Protocols and Generics in Swift

• Protocols can only be used as a generic constraint because they have Self or associated type requirements.
• Protocols do not allow the use of generic parameters, but instead use associated types.
• Specialization cannot be applied to non-generic types.

import Foundation

protocol BaseReq {
    // Generic Type T in protocol
    associatedtype T
    func obj(_ s:String) -> T
    init(completion:((_ res: Result<T>)->())?)
}

extension BaseReq {
    func launch() {
        let s = "network return string"
        obj(s)
    }
}

class AReq: BaseReq {
    // declare the Type of T
    typealias T = AObj
    required init(completion:((_ res: Result<T>)->())?) {

    }

    func obj(_ s: String) -> T {
        return T(s)
    }
}

class BReq: BaseReq {
    typealias T = BObj
    required init(completion:((_ res: Result<T>)->())?) {

    }

    func obj(_ s:String) -> T {
        return T(s)
    }
}

class BaseObj {

}

class AObj: BaseObj {

    var a = "a"

    init(_ s: String) {
        print("a", s)
    }
}

class BObj: BaseObj {

    var b = "b"

    init(_ s: String) {
        print("b", s)
    }
}

class Launch {

    // static func launch(req: BaseReq) -> BaseReq {
    // If you write like the one above, you will get:
    // Error: Protocol 'BaseReq' can only be used as a generic constraint because it has Self or associated type requirements.
    static func launch<U: BaseReq>(req: U) -> U {
        let s = "network return string"
        print(req)

        req.obj(s)

        return req
    }
}

// Cannot specialize non-generic type
// enum Result {
enum Result<T> {
    case suc(T)
    case err(Error)
}


let areq = AReq { (r) in
    switch r {
    case .suc(let s):
        // S is AObj, we can access s.a
        s.a
    case .err(let r):
        break
    }
}
areq.launch()
// or
Launch.launch(req: areq)

let breq = BReq { (r) in
    switch r {
    case .suc(let s):
        // S is BObj, we can access s.b
        s.b
    case .err(let r):
        break
    }
}
breq.launch()
Launch.launch(req: breq)
s

Translated by gpt-3.5-turbo