TypeConversion.swift

//: Playground - noun: a place where people can play

import UIKit

// Simple struct to be fullfilled from JSON
struct User {
    let name: String
    let age: Int
}

// Simple JSON response object
let response = [
    "user" : [
        "username" : "Foxy",
        "age" : "18"
    ]
] as Dictionary<String, AnyObject>

// Simple way to do parsing
let user = response["user"] as! Dictionary<String, AnyObject>

User(
    name: user["username"] as! String, // Dealing with casting
    age: Int(user["age"] as! String)! // Type conversions
)

// Protocol of things that can convert to other things
// Any can convert to itself at least
protocol Convertable {
    static func convert<T>(value: T) -> Self?
}

// Generic method that will return expected type.
extension Dictionary where Value: AnyObject {
    func tryConvert<T : Convertable>(key: Key) -> T? {
        return self[key].flatMap { T.convert($0) }
    }
    
    func convert<T : Convertable>(key: Key) -> T {
        return tryConvert(key)!
    }
}

// Convertable support
extension String : Convertable {
    static func convert<T>(value: T) -> String? {
        switch value {
            
        // Identity conversion
        case let string as String: return string
            
        default: return nil}
    }
}

extension Int : Convertable {
    static func convert<T>(value: T) -> Int? {
        switch value {
        case let int as Int: return int
        
        // Type conversion
        case let string as String: return Int(string)
            
        default: return nil}
    }
}

// Usage with type casting
let user2 = User(
    name: user.convert("username"),
    age: user.convert("age")
)