swift - 扩展 Dictionary，其中 Key 是 String 类型

Question

我想扩展 Dictionary 的方法，但前提是 Key 是 String 类型。

我试着这样做:

extension Dictionary where Key: String {
    mutating func lowercaseKeys() {
        for key in self.keys {
            self[key.lowercase] = self.removeValueForKey(key)
        }
    }
}

并得到错误:

Type 'Key' constrained to non-protocol type 'String'

根据这个错误消息，我可以知道我只能使用协议(protocol)进行这种过滤...有没有办法绕过这个？

Answer 1

我相信最能满足您需求的协议(protocol)是 StringLiteralConvertible，只需多加几行，您就可以完成此任务

extension Dictionary where Key: StringLiteralConvertible {
    mutating func setAllKeysLowercase() {
        for key in self.keys {
            if let lowercaseKey = String(key).lowercaseString as? Key {
                self[lowercaseKey] = self.removeValueForKey(key)
            }
        }
    }
}

var stringKeyDictionary = [ "Hello" : NSObject(), "World" : NSObject() ]
stringKeyDictionary.setAllKeysLowercase()
print( stringKeyDictionary )

// Prints: ["hello": <NSObject: 0x1007033c0>, "world": <NSObject: 0x1007033d0>]

var numberKeyDictionary = [ 0 : NSObject(), 1: NSObject() ]
numberKeyDictionary.setAllKeysLowercase() //< Won't compile, keys are not strings