any Identifiable can't conform to 'Identifiable'

Question

update: add same error about Hashable

I have created an Identifiable compliant protocol and compliant structures. Then, when I create the list and reference it in ForEach, I get the error Type 'any TestProtocol' cannot conform to 'Identifiable'(I get the same error about Hashable).

How should I fix this program?

If I write ForEach(list, id: \.id) , it works, but I don't think it makes sense to be Identifiable compliant.

import SwiftUI

protocol TestProtocol: Identifiable, Hashable {
    var id: UUID { get set }
    var name: String { get set }
    
    func greeting() -> String
    static func == (lhs: Self, rhs: Self) -> Bool
}

extension TestProtocol {
    static func == (lhs: Self, rhs: Self) -> Bool {
        return lhs.id == rhs.id
    }
}

struct Person: TestProtocol {
    var id = UUID()
    var name: String
    
    func greeting() -> String {
        return "my name is \(name) and I'm a human."
    }
}

struct Dog: TestProtocol {
    var id = UUID()
    var name: String
    
    func greeting() -> String {
        return "my name is \(name) and I'm a dog."
    }
}

struct ContentView: View {
    var list: [any TestProtocol] = [Person(name: "p1"), Dog(name: "d1")]
    @State var selected: any TestProtocol
    
    var body: some View {
        VStack {
            Picker(selection: $selected) { // Type 'any TestProtocol' cannot conform to 'Hashable'
                ForEach(list) { l in // Type 'any TestProtocol' cannot conform to 'Identifiable'
                    Text(l.greeting()).tag(l) // Type 'any TestProtocol' cannot conform to 'Hashable'
                }
            } label: {
                Text("select")
            }
        }
    }
}

Not sure if I understand what you mean with _"... but I don't think it makes sense to be Identifiable compliant."_. So you don't want your protocol to extend `Identifiable` at all or is it using the `id` property that doesn't make sense? — Joakim Danielson, Sep 19 '22 at 14:27
To design it this way, you actually need to implement `RandomAccessCollection` instead of `var list: [any TestProtocol]`, which can contain your types like `Person`, `Dog`. Or, you can go a simpler route: create `class ListItem: Identifiable, Hashable {` with `open func greeting() -> String { fatalError("Must be implemented") }` and then make descendants `Person` and `Dog` implement this function (or any other such functions) — timbre timbre, Sep 19 '22 at 19:48

burnsi · Accepted Answer · 2022-09-20T09:28:35.030

Your error message complaining about Hashable is a "red hering". The protocol TestProtocol, and therefor all structs conforming to it, conforms to Hashable.

let person = Person(name: "IAmHashable")
print(person.hashValue)

The reason this is failing is within the Picker. It needs a concrete type and not a protocol. One solution would be to create a "Container" type and a custom binding that handles this.

struct Container: Identifiable, Hashable{
    //implement the same equality as in your TestProtocol
    static func == (lhs: Container, rhs: Container) -> Bool {
        rhs.wrapped.id == lhs.wrapped.id
    }
    
    func hash(into hasher: inout Hasher) {
        hasher.combine(wrapped)
    }
    
    var wrapped: any TestProtocol
    //for convenience
    var id: UUID {wrapped.id}
}

and the ContentView:

struct ContentView: View {
    
    let startArr: [any TestProtocol] = [Person(name: "p1"), Dog(name: "d1")]
    @State private var selected: (any TestProtocol)?

    var body: some View {
        // list of wrapped protocols
        var list: [Container] = { startArr.map{Container(wrapped: $0)}}()
        // binding
        let selectionBinding: Binding<Container> = .init {
            let returninstance = list.first { cont in
                cont.id == selected?.id
            }
            return returninstance ?? list[0]
        } set: { container in
            selected = container.wrapped
        }
        
        // viewCode
        VStack {
            Picker(selection: selectionBinding) {
                ForEach(list) { l in
                    Text(l.wrapped.greeting())
                        .tag(l)
                }
            } label: {
                Text("select")
            }
            // confirmation selection changed
            Text(selected?.name ?? "no Selection")
        }
    }
}

Remarks:

This solution has a few drawbacks:

your initial array startArr should never be empty, else return returninstance ?? list[0] will break you code. This can be handled, but I think this is out of the scope of this question.
the equality comparison of Container needs to be the same as in TestProtocol as you cannot compare two any Protocols
on the appearance of ContainerView selected will be nil until something is selected. Any usage, e.g.: the Text element in this case, needs to deal with this. But you could probably set this in .onApear.

any Identifiable can't conform to 'Identifiable'

1 Answers1

Remarks: