使用 Swift 5 将字典数据转换为 JSON 格式

Question

At play ground I used following code to convert dictionary data to create JSON file. It worked but not exactly as I wanted.

var topLevel: [AnyObject] = []
var myDict : [String:Any] = [:]


myDict ["label"] = "label: carrots" as AnyObject
myDict ["coordinates"] =  [ "x: 120" as AnyObject,
                           "y: 164" as AnyObject,
                           "width: 230" as AnyObject,
                           "height: 119" as AnyObject]

let jsonData = try JSONSerialization.data(withJSONObject: myDict, options: .prettyPrinted)
let fileManager = FileManager.default
let url = FileManager.default.urls(for: .documentDirectory, in: .userDomainMask).first!
let jsonUrl = url.appendingPathComponent("test.json")  

try? jsonData.write(to: jsonUrl )

When we open created test.json file we see that.

 {
 "label" : "label: carrots",
"coordinates" : [
  "x: 120",
  "y: 164",
  "width: 230",
  "height: 119"
 ]
}

But we want to see is that, instead of "{" we are getting "[" after coordinates.

          {
             "label": "carrots",
             "coordinates": {
             "x": 120
             "y": 164
             "width": 230
              "height": 119
            }

Answer 1

You could use the following:

myDict ["coordinates"] = [ "x" : 120,
                           "y" : 164,
                           "width" : 230,
                           "height": 119 ]

So you are supplying a dictionary with key value pairs in Swift.

If you print the jsonData with the above code, you will get:

"label" : "label: carrots",
"coordinates" : {
    "height" : 119,
    "y" : 164,
    "x" : 120,
    "width" : 230
}

JSONEncoder Alternative

In general you might want to take a look at JSONEncoder and use it instead, see the documentation here: https://developer.apple.com/documentation/foundation/jsonencoder

Your example adapted for use with JSONEncoder would look something like this:

struct TopLevel: Encodable {
    let label: String
    let coordinates: Coordinates
}

struct Coordinates: Encodable {
    let x: Int
    let y: Int
    let witdth: Int
    let height: Int
}

...


let topLevel = TopLevel(label: "carrots",
                        coordinates: Coordinates(x: 120,
                                                 y: 164,
                                                 witdth: 230,
                                                 height: 119))

let encoder = JSONEncoder()
encoder.outputFormatting = .prettyPrinted

if let data = try? encoder.encode(topLevel) {
    print(String(data: data, encoding: .utf8) ?? "")
}

The advantage is type checking during compile time: the compiler enforces type constraint rules.

Answer 2

I listen @Stephan Schlecht and changed code like this it worked. And for the people who wonder why I am trying to do this;

I will create object detection model with Apple's Create ML it needs this kind data as JSON along with the images.

I was able to get positions and dimensions of the objects images in pictures by using Image Classifier. But know I am planing to use more automation to get images in pictures to produce this type of data.

struct Coordinates: Codable {
  var x: Int
  var y: Int
  var width: Int
  var height: Int
}


struct ID: Codable {    
  var label: String = "carrot"
  var coordinates = Coordinates(x: 10, y: 10, width: 100, height: 100)
}




let  myData = ID()
let encoder = JSONEncoder()
encoder.outputFormatting = .prettyPrinted
let data = try encoder.encode(myData)
print(String(data: data, encoding: .utf8)!)



let fileManager = FileManager.default
let url = FileManager.default.urls(for: .documentDirectory, in: .userDomainMask).first!
let jsonUrl = url.appendingPathComponent("test.json")  

try? data.write(to: jsonUrl )

{
  "label" : "carrot",
  "coordinates" : {
  "y" : 10,
  "x" : 10,
  "width" : 100,
  "height" : 100
  }
}