Swift Property Requirements

Protocols.swift

/**
  [Swift Protocols](https://docs.swift.org/swift-book/LanguageGuide/Protocols.html#ID269)

  # Property Requirements
  
  A protocol can require any conforming type to provide an instance property or
    type property with a particular name and type. The protocol doesn’t specify
    whether the property should be a stored property or a computed property it
    only specifies the required property name and type. The protocol also specifies
    whether each property must be gettable or gettable and settable.

  If a protocol requires a property to be gettable and settable, that property
    requirement can’t be fulfilled by a constant stored property or a read-only
    computed property. If the protocol only requires a property to be gettable,
    the requirement can be satisfied by any kind of property, and it’s valid for
    the property to be also settable if this is useful for your own code.

  Property requirements are always declared as variable properties, prefixed with
    the var keyword. Gettable and settable properties are indicated by writing
    { get set } after their type declaration, and gettable properties are
    indicated by writing { get }.
*/

protocol GetSetProtocol {
    var email: String { get set }
    var username: String { get set }
    var firstName: String? { get set }
    var lastName: String? { get set }
}

protocol GetProtocol {
    var email: String { get }
    var username: String { get }
    var firstName: String? { get }
    var lastName: String? { get }
}

// Must be `var` or `{ get set }`
// Can not be `let` or `{ T }`
struct User: GetSetProtocol {
    private var hiddenEmailVariable: String = ""
    var email: String {
        set {
            print(newValue)
            hiddenEmailVariable = newValue
        }
        get {
            return hiddenEmailVariable
        }
    }
    var username: String {
        didSet {
            print(username)
        }
    }
    var firstName: String?
    var lastName: String?
    
    init(email: String, username: String,
         firstName: String?, lastName: String?) {
        self.username = username
        self.firstName = firstName
        self.lastName = lastName
        
        // set `hiddenEmailVariable` to the value of `email`
        self.email = email
    }
}

// Can be any: `{ }`, `let`, `var`
struct UserResponse: GetProtocol {
    private var hiddenEmailVariable: String = ""
    var email: String {
        set {
            print(newValue)
            hiddenEmailVariable = newValue
        }
        get {
            return hiddenEmailVariable
        }
    }
    var username: String {
        didSet {
            print(username)
        }
    }
    let firstName: String?
    var lastName: String? {
        nil
    }
}