Is it possible to use a protocol to require a nested struct?

Using Swift
1

Is it possible to use a protocol to require a nested struct?

I have a struct like so:

struct DraftsURL {
    static let scheme = "drafts5"
    static let host = "x-callback-url"
    struct actions {
        static let create = "/create"
        static let open = "/open"
        static let get = "/get"
        static let prepend = "/prepend"
        static let append = "/append"
        static let replaceRange = "/replaceRange"
        static let search = "/search"
        static let workspace = "/workspace"
        static let runAction = "/runAction"
        static let dictate = "/dictate"
        static let arrange = "/arrange"
    }
}

I'd like to generalize some of this with a protocol so that other structs can do the same thing. Something like this:

protocol URLBuilder {
    static var scheme: String { get }
    static var host: String { get }
    struct actions { } //Type 'actions' cannot be nested in protocol 'URLBuilder'
}

For my purposes, I just want to require a struct called actions without dictating anything else in the implementation of that struct.

So is it possible to use a protocol to require a nested struct?

Or perhaps am I not doing this in the "right" way? Is this a sign I should be using a class instead?

2

It sounds like you want associatedtype

1 Like
3

Why is it a struct with a lowercase plural name instead of an enum named Action though?

If you do what Nevin said, any types will work.

4

Thank you guys. I’ll try both of those.

5

So an enum sounds more type safe than just an associatedtype (since associatedtype will accept any type). But I'm still playing around with enums inside of protocols and I haven't figured out how to nest an enum inside a protocol since Types are not allowed to be nested in protocols.

6

You're allowed to have typealiases inside protocols, so you could do:

enum URLBuilderAction { ... }

protocol URLBuilder {
  typealias Action = URLBuilderAction
  ...
}
7

Hmm, I'm not sure this works either. If I understand correctly, then with that approach the enum is already statically defined outside of the protocol. With a typealias now that enum is already defined, plus I don't think swift will let me extension the enum.

I'm trying to figure out how to say in the protocol:

"I'd like some enum named Action, but you can fill that enum with whatever you want."

8

Why does it matter that it's an enum? There's no way to require that, and if you could, there would be nothing you could do with that restriction anyway.

2 Likes
9

I was trying to implement your advice when you said:

10

No, you can not create a protocol that requires conforming types to have nested types defined inside of them. Why would you need this? Do you have an example use case?