How is init() relevant in Swift protocol?

Question

Protocols in Swift can declare the init() method in their definition. However, I can't think of any use case where this solves any problem other than forcing the conforming classes to define the init() as in the protocol. We can call the declared methods on the protocol type but init on protocol cannot be used to instantiate its object, which is its only purpose.

What problem does declaring init() method in a protocol solve?

Answer 1

I think the real utility comes when it's used as a constraint in a generic class o function. This is real code from one of my projects.

I declare a protocol with a init:

protocol JSONCreatable {
    init(fromJson json: JSON)
}

Then, in a generic function where I return a class that conforms to that protocol:

import SwiftyJSON

extension JSON {
    func asObject<T>() -> T? where T: JSONCreatable {
        if isEmpty {
            return nil
        }
        return T(fromJson: self)
    }

    func asArray<T>() -> [T] where T: JSONCreatable {
        return array?.map{ json in T(fromJson: json) } ?? []
    }
}

This allows me to do things like this:

let user: User = json["user"].asObject()
let results: [Element] = json["elements"].asArray()