Swift Struct Composition

composition.swift

// Value types are awesome, but we lose the ability to inherit functionality if we use structs in swift.
//
// Below are some experiments where I've been playing with composition to solve this for a common use case
//   where we want to have some common base model fields. 

import Foundation

// Create a struct to hold our common fields
struct CommonFields {
  let id: Int

  init(dictionary: [String: AnyObject]) {
    id = dictionary["id"] as! Int
  }
}

// Our 'child' model
struct User {
  let common: CommonFields
  let name: String

  init(dictionary: [String: AnyObject]) {
    // Utilise common initialisation
    common = CommonFields(dictionary: dictionary)
    name = dictionary["name"] as! String
  }
}

// Create user
let user = User(dictionary: ["id": 5, "name": "john testington"])
user.common.id // 5

=====================================================================

// How about if we want some polymorphism?
protocol BaseModel {
  var common: CommonFields { get }
}

// Identical to before but with protocol conformance
struct User: BaseModel {
  let common: CommonFields
  let name: String

  init(dictionary: [String: AnyObject]) {
    common = CommonFields(dictionary: dictionary)
    name = dictionary["name"] as! String
  }
}

let someModel: BaseModel = User(dictionary: ["id": 5, "name": "john testington"])
someModel.common.id // 5

=====================================================================

// Protocol extensions for convenience
protocol BaseModel {
  var common: CommonFields { get }
  
  var id: Int
}

extension BaseModel {
  var id: Int {
    return common.id
  }
}

// User as before
let user = User(dictionary: ["id": 5, "name": "john testington"])
user.id // 5

// There are a few issues with this
// 1. Our 'common' struct is exposed as public (we can't make protocol vars private)
// 2. If we add a field to our common fields we need to add a corresponding field to 
//    the protocol if we want the convenient getter