swift - 在 repeat while 循环中使用 while case 语句

Question

我想弄清楚如何在 repeat while 循环中使用 case 语句。它在普通 while 循环中工作，但在 repeat while 循环中不工作。

indirect enum Ancestor {
    case son(name: String)
    case father(name: String, Ancestor)
}

创建一些枚举对象并设置递归关系

let greatGrandson: Ancestor = .son(name: "Sam")
let grandson: Ancestor = .father(name: "David", greatGrandson)
let father: Ancestor = .father(name: "John", grandson)
let grandFather: Ancestor = .father(name: "Robert", father)

递归处理并打印枚举

var relation = grandFather
while case Ancestor.father = relation  {
    switch relation {
    case .son(let name):
        fatalError("this should not happen")
    case .father(let name, let thisRelation):
        print("father - \(name)")
        relation = thisRelation
    }
}
if case .son(let name) = relation {
    print("son \(name)")
}

问题是doing a while并不能正常工作，因为它会在处理之前检查条件并踢出儿子。所以我想用大小写检查重复 while 循环

以下 repeat while 循环无法编译。

repeat {
    switch relation {
    case .son(let name):
        print("son - \(name)")
    case .father(let name, let thisRelation):
        print("father - \(name)")
        relation = thisRelation
    }
} while case Ancestor.father = relation

即使在直接 while 循环 中 while case 语句有效，但在repeat while 循环 中我得到以下错误

Enum 'case' is not allowed outside of an enum

我做错了什么，为什么我不能在 repeat while 循环中使用 while case？

Answer 1

根据 The Swift Programming Language , while 语句 比 repeat-while 语句 更有能力。

repeat-while 需要一个 Bool 表达式。

一个非常丑陋的解决方法是将case 条件放入闭包 中的if 语句 并调用它:

repeat {
    switch relation {
    case .son(let name):
        print("son - \(name)")
    case .father(let name, let thisRelation):
        print("father - \(name)")
        relation = thisRelation
    }
} while ({ if case Ancestor.father = relation { return true }; return false }())