Swift extension: Dictionary<String, AnyObject> to JSON string for URL parameter

Question

Here's a quick extension for [String: AnyObject] dictionaries in Swift. The output needs to be in the following format (order and spacing don't matter):

{"key": "val", "keywdict": {"anotherKey": 100, "Key2": "Val2"}, "strings": ["string", "another"]}

I am wondering if there's a better way, or if this can be improved.

extension Dictionary //...
func toJSONString() -> String {
 var pcs = [String]()
 for (key, val) in self {
 var valStr = ""
 if let val = val as? String {
 valStr = "\"\(val)\""
 } else if let val = val as? Dictionary<String, AnyObject> {
 valStr = val.toJSONString() // recursion for nested dictionaries
 } else if let val = val as? Array<String> {
 let tmpStr = "\",\"".join(val)
 valStr = "[\"\(tmpStr)\"]"
 } else if let val = val as? NSNumber {
 valStr = "\(val)"
 }
 pcs.append("\"\(key)\":\(valStr)")
 }
 return "{" + ",".join(pcs) + "}"
}

• Legato: Welcome to CodeReview, Andrew. I hope you get some fine answers.

Answer 1

Your dictionary to JSON converter has some serious issues:

• The special characters " and \in strings are not properly escaped:

  [ "a\"b" : "c\\d"].toJSONString()
  

  produces "{"a"b":"c\d"}", correct would be {"a\"b":"c\\d"} (see for example http://json.org).

• Only arrays of strings are implemented, but not arrays of numbers or arrays of arrays or dictionaries.
• Value types which are not covered by your code just print as an empty string instead of causing an error.

• Boolean values are simply treated as numbers:

  [ "key" : true].toJSONString()
  

  gives {"key":1}, correct would be {"key":true}

While this could all be fixed, it probably isn't worth the effort because there is a Foundation class NSJSONSerialization which already does exactly this:

var dict : [String: AnyObject] = [:]
dict["a\"b"] = "c\\d"
dict["strings"] = ["string", "another"]
dict["keywdict"] = [ "anotherKey": 100, "Key2": "Val2"]
dict["numbers"] = [ 1, 2, 3]
dict["bools"] = [ true, false]
var error : NSError?
if let jsonData = NSJSONSerialization.dataWithJSONObject(dict, options: nil, error: &error) {
 let jsonString = NSString(data: jsonData, encoding: NSUTF8StringEncoding)! as String
 println(jsonString)
} else {
 println("Error in JSON conversion: \(error!.localizedDescription)")
}

Output:

{"keywdict":{"anotherKey":100,"Key2":"Val2"},"bools":[true,false],"a\"b":"c\\d","strings":["string","another"],"numbers":[1,2,3]}

• Andrew Robinson: Thanks for the input, @Martin R. For my implementation (which I should have stated) I knew that I only needed to handle [String] arrays and numerics, though I totally goofed on escaping characters! The NSJSONSerialization implementation is CLEARLY the better solution, and I totally missed it. Thanks again.