W.venus W.venus - 1 month ago 121
JSON Question

Crash : Convert dictionary to Json string in Swift 3

I'm trying to convert my swift dictionary to Json string but getting strange crash by saying

Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: 'Invalid type in JSON write (_SwiftValue)'

My code:

let jsonObject: [String: AnyObject] = [
"firstname": "aaa",
"lastname": "sss",
"email": "my_email",
"nickname": "ddd",
"password": "123",
"username": "qqq"
]

do {
let jsonData = try JSONSerialization.data(withJSONObject: jsonObject, options: .prettyPrinted)
// here "jsonData" is the dictionary encoded in JSON data

let decoded = try JSONSerialization.jsonObject(with: jsonData, options: [])
// here "decoded" is of type `Any`, decoded from JSON data

// you can now cast it with the right type
if let dictFromJSON = decoded as? [String:String] {
// use dictFromJSON
}
} catch {
print(error.localizedDescription)
}


Please help me!

Regards.

Answer

String is not of type AnyObject. Objects are reference types, but String in swift has value semantics. A String however, can be of type Any, so the code below works. I suggest you read up on reference types and value semantic types in Swift; its a subtle but important distinction and its also different from what you expect from most other languages, where String is often a reference type (including objective C).

    let jsonObject: [String: Any] = [
        "firstname": "aaa",
        "lastname": "sss",
        "email": "my_email",
        "nickname": "ddd",
        "password": "123",
        "username": "qqq"
    ]

    do {
        let jsonData = try JSONSerialization.data(withJSONObject: jsonObject, options: .prettyPrinted)
        // here "jsonData" is the dictionary encoded in JSON data

        let decoded = try JSONSerialization.jsonObject(with: jsonData, options: [])
        // here "decoded" is of type `Any`, decoded from JSON data

        // you can now cast it with the right type
        if let dictFromJSON = decoded as? [String:String] {
            print(dictFromJSON)
        }
    } catch {
        print(error.localizedDescription)
    }