扩展字典，其中Key是String类型 [英] Extend Dictionary where Key is of type String

问题描述

我想扩展一个Dictionary的方法，但只有当Key是String类型时。

I want to have extend a method of Dictionary but only if Key is of type String.

我尝试这样做：

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

并得到错误：

键入'Key'约束到非协议类型'String'

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

根据这个错误信息，我可以告诉我只能用协议进行这种过滤...有没有办法绕过这个？ p>

Based on this error message I can tell that I can only do this kind of filtering with protocols... Is there a way to by-pass this?

推荐答案

我相信最符合您需求的协议是 StringLiteralConvertible 有几条额外的行，将让你完成这个

I believe the closest protocol that meets your needs is StringLiteralConvertible, which, with a few extra lines, will let you accomplish this

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

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