How to convert array of dictionary to JSON?

Question

I have an array of dictionaries that I'd like to convert to JSON. My object is of type [[String: AnyObject]] and would like to end up with a sample like this:

[
  { "abc": 123, "def": "ggg", "xyz": true },
  { "abc": 456, "def": "hhh", "xyz": false },
  { "abc": 789, "def": "jjj", "xyz": true }
]

This is what I'm trying, but the compiler is not liking my declaration:

extension Array where Element == Dictionary<String, AnyObject> {
    var json: String {
        do { return try? NSJSONSerialization.dataWithJSONObject(self, options: []) ?? "[]" }
        catch { return "[]" }
    }
}

How can I do this?

so your question isn't about JSON generation but constraining Element. — vikingosegundo
– vikingosegundo, Commented Sep 11, 2016 at 18:36
dataWithJSONObject returns NSData on success, so at least you have to create a string to return. — vadian
– vadian, Commented Sep 11, 2016 at 18:46

Eric Aya · Accepted Answer · 2016-09-11 19:48:36Z

14

A simple way to achieve that is to just extend CollectionType.

Use optional binding and downcasting, then serialize to data, then convert to string.

extension CollectionType where Generator.Element == [String:AnyObject] {
    func toJSONString(options: NSJSONWritingOptions = .PrettyPrinted) -> String {
        if let arr = self as? [[String:AnyObject]],
            let dat = try? NSJSONSerialization.dataWithJSONObject(arr, options: options),
            let str = String(data: dat, encoding: NSUTF8StringEncoding) {
            return str
        }
        return "[]"
    }
}

let arrayOfDictionaries: [[String:AnyObject]] = [
    ["abc":123, "def": "ggg", "xyz": true],
    ["abc":456, "def": "hhh", "xyz": false]
]

print(arrayOfDictionaries.toJSONString())

Output:

[
  {
    "abc" : 123,
    "def" : "ggg",
    "xyz" : true
  },
  {
    "abc" : 456,
    "def" : "hhh",
    "xyz" : false
  }
]

edited Sep 11, 2016 at 19:48

answered Sep 11, 2016 at 19:36

Eric Aya

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3 Comments

Cristik Over a year ago

This will solve the asker's issue, but introduce subtle bugs, like dictionaries being converted to [] instead of their JSON equivalent. I think the asker also seeks for type safety, which would prevent runtime issues like this.

Eric Aya Over a year ago

@Cristik Good thinking. I've added a type constraint to the extension. It should be closer to what OP wanted.

teradyl Over a year ago

I'm running into Type of expression is ambiguous without more context errors when I create the arrayOfDictionaries. This may require a different question but it's making this solution not work...

Alvin George · Accepted Answer · 2017-10-12 12:32:39Z

10

Swift 4:

extension Collection where Iterator.Element == [String:AnyObject] {
    func toJSONString(options: JSONSerialization.WritingOptions = .prettyPrinted) -> String {
        if let arr = self as? [[String:AnyObject]],
            let dat = try? JSONSerialization.data(withJSONObject: arr, options: options),
            let str = String(data: dat, encoding: String.Encoding.utf8) {
            return str
        }
        return "[]"
    }
}

Usage:

let arrayOfDictionaries = [
  { "abc": 123, "def": "ggg", "xyz": true },
  { "abc": 456, "def": "hhh", "xyz": false },
  { "abc": 789, "def": "jjj", "xyz": true }
]

print(arrayOfDictionaries.toJSONString())

answered Oct 12, 2017 at 12:32

Alvin George

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1 Comment

J.Arji Over a year ago

return error (key: String, value: Any)' is not convertible to '[String : Any]'

teradyl · Accepted Answer · 2021-05-29 04:51:11Z

2

Swift 5, I had to change it to Any instead of AnyObject, building off solutions of Eric & A.G.

extension Collection where Iterator.Element == [String: Any] {
  func toJSONString(options: JSONSerialization.WritingOptions = .prettyPrinted) -> String {
    if let arr = self as? [[String: Any]],
       let dat = try? JSONSerialization.data(withJSONObject: arr, options: options),
       let str = String(data: dat, encoding: String.Encoding.utf8) {
      return str
    }
    return "[]"
  }
}

Usage:

let arrayOfDictionaries = [
  { "abc": 123, "def": "ggg", "xyz": true },
  { "abc": 456, "def": "hhh", "xyz": false },
  { "abc": 789, "def": "jjj", "xyz": true }
]

print(arrayOfDictionaries.toJSONString())

answered May 29, 2021 at 4:51

teradyl

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Comments

Cristik · Accepted Answer · 2016-09-11 19:55:52Z

0

I think you're out of luck trying to extend Array, because same-type requirements are not possible when extending an entity.

What you can make is to make use of free functions:

func jsonify(array: [[String:AnyObject]]) { 
...
}

This way you'll keep the type safety you want to, with the expense of moving the function location.

answered Sep 11, 2016 at 19:55

Cristik

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