Nested Types

Nested Types

Define types inside the scope of another type.

Enumerations are often created to support a specific class or structure's functionality. Similarly, it can be convenient to define utility structures purely for use within the context of a more complex type, and protocols that are normally used in conjunction with a specific type. To accomplish this, Swift enables you to define nested types, whereby you nest supporting types like enumerations, structures, and protocols within the definition of the type they support.

To nest a type within another type, write its definition within the outer braces of the type it supports. Types can be nested to as many levels as are required.

Nested Types in Action

The example below defines a structure called BlackjackCard, which models a playing card as used in the game of Blackjack. The BlackjackCard structure contains two nested enumeration types called Suit and Rank.

In Blackjack, the Ace cards have a value of either one or eleven. This feature is represented by a structure called Values, which is nested within the Rank enumeration:

struct BlackjackCard {
    // nested Suit enumeration
    enum Suit: Character {
        case spades = "♠", hearts = "♡", diamonds = "♢", clubs = "♣"
    }

    // nested Rank enumeration
    enum Rank: Int {
        case two = 2, three, four, five, six, seven, eight, nine, ten
        case jack, queen, king, ace
        struct Values {
            let first: Int, second: Int?
        }
        var values: Values {
            switch self {
            case .ace:
                return Values(first: 1, second: 11)
            case .jack, .queen, .king:
                return Values(first: 10, second: nil)
            default:
                return Values(first: self.rawValue, second: nil)
            }
        }
    }

    // BlackjackCard properties and methods
    let rank: Rank, suit: Suit
    var description: String {
        var output = "suit is \(suit.rawValue),"
        output += " value is \(rank.values.first)"
        if let second = rank.values.second {
            output += " or \(second)"
        }
        return output
    }
}

The Suit enumeration describes the four common playing card suits, together with a raw Character value to represent their symbol.

The Rank enumeration describes the thirteen possible playing card ranks, together with a raw Int value to represent their face value. (This raw Int value isn't used for the Jack, Queen, King, and Ace cards.)

As mentioned above, the Rank enumeration defines a further nested structure of its own, called Values. This structure encapsulates the fact that most cards have one value, but the Ace card has two values. The Values structure defines two properties to represent this:

• first, of type Int
• second, of type Int?, or “optional Int”

Rank also defines a computed property, values, which returns an instance of the Values structure. This computed property considers the rank of the card and initializes a new Values instance with appropriate values based on its rank. It uses special values for jack, queen, king, and ace. For the numeric cards, it uses the rank's raw Int value.

The BlackjackCard structure itself has two properties --- rank and suit. It also defines a computed property called description, which uses the values stored in rank and suit to build a description of the name and value of the card. The description property uses optional binding to check whether there's a second value to display, and if so, inserts additional description detail for that second value.

Because BlackjackCard is a structure with no custom initializers, it has an implicit memberwise initializer, as described in doc:Initialization#Memberwise-Initializers-for-Structure-Types. You can use this initializer to initialize a new constant called theAceOfSpades:

let theAceOfSpades = BlackjackCard(rank: .ace, suit: .spades)
print("theAceOfSpades: \(theAceOfSpades.description)")
// Prints "theAceOfSpades: suit is ♠, value is 1 or 11".

Even though Rank and Suit are nested within BlackjackCard, their type can be inferred from context, and so the initialization of this instance is able to refer to the enumeration cases by their case names (.ace and .spades) alone. In the example above, the description property correctly reports that the Ace of Spades has a value of 1 or 11.

Referring to Nested Types

To use a nested type outside of its definition context, prefix its name with the name of the type it's nested within:

let heartsSymbol = BlackjackCard.Suit.hearts.rawValue
// heartsSymbol is "♡"

For the example above, this enables the names of Suit, Rank, and Values to be kept deliberately short, because their names are naturally qualified by the context in which they're defined.