Swift: flatMap to Dictionary

Question

I am trying to parse the following JSON:

{ 
  "String For Key 1": [
    "Some String A",
    "Some String B",
    "Some String C",
  ],
  "String For Key 2": [
    "Some String D",
    "Some String E",
    "Some String F"
  ],
  "String For Key 3": [
    "Some String G",
    "Some String H",
    "Some String I",
  ],
  "String For Key 4": [
    "Some String J",
    "Some String K",
    "Some String L"
  ],
  "String For Key 5": [
    "Some String M",
    "Some String N",
    "Some String O"
  ],
  "String For Key 6": [
    "Some String P",
    "Some String Q",
    "Some String R"
  ]
}

I am also following this tutorial. It's on Github in Playground

In the example, they are parsing an Array of dictionaries using typealias JSONDictionary = [String: AnyObject]

I need to parse a Dictionary that has a Key which is String, and Value that is an Array. Hence: typealias JSONDictionary = [String: [AnyObject]] and I am getting an error when returning a flatMap.

extension Resource {
    init(url: NSURL, parseJSON: AnyObject -> A?) {
        self.url = url
        self.parse = { data in
            let json = try? NSJSONSerialization.JSONObjectWithData(data, options: []) as? [String: [AnyObject]]
           //This is where I get an error
            return json.flatMap(parseJSON)
        }
    }
}

Error:

Cannot convert value of type `AnyObject -> A? to expected argument type `([String: [AnyObject]]?) -> _?

How can I get a dictionary with Keys as String and Values as an Array? I would like to keep their Generic approach.

Answer 1

Type of jsonString is Dictionary<String, Array<String>> You can give it a type like Dictionary<String, AnyObject>

AnyObject can contain class and struct i.e Array, Dictionary, String, Int, Obj C Classes Hence an Array<String> is represented by AnyObject not [AnyObject]

let jsonString:[String: AnyObject] =
[
    "String For Key 1": [
    "http://www.urlToSomePathA.jpg",
    "http://www.urlToSomePathB.jpg",
    "http://www.urlToSomePathC.jpg",
    ],
    "String For Key 2": [
    "http://www.urlToSomePathD.jpg",
    "http://www.urlToSomePathE.jpg",
    "http://www.urlToSomePathF.jpg"
    ]
]

// Now you can map a function on the value type
let values = jsonString.flatMap { $0.1 as? [String] }
print(values)
// RESULT: [["http://www.urlToSomePathA.jpg", "http://www.urlToSomePathB.jpg", "http://www.urlToSomePathC.jpg"], ["http://www.urlToSomePathD.jpg", "http://www.urlToSomePathE.jpg", "http://www.urlToSomePathF.jpg"]]

Update

let keys = jsonString.flatMap { $0.0 } // ["String For Key 1", "String For Key 2"]

As commented by @dffri, you can use keys property on dictionary which will return LazyMapCollection which will realise only if you access the object inside.

let keys = jsonString.keys