ios - 下标:使用 String 枚举访问我的字典值

Question

我想做类似的事情:使用 String 枚举访问我的字典值。我试图重载字典的下标但没有成功。

访问字典:

let district = address[JsonKeys.district]

JsonKeys 在哪里:

enum JsonKeys: String {
    case key1
    case key2
    case key...
}

我的下标重载如下:

extension Dictionary where Key: StringLiteralConvertible, Value: AnyObject {
    subscript(index: FOJsonKeys) -> AnyObject {
        get {
            return self[ index.rawValue] as! AnyObject
        }
    }
}

我收到以下消息:

**Cannot subscript a value of type 'Dictionary<Key, Value>' with an index of type 'String'**

我哪里错了？

PS:不想这样做(这样可以更正错误，但是这样代码是不可读的):

let district = address[JsonKeys.district.rawValue]

字典是AlamoFire给我的Json解析字典。我很确定我不能改变它的类型。

Answer 1

最简单的方法是将字典提升到更多上下文中。这种情况下的上下文是“它只有来自这个枚举的键”。在 Swift 中提升类型非常简单。只需将其包装在一个结构中即可。

// This could be a nested type inside JSONObject if you wanted.
enum JSONKeys: String {
    case district
}

// Here's my JSONObject. It's much more type-safe than the dictionary,
// and it's trivial to add methods to it.
struct JSONObject {
    let json: [String: AnyObject]
    init(_ json: [String: AnyObject]) {
        self.json = json
    }

    // You of course could make this generic if you wanted so that it
    // didn't have to be exactly JSONKeys. And of course you could add
    // a setter.
    subscript(key: JSONKeys) -> AnyObject? {
        return json[key.rawValue]
    }
}

let address: [String: AnyObject] = ["district": "Bob"]

// Now it's easy to lift our dictionary into a "JSONObject"
let json = JSONObject(address)

// And you don't even need to include the type. Just the key.
let district = json[.district]