1. 使用 CaseIterable 获取枚举的所有 case

作者:南峰子

在写 Swift 代码时,可能需要获取枚举的所有 case,在之前的版本中,我们可能需要自己来实现一个静态方法 allCases,如下代码所示:

enum CompassDirection {
    case north, south, east, west

    static let allCases = [north, south, east, west]
}

而在 Swift 4.2 中,标准库为我们提供了 CaseIterable 协议,可以实现这个需求,如下代码所示:

enum CompassDirection: CaseIterable {
    case north, south, east, west
}

print("There are \(CompassDirection.allCases.count) directions.")
// Prints "There are 4 directions."
let caseList = CompassDirection.allCases
                               .map({ "\($0)" })
                               .joined(separator: ", ")
// caseList == "north, south, east, west"

实现 CaseIterable 的枚举在编译器会自动合成 allCases 方法,且 allCases 数组中的元素按 case 声明的顺序。不过如果枚举中的关联值或者 @available 属性,则需要自己重写 allCases,如下代码所示:

enum CompassDirection: CaseIterable {

    case north, south, east
    case west(Int)

    static var allCases: [CompassDirection] {
        return [north, south, east, west(0)]
    }
}

print("There are \(CompassDirection.allCases.count) directions.")
// Prints "There are 4 directions."
let caseList = CompassDirection.allCases
    .map({ "\($0)" })
    .joined(separator: ", ")
// caseList == "north, south, east, west(0)"

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