A draft on how to use Protocol-Oriented Programming in Swift 2 with a web API

popWebAPI.swift

import UIKit


// We define a APIResultType as something that can be generated from a JSON

protocol APIResultType {
    
    init?(json: String)
    
}


// A APIRequest is something that associates an URL with a APIResultType

protocol APIRequest {
    
    typealias T: APIResultType
    var url: String { get }
    
}


// We extend APIRequest with a function to load data from a webservice.
// Notice that the load method will init and return an APIResultType.

extension APIRequest {
    
    private func loadJSON(url: String) -> String {
        return "DATA FROM SERVER"
    }
    
    func load() -> T? {
        let json = loadJSON(url)
        return T(json: json)
    }
    
}


// Now we want to make a music app, so we need a artist type

struct Artist {

    var name: String

}

// First we make the artist type conform to the APIResultType so we can generate it from JSON

extension Artist : APIResultType {
    
    init?(json: String) {
        // Build self from data
        // We use a failable initializer, because validation might fail
        self.name = json
    }
    
}


// To request an artist we create the ArtistRequest type that conforms to APIRequest
// and we associate this APIRequest with the Artist type.
// The ArtistRequest only needs an ID to make the URL for the request.

struct ArtistRequest : APIRequest {
    
    typealias T = Artist
    
    var id: Int
    var url: String { return "http://api.mymusic.com/artists/\(id)" }
    
}


// To get an Artist type from the API we now only need to create a ArtistRequest,
// run it and we get the type we wanted.

if let artist = ArtistRequest(id: 123).load() {
    print(artist.name)
}