Strongly typed NSUserDefaults using Phantom Types.

NSUserDefaults+Keys.swift

// We can use Phantom Types to provide strongly typed acces to NSUserDefaults
// Similar to: http://www.objc.io/blog/2014/12/29/functional-snippet-13-phantom-types/


// A key to UserDefaults has a name and phantom type
struct Key<T> {
  let name: String
}


// Extensions to NSUserDefaults for subscripting using keys.
// Apparently generics don't work with subscript.
// But this code should be in a library somewhere, so who cares about a little repetition.
extension NSUserDefaults {
  subscript(key: Key<String>) -> String? {
    get {
      return stringForKey(key.name)
    }
    set {
      if let value = newValue {
        setValue(value, forKey: key.name)
      }
      else {
        removeObjectForKey(key.name)
      }
    }
  }

  subscript(key: Key<Int>) -> Int? {
    get {
      return integerForKey(key.name)
    }
    set {
      if let value = newValue {
        setInteger(value, forKey: key.name)
      }
      else {
        removeObjectForKey(key.name)
      }
    }
  }

  subscript(key: Key<Double>) -> Double? {
    get {
      return doubleForKey(key.name)
    }
    set {
      if let value = newValue {
        setDouble(value, forKey: key.name)
      }
      else {
        removeObjectForKey(key.name)
      }
    }
  }

  subscript(key: Key<Bool>) -> Bool? {
    get {
      return boolForKey(key.name)
    }
    set {
      if let value = newValue {
        setBool(value, forKey: key.name)
      }
      else {
        removeObjectForKey(key.name)
      }
    }
  }

  subscript(key: Key<NSData>) -> NSData? {
    get {
      return dataForKey(key.name)
    }
    set {
      if let value = newValue {
        setValue(value, forKey: key.name)
      }
      else {
        removeObjectForKey(key.name)
      }
    }
  }

  subscript(key: Key<NSDate>) -> NSDate? {
    get {
      return objectForKey(key.name) as? NSDate
    }
    set {
      if let value = newValue {
        setValue(value, forKey: key.name)
      }
      else {
        removeObjectForKey(key.name)
      }
    }
  }
}



// We create three keys for values to be stored in NSUserDefaults
//
// The 'Keys' struct is merely used as a namespace, to avoid poluting the global namespace
struct Keys {
  static let name = Key<String>(name: "nameKey")
  static let birthdate = Key<NSDate>(name: "birthdateKey")
  static let lastCompletedOnboardingVersion = Key<Int>(name: "lastCompletedOnboardingVersion")
}


// Example of using NSUserDefaults with phantom type Keys:
let userDefaults = NSUserDefaults.standardUserDefaults()

if userDefaults[Keys.lastCompletedOnboardingVersion] < 3 {
  presentAwesomeOnboardingFlowVersion3()
  userDefaults[Keys.lastCompletedOnboardingVersion] = 3
}

if let name = userDefaults[Keys.name] {
  if let birthdate = userDefaults[Keys.birthdate] where birthdate.isToday() {
    println("Hi \(name), happy birthday!")
  }
  else {
    println("Hi \(name)")
  }
}