Swift: Convert struct to JSON? - json

Question

I created a struct and want to save it as a JSON-file.
struct Sentence {
var sentence = ""
var lang = ""
}
var s = Sentence()
s.sentence = "Hello world"
s.lang = "en"
print(s)
...which results in:
Sentence(sentence: "Hello world", lang: "en")
But how can I convert the struct object to something like:
{
"sentence": "Hello world",
"lang": "en"
}

Answer 1

Swift 4 introduces the Codable protocol which provides a very convenient way to encode and decode custom structs.
struct Sentence : Codable {
let sentence : String
let lang : String
}
let sentences = [Sentence(sentence: "Hello world", lang: "en"),
Sentence(sentence: "Hallo Welt", lang: "de")]
do {
let jsonData = try JSONEncoder().encode(sentences)
let jsonString = String(data: jsonData, encoding: .utf8)!
print(jsonString) // [{"sentence":"Hello world","lang":"en"},{"sentence":"Hallo Welt","lang":"de"}]
// and decode it back
let decodedSentences = try JSONDecoder().decode([Sentence].self, from: jsonData)
print(decodedSentences)
} catch { print(error) }

Answer 2

Swift 4 supports the Encodable protocol e.g.
struct Sentence: Encodable {
var sentence: String?
var lang: String?
}
let sentence = Sentence(sentence: "Hello world", lang: "en")
Now you can automatically convert your Struct into JSON using a JSONEncoder:
let jsonEncoder = JSONEncoder()
let jsonData = try jsonEncoder.encode(sentence)
Print it out:
let jsonString = String(data: jsonData, encoding: .utf8)
print(jsonString)
{
"sentence": "Hello world",
"lang": "en"
}
https://developer.apple.com/documentation/foundation/archives_and_serialization/encoding_and_decoding_custom_types

Answer 3

Use the NSJSONSerialization class.
Using this for reference, you may need to create a function which returns the JSON serialized string. In this function you could take the required properties and create a NSDictionary from them and use the class mentioned above.
Something like this:
struct Sentence {
var sentence = ""
var lang = ""
func toJSON() -> String? {
let props = ["Sentence": self.sentence, "lang": lang]
do {
let jsonData = try NSJSONSerialization.dataWithJSONObject(props,
options: .PrettyPrinted)
return String(data: jsonData, encoding: NSUTF8StringEncoding)
} catch let error {
print("error converting to json: \(error)")
return nil
}
}
}
Because your struct only has two properties it might be easier to just build the JSON string yourself.

Answer 4

Here's a nice extension and a method for JSON encoding/decoding:
extension Encodable {
func toJSONString() -> String {
let jsonData = try! JSONEncoder().encode(self)
return String(data: jsonData, encoding: .utf8)!
}
}
func instantiate<T: Decodable>(jsonString: String) -> T? {
return try? JSONDecoder().decode(T.self, from: jsonString.data(using: .utf8)!)
}
Sample usage:
struct Sentence: Codable {
var sentence = ""
var lang = ""
}
let sentence = Sentence(sentence: "Hello world", lang: "en")
let jsonStr = sentence.toJSONString()
print(jsonStr) // prints {"lang":"en","sentence":"Hello world"}
let sentenceFromJSON: Sentence? = instantiate(jsonString: jsonStr)
print(sentenceFromJSON!) // same as original sentence