swift - NSMutableDictionary:强制转换为不相关的 Swift 类型？

Question

谁能帮我解决这个问题，我尝试了各种版本，但总是遇到错误或警告。下面的代码和输出来自 Swift Playground。

var foundationDict = NSMutableDictionary()
foundationDict.setObject("Bilbo", forKey: "FirstName")
foundationDict.setObject("Baggins", forKey: "LastName")

var swiftDict = foundationDict as! Dictionary<String, String>
for (key, value) in swiftDict {
    print("KEY: \(key) VALUE: |\(value)")
}

输出:

KEY: FirstName VALUE: |Bilbo
KEY: LastName VALUE: |Baggins

警告:

Cast from NSMutableDictionary to unrelated type Dictionary<String String> always fails

Answer 1

我不确定为什么，但这样表述可以消除警告:

var swiftDict = (foundationDict as NSDictionary) as! Dictionary<String, String>

在这种情况下，类型系统似乎与 NSMutableDictionary/NSDictionary 斗争。