遍历enum的case

• 可以将自定义的枚举类型遵循CaseIterable来允许枚举被遍历.
• Swift会暴露一个包含对应枚举类型所有情况的集合名为allCases

e.g.

enum CompassPoint: CaseIterable {
    case north
    case south
    case east
    case west
}

let numberOfDirections = CompassPoint.allCases.count
print("\(numberOfDirections) directions")

for direction in CompassPoint.allCases {
    print("direction: \(direction)")
}

输出结果:

4 directions
direction: north
direction: south
direction: east
direction: west

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