recursive).

Thanks in advance.

Answer 1

It seems you want to add a stored property to a type via protocol extension. However this is not possible because with extensions you cannot add a stored property.

I can show you a couple of alternatives.

Subclassing (Object Oriented Programming)

The easiest way (as probably you already imagine) is using classes instead of structs.

class IdentifiableBase {     var id = 0     var name = "default" }  class A: IdentifiableBase { }  let a = A() a.name  = "test" print(a.name) // test 

Cons: In this case your A class needs to inherit from IdentifiableBase and since in Swift theres is not multiple inheritance this will be the only class A will be able to inherit from.

Components (Protocol Oriented Programming)

This technique is pretty popular in game development

struct IdentifiableComponent {     var id = 0     var name = "default" }  protocol HasIdentifiableComponent {     var identifiableComponent: IdentifiableComponent { get set } }  protocol Identifiable: HasIdentifiableComponent { }  extension Identifiable {     var id: Int {         get { return identifiableComponent.id }         set { identifiableComponent.id = newValue }     }     var name: String {         get { return identifiableComponent.name }         set { identifiableComponent.name = newValue }     } } 

Now you can make your type conform to Identifiable simply writing

struct A: Identifiable {     var identifiableComponent = IdentifiableComponent() } 

Test

var a = A() a.identifiableComponent.name = "test" print(a.identifiableComponent.name) // test