How to fix? Expected to decode Dictionary but found a string/data instead

Question

What is wrong here? Or how else I should decode, I would NOT use JSONSerialize.

let jsonData = try! Data(contentsOf: urls[0])
let decoder = JSONDecoder()
let d = try decoder.decode([String: JSON].self, from: jsonData)

file content is a simple JSON:

{"name": "fff", "price": 10}

And my JSON code:

public enum JSON: Decodable {
    case string(String)
    case number(Float)
    case object([String:JSON])
    case array([JSON])
    case bool(Bool)
}

If *file content is a simple* why do you decode so complicated? Consider that you have to *extract* the cases later which is unnecessarily expensive. If you have a generic way in mind, in terms of Swift this is not *generic*. — vadian, Sep 09 '22 at 15:23

Joakim Danielson · Accepted Answer · 2022-09-09T15:49:23.930

1

You need to add a custom init(from:) where you try to decode into each possible enum case until you are successful or throw an error

Here is a short version that handles three of the cases

struct EnumDecoderError: Error {}

public init(from decoder: Decoder) throws {
    let container = try decoder.singleValueContainer()
    if let string = try? container.decode(String.self) {
        self = JSON.string(string)
    } else if let number = try? container.decode(Float.self) {
        self = JSON.number(number)
    } else if let array = try? container.decode([JSON].self) {
        self = JSON.array(array)
    } else {
        throw EnumDecoderError()
    }
}

as mentioned in the comments by @LeoDabus we can catch typeMismatch errors (and throw any other error directly) or as before throw an error at the end if no decoding worked. (Again a shortened version)

public init(from decoder: Decoder) throws {
    let container = try decoder.singleValueContainer()

    do {
        let string = try container.decode(String.self)
        self = JSON.string(string)
    } catch DecodingError.typeMismatch {
        do {
            let number = try container.decode(Float.self)
            self = JSON.number(number)
        } catch DecodingError.typeMismatch {
            do {
                let array = try container.decode([JSON].self)
                self = JSON.array(array)
            } catch DecodingError.typeMismatch {
                throw DecodingError.typeMismatch(JSON.self, .init(codingPath: decoder.codingPath, debugDescription: "Data type is not supported"))
            }
        }
    }
}

edited Sep 09 '22 at 15:49

answered Sep 09 '22 at 14:58

Joakim Danielson

43,251
5
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I think it would be better to catch the error instead of discarding it. – Leo Dabus Sep 09 '22 at 15:00
What error do you refer to, simply using `try` instead of `try?` for the last attempt? – Joakim Danielson Sep 09 '22 at 15:05
1

For all of them. You need to catch only typeMismatch errors. Check this [post](https://stackoverflow.com/a/71370811/2303865) – Leo Dabus Sep 09 '22 at 15:07
You can simply not catch the last try AFAIR. – Leo Dabus Sep 09 '22 at 17:04
It works fine in my test and not catching it gives an error message that is a bit confusing, "Expected to decode Array but found..." – Joakim Danielson Sep 09 '22 at 17:32
Can you create a gist and post the link? – Leo Dabus Sep 09 '22 at 17:46
Gist created (first time for me so I hope it works), https://gist.github.com/joadan990/9af1ffdc29a279d26e27e630f2e71664 – Joakim Danielson Sep 09 '22 at 18:06
That is what I meant https://gist.github.com/leodabus/4c1e3e6790562a9cb28d519f14c6a5b6 – Leo Dabus Sep 09 '22 at 20:00

score 0 · Answer 2 · edited Nov 22 '22 at 20:52

0

You need to decode it into a structure

private struct MyData: Codable {
    var name: String?
    var price: Int?
}

...
let jsonData = try! Data(contentsOf: urls[0])
let d = try JSONDecoder().decode(MyData.self, from: jsonData)
...

edited Nov 22 '22 at 20:52

fmaccaroni

3,846
1
20
35

answered Sep 09 '22 at 14:55

Suyash Medhavi

1,135
6
18

Namig your struct `Data` is definitely not a good idea. – Leo Dabus Sep 09 '22 at 14:58
@LeoDabus you are right. Changed to `MyData` – Suyash Medhavi Sep 09 '22 at 14:59
JSON is always a string. Data init contentsOf should never be used to download remote resources. Btw you should decode MyData not Data – Leo Dabus Sep 09 '22 at 15:01
I can't see how this solution handles array or dictionary properties that are part of the enum? – Joakim Danielson Sep 09 '22 at 15:15
@JoakimDanielson I have created the struct from your sample JSON. You will need to modify the struct based on what JSON key the array (or other types) are coming – Suyash Medhavi Sep 09 '22 at 15:19
I am not OP. Your answer should include this as well since the enum has a dynamic structure and it isn’t self explanatory how your solution handles this from the current answer. – Joakim Danielson Sep 09 '22 at 16:43

Vikram Parimi · Answer 3 · 2022-09-09T15:13:39.677

First of all you don't need to maintain datatypes in the JSON enum to parse your response.

The JSONDecoder will be able to parse with the appropriate datatype if you match your object to the response structure that you receive from APIs or JSON files maintained locally

Taking you json file as example:

{"name": "fff", "price": 10}

The recommended way to parse this structure would be as follows

Create a struct or class according to your needs. For this I will use a struct
```
struct Product: Decodable {
  var name: String?
  var price: Int?
```
}

I have marked both the vars optionals just in case to handle the failures if the field does not exist in the JSON response.
Parse the Structure

Use the product struct that was created in the previous step by creating a decoder instance and setting the Product.Self to parse the object

let decoder = JSONDecoder()
let productObject = try decoder.decode(Product.self, from: jsonData)

If you have an array of objects of the same structure in the JSON response use below:

let productObjects = try decoder.decode([Product].self, from: jsonData)

Just include [] around the product object

Please clean up your code, struct is with a lowercase s and Product needs to conform to Decodable and there are no comma between property declarations. — Joakim Danielson, Sep 09 '22 at 15:12
Also, I can't see how this solution handles array or dictionary properties that are part of the enum? — Joakim Danielson, Sep 09 '22 at 15:14

How to fix? Expected to decode Dictionary but found a string/data instead

3 Answers3