"The data couldn’t be read because it is missing" error when decoding JSON in Swift

I am getting the following error :

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

When I run the following code:

struct Indicator: Decodable {
let section: String
let key: Int
let indicator: Int
let threshold: Int
}
var indicators = [Indicator]()


do {
if let file = Bundle.main.url(forResource: "indicators", withExtension: "json") {
indicators = try JSONDecoder().decode([Indicator].self, from: try Data(contentsOf: file))
}
} catch {
print(error.localizedDescription)
}

These are in a function, but I have removed them for clarity. I have a code block which is very similar in a different file (I copied this code from there and changed the names essentially) so I'm not sure why it's happening. The json file is valid json and has it's target properly set.

Thanks

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最佳答案

I just solved a similar issue on my end but for the property list decoder.

The error in this case seems to mean that a key wasn't found and not the data as a whole.

Try making the variables in your struct optional and it should return a nil value where the problem lies.

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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.

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

So never use ABC0 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.

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"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:...
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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)
}`
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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)
}

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