Swift:使用变量/参数访问元组元素？

Question

这个问题已经有答案了:

Swift Tuple index using a variable as the index? (5 个回答)

已关闭 6 年前。

这是[通用版本]情况:

let tuple: (first: Int, second: Int, third: Int) // tuple.0 is equivalent to  tuple.first

enum TupleIndex: Int { 
    case first = 0, second, third 
}

func selectTupleElement (index: TupleIndex) {
    let indexNum = index.rawValue
    let tupleElement = tuple.indexNum // Oh noooooo!!!
}

编译器将上面最后一行所示的问题点读取为“indexNum 的 tuple 属性或元素”(当然不存在)，而不是“tuple 的元素”索引等于 indexNum "

有没有办法使用元组来做我想做的事情？

Answer 1

您可以利用运行时自省(introspection)来有条件地提取固定大小和相同类型成员的元组中的第n个成员 (例如下面:为具有统一类型成员的 arity 3 元组实现)并(有条件地)将其转换为成员的类型。例如:

func selectTupleElementFor<T>(memberNumber: Int, inTuple tuple: (T, T, T)) -> T? {
    return Mirror(reflecting: tuple).children.enumerated()
        .first(where: { $0.0 == memberNumber - 1 }).flatMap { $1.1 as? T }
}

// example usage
let tuple: (first: Int, second: Int, third: Int) = (10, 20, 30)
if let tupleMember = selectTupleElementFor(memberNumber: 2, inTuple: tuple) {
    print(tupleMember) // 20
}

或者，使用您的 TupleIndex enum:

enum TupleIndex: Int { 
    case first = 0, second, third 
}

func selectTupleElementFor<T>(tupleIndex: TupleIndex, inTuple tuple: (T, T, T)) -> T? {
    return Mirror(reflecting: tuple).children.enumerated()
        .first(where: { $0.0 == tupleIndex.rawValue }).flatMap { $1.1 as? T }
}

// example usage
let tuple: (first: Int, second: Int, third: Int) = (10, 20, 30)
if let tupleMember = selectTupleElementFor(tupleIndex: .second, inTuple: tuple) {
    print(tupleMember) // 20
}