I would like to create an immutable value that is assigned different values depending on a condition. In Scala I would be able to write the following:

我希望創建一個不可變的值，根據條件指定不同的值。在Scala里，我可以寫下以下內容:

    let attribs = if #available(iOS 8.2, *) {
        [ NSFontAttributeName: UIFont.systemFontOfSize(30, weight: UIFontWeightLight) ]
    } else {
        [ NSFontAttributeName: UIFont.systemFontOfSize(30) ]
    }

but since Swift if statements don't return the result of the executed code block that doesn't work.

但是由於Swift if語句不返回已執行代碼塊的結果，所以無法工作。

If the test was for a normal Bool I could use the ternary conditional operator

如果測試的是一個正常的Bool，我可以使用三元條件運算符

    let attribs = #available(iOS 8.2, *) ?
        [ NSFontAttributeName: UIFont.systemFontOfSize(30, weight: UIFontWeightLight) ]
    :
        [ NSFontAttributeName: UIFont.systemFontOfSize(30) ]

but that doesn't work for the iOS version tests, I get the following error:

但這對iOS版本測試不適用，我得到如下錯誤:

#available may only be used as condition of an 'if', 'guard' or 'while' statement.

I feel like I'm stuck with a var, but unless I also make it optional I end up with a double assignment in most cases, which seems so ugly and unnecessary?

我覺得我被困在一個var中了，但是除非我也把它設定為可選的，否則我在大多數情況下都會得到一個雙重的賦值，這看起來是如此的丑陋和不必要?

    var attribs = [ NSFontAttributeName: UIFont.systemFontOfSize(30) ]
    if #available(iOS 8.2, *) {
        [ NSFontAttributeName: UIFont.systemFontOfSize(30, weight: UIFontWeightLight) ]
    }

Thanks for any help!

感謝任何幫助!

3 个解决方案

let attribs: [String: Any]
if #available(iOS 8.2, *) {
    attribs = [NSFontAttributeName: UIFont.systemFontOfSize(30, weight: UIFontWeightLight)]
} else {
    attribs = [NSFontAttributeName: UIFont.systemFontOfSize(30)]
}

let value: String
if #available(iOS 9, *) {
    value = "iOS 9 is available"
} else {
    value = "iOS 9 and up only"
}
print(value) // iOS 9 is available

var someConstant:String {
    if #available(iOS 9, *) {
        return "iOS 9"
    } else {
        return"not iOS 9"
    }
}