斯威夫特中的元组“向上倾斜”

如果我有一个带签名的元组(String,Bool),我无法将其转换为(String,Any).编译器说：

error: cannot express tuple conversion ‘(String,Bool)’ to ‘(String,
Any)’

但这应该可行,因为Bool可以安全地转换为Any with as.如果您执行类似的操作,几乎会抛出相同的错误：

let any: Any = ("String",true)
any as! (String,Any) // error
any as! (String,Bool) // obvIoUsly succeeds

错误：

Could not cast value of type ‘(Swift.String,Swift.Bool)’ to
‘(protocol<>,protocol<>)’

那么有没有针对第二种情况的解决方法？因为您甚至无法将Any转换为任何可以单独转换元素的元组(Any,Any).

即使它们包含的类型可以,也无法强制转换元组.例如：

let nums = (1,5,9)
let doubleNums = nums as (Double,Double,Double) //fails

但：

let nums : (Double,Double) = (1,9) //succeeds

在你的情况下,解决方法是强制转换单个元素,而不是元组本身：

let tuple = ("String",true)
let anyTuple = (tuple.0,tuple.1 as Any)
// anyTuple is (String,Any)

这是the Swift documentation注意到的原因之一：

Tuples are useful for temporary groups of related values. They are not suited to the creation of complex data structures. If your data structure is likely to persist beyond a temporary scope,model it as a class or structure,rather than as a tuple.

我认为这是一个实现限制,因为元组是像函数一样的复合类型.同样,你不能创建元组的扩展(例如扩展(String,Bool){…}).

如果您实际使用的是返回(String,Any)的API,请尝试将其更改为使用类或结构.但是如果你无力改进API,你可以打开第二个元素的类型：

let tuple : (String,Any) = ("string",true)

switch tuple.1 {

case let x as Bool:
    print("It's a Bool")
    let boolTuple = (tuple.0,tuple.1 as! Bool)

case let x as Double:
    print("It's a Double")
    let doubleTuple = (tuple.0,tuple.1 as! Double)

case let x as NSDateFormatter:
    print("It's an NSDateFormatter")
    let dateFormatterTuple = (tuple.0,tuple.1 as! NSDateFormatter)

default:
    print("Unsupported type")
}

如果API返回Any并且不保证元组(String,Any),那么你运气不好.