Decoding JSON strings containing arrays - json

Question

I'm encoding and decoding to and from JSON strings using JSONSerialization in the class below. I can encode both NSDictionaries & NSArrays and I can decode strings that have been encoded using NSDictionaries but not strings that were encoded from arrays, it barfs at JSONSerialization.jsonObject( ...
I can work without arrays, at a pinch but it would be nice to know why this is happening. Thoughts appreciated
let a = [1,2,3,4,5]
let s = JSON.Encode( a )!
JSON.Decode( s ) // fails
let a = ["a" : 1, "b" : 2, "c" : 3 ]
let s = JSON.Encode( a )!
JSON.Decode( s ) // works
-
class JSON: NSObject {
static func Encode( _ obj: Any ) -> String? {
do {
let data = try JSONSerialization.data(withJSONObject: obj, options:JSONSerialization.WritingOptions(rawValue: 0) )
if let string = NSString(data: data, encoding: String.Encoding.utf8.rawValue) {
return string as String
}
return nil
} catch let error as NSError {
return nil
}
}
static func Decode( _ s: String ) -> (NSDictionary?) {
let data = s.data(using: String.Encoding.utf8, allowLossyConversion: false)!
do {
// fails here to work on anything other than "{ a : b, c : d }"
// hates any [1,2,3] arrays in the string
let json = try JSONSerialization.jsonObject(with: data, options:JSONSerialization.ReadingOptions.mutableContainers) as! NSDictionary
return json
} catch let error as NSError {
return nil
}
}
}

Answer 1

You are casting the result to dictionary so it cannot work with something else.
A solution is to make the function generic and specify the expected return type.
I cleaned up the code a bit, basically it’s recommended to make a function throw if it contains a throwing API.
class JSON {
enum JSONError : Error { case typeMismatch }
static func encode( _ obj: Any ) throws -> String {
let data = try JSONSerialization.data(withJSONObject: obj)
return String(data: data, encoding: .utf8)!
}
static func decode<T>( _ s: String ) throws -> T {
let data = Data(s.utf8)
guard let result = try JSONSerialization.jsonObject(with: data) as? T else {
throw JSONError.typeMismatch
}
return result
}
}
let a = [1,2,3,4,5]
do {
let s = try JSON.encode(a)
let u : [Int] = try JSON.decode(s)
print(u)
} catch { print(error) }
Note (as always):
In Swift do not use NSDictionary, NSString and NSErrorand .mutableContainers is completely pointless.