"The Data Couldn’t Be Read Because It Is Missing" Error When Decoding Json in Swift

Printing error.localizedDescription is misleading because it displays only a quite meaningless generic error message.

So never use localizedDescription in Decodable catch blocks.

In the simple form just

print(error)

It shows the full error including the crucial information debugDescription and context.Decodable errors are very comprehensive.

While developing code you could catch each Decodable error separately for example

} catch let DecodingError.dataCorrupted(context) {
    print(context)
} catch let DecodingError.keyNotFound(key, context) {
    print("Key '\(key)' not found:", context.debugDescription)
    print("codingPath:", context.codingPath)
} catch let DecodingError.valueNotFound(value, context) {
    print("Value '\(value)' not found:", context.debugDescription)
    print("codingPath:", context.codingPath)
} catch let DecodingError.typeMismatch(type, context)  {
    print("Type '\(type)' mismatch:", context.debugDescription)
    print("codingPath:", context.codingPath)
} catch {
    print("error: ", error)
}

It shows only the most significant information.

"The data couldn’t be read because it is missing"

the error coming from this code:

...catch {
    print(error.localizedDescription)
}

Because: seems that a key is missing or mistyped.

You can check which key is missing by coding like this:

...catch {
    debugPrint(error)
}

Note: If struct keys are different from JSON data keys, see below example: the key in struct is 'title' but in data is 'name'.

struct Photo: Codable {
    var title: String
    var size: Size

    enum CodingKeys: String, CodingKey
    {
        case title = "name"
        case size
    }
}

If you mistype 'name', the error will pop up.

Also, if you mistype this 'CodingKeys', you will get the error.

enum CodingKeys:...

Try printing the actual error instead of just the description. It should give you a message like "No value associated with key someKey (\"actual_key_if_you_defined_your_own\").", which is much more useful than the localizedDescription.

The thing most people use but they should not is

print(error.localizedDescription) 

As it is extremely misleading when it comes to JSON files. Instead, just use

print(error)

Just had a same error. I had an error in manual code for decoder. In my code property completedOn was optional, but I was using try instead of try? when decoding it. When the value was missing in json, the decoding would fail for that property. See the code bellow to better understand what I mean.

public var uuid: UUID
public var completedOn: Date?

...

required public convenience init(from decoder: Decoder) throws {
    self.init()

    let container = try decoder.container(keyedBy: CodingKeys.self)
    self.uuid = try container.decode(UUID.self, forKey: .uuid)
    self.completedOn = try? container.decode(Date.self, forKey: .completedOn)
}

First make the properties optional then

If your case similar to this try this decodeIfPresent

    `public init(from decoder: Decoder) throws {
    let container = try decoder.container(keyedBy: CodingKeys.self)
    firstName = try container.decodeIfPresent(String.self, forKey: .firstName)
    }`