Swift：具有多个类型选项的未声明变量

Question

I'm learning swift through Apple's App Development book. There is a project for designing a Favorite Athlete App to view, add and edit athletes.

I'm wondering if there is a way to have a variable in a struct/class that is of a type that is dependent on the literal of another variable....

Ok, so this is what I'm thinking.

enum League {
    case MLB, NFL, NBA
}
enum MLB {
    case Braves, Yankees, Red Sox
}
enum NFL {
    case Falcons, Giants, Patriots
}
enum NBA {
    case Hawks, Knicks, Celtics
}

struct Athlete {
    var name: String
    var age: Int
    var league: League
    var Team: switch league{
            case .MLB:
                return MLB enum
            case .NFL:
                return NFL enum
            case .NBA:
                return NBA enum
    } 
}

Answer 1

You can't have spaces in enum cases. Per convention, you should use lowerCamelCase. Just from looking at your code, it looks like you're looking for something like this:

enum League {
    case mlb, nfl, nba
}

protocol LeagueTeam {
    static var league: League { get }
}

enum MLBTeams: LeagueTeam {
    case braves, yankees, redSox
    static let league = League.mlb
}
enum NFLTeams: LeagueTeam {
    case falcons, giants, patriots
    static let league = League.nfl
}
enum NBATeams: LeagueTeam {
    case hawks, knicks, celtics
    static let league = League.nba
}

struct Athlete {
    var name: String
    var age: Int
    var team: LeagueTeam
    var league: League { return type(of: team).league }
}

Answer 2

I like Alexander's solution, but this also lends itself nicely to enums with associated data:

enum MLBTeam {
    case braves, yankees, redSox
}

enum NFLTeam {
    case falcons, giants, patriots
}

enum NBATeam {
    case hawks, knicks, celtics
}

enum Team {
    case mlb(MLBTeam)
    case nfl(NFLTeam)
    case nba(NBATeam)
}

struct Athlete {
    var name: String
    var age: Int
    var team: Team
}

let athlete = Athlete(name: "Julio Teherán", age: 26, team: .mlb(.braves))

With this, you can use the team property to switch both on specific teams and on leagues.

switch athlete.team {
case .mlb: print("Baseball")
case .nfl: print("Football")
case .nba: print("Basketball")
}

switch athlete.team {
case .mlb(.braves): print("Braves")
default: print("Not Braves")
}

The one sad thing in Swift 4 is that it still can't auto-generate Equatable for enums with associated values, so you have to do that by hand if you want it.

extension Team: Equatable {
    static func ==(lhs: Team, rhs: Team) -> Bool {
        switch (lhs, rhs) {
        case let (.mlb(lhsTeam), .mlb(rhsTeam)): return lhsTeam == rhsTeam
        case let (.nfl(lhsTeam), .nfl(rhsTeam)): return lhsTeam == rhsTeam
        case let (.nba(lhsTeam), .nba(rhsTeam)): return lhsTeam == rhsTeam
        default: return false
        }
    }
}

It's difficult to get Alexander's solution to allow == on LeagueTeam . Making LeagueTeam conform to Equatable creates all kinds of problems, and you can't create an exhaustive switch over LeagueTeam because it can have arbitrary implementations. So that's one nice thing about using an enum here.

On the other hand, using an enum pushes you into exhaustive switch statements, which may be a problem if the leagues or team list may change. (On the other hand, it may be a benefit…)