json.swift

import Foundation

public enum Error: Int, ErrorType {
    public static let Domain = "Italmass.Error.Domain"
    
    case Missing
    case Invalid
}

extension NSError {
    convenience init(_ code: Error, _ description: String) {
        let userInfo = [NSLocalizedDescriptionKey: description]
        self.init(domain: Error.Domain, code: code.rawValue, userInfo: userInfo)
    }
}

extension Dictionary where Key: StringLiteralConvertible, Value: AnyObject {
    
    public func valueFor<T>(key key: Key) throws -> T {
        guard let value = self[key] else {
            throw NSError(.Missing, "Missing value for key `\(key)`.")
        }
        guard let correct = value as? T else {
            throw NSError(.Invalid, "Invalid value for key `\(key)`. Expected type `\(T.self)`. Unexpected type `\(value.dynamicType)`.")
        }
        return correct
    }
    
    public func valueFor<T>(keyPath keys: [Key]) throws -> T {
        var keyPath = keys
        guard let lastKey = keyPath.popLast() else {
            throw NSError(.Missing, "Key path is empty.")
        }
        let dict = try keyPath.reduce(self) { try $0.valueFor(key: $1) }
        
        return try dict.valueFor(key: lastKey)
    }
    
    public func valueFor<T, U>(key key: Key, transform: (U throws -> T?)) throws -> T {
        let origin: U = try valueFor(key: key)
        guard let value = try transform(origin) else {
            throw NSError(.Missing, "Value: \(origin) for key: `\(key)` not transformed.")
        }
        return value
    }
}

struct User {
    let id: UInt
    let name: String
    let birthday: NSDate
    let cars: [Car]
    
    init(dictyonary: [String: AnyObject]) throws {
        id = try dictyonary.valueFor(key: "object_id")
        name = try dictyonary.valueFor(key: "first_name")
        birthday = try dictyonary.valueFor(key: "birthday") { NSDate(timeIntervalSince1970: $0) }
        
        let array: [[String: AnyObject]] = try dictyonary.valueFor(key: "cars")
        cars = try array.map { try Car(dictyonary: $0) }
    }
}

struct Car {
    let name: String
    
    init(dictyonary: [String: AnyObject]) throws {
        name = try dictyonary.valueFor(keyPath: ["model", "name"])
    }
}