swift - extension of Dictionary where <String, AnyObject>

Question

I am trying to create a dictionary extension where Dictionary is of the type <String, AnyObject>.

Was looking in many places and trying different approaches, but none of them seemed to work. This was one of them:

extension Dictionary where <String, AnyObject>{
    var jsonString:String {
        return ""
    }
}

Another method that didn't actually work for some reason:

extension Dictionary where Key:Hashable, Value:AnyObject {

    var jsonString:String {

        do {
           let stringData = try NSJSONSerialization.dataWithJSONObject(self, options: NSJSONWritingOptions.PrettyPrinted)
            if let string = String(data: stringData, encoding: NSUTF8StringEncoding){
                return string
            }
        }catch _ {

        }
        return ""
    }
}

Got: Argument type 'Dictionary' does not conform to expected type of 'AnyObject'

Answer 1

>=3.1

From 3.1, we can do concrete extensions, ie:

extension Dictionary where Key == String {}

<3.1

We can not conform concrete types w/ concrete generics, ie:

extension Dictionary where Key == String

However, because Dictionary conforms to sequence and we can conform protocol types w/ concrete generics, we could do:

extension Sequence where Iterator.Element == (key: String, value: AnyObject) {
    func doStuff() {...

Otherwise, we can constrain our key to a protocol that string conforms to like this:

extension Dictionary where Key: StringLiteralConvertible, Value: AnyObject {
    var jsonString: String {
        return ""
    }
}

As per your updated answer. Json serialization needs an object, Swift Dictionaries are structs. You need to convert to an NSDictionary You must specify Key to conform to NSObject to properly convert to an NSDictionary.

Small note: Dictionaries already type constrain Key to be Hashable, so your original constraint wasn't adding anything.

extension Dictionary where Key: NSObject, Value:AnyObject {

    var jsonString:String {

        do {
            let stringData = try NSJSONSerialization.dataWithJSONObject(self as NSDictionary, options: NSJSONWritingOptions.PrettyPrinted)
            if let string = String(data: stringData, encoding: NSUTF8StringEncoding){
                return string
            }
        }catch _ {

        }
        return ""
    }
}

Note, that the dictionaries must conform to this type to access the extension.

let dict = ["k" : "v"]

Will become type [String : String], so you must be explicit in declaring:

let dict: [NSObject : AnyObject] = ["k" : "v"]