How Can I Decode a JSON String Containing an Embedded JSON Object in Swift?

Below is an explanation of both issues along with one way to solve them.

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Why Does Decoding Fail?

When your JSON response’s token value is a plain string, the decoder can simply read it. However, when it’s a JSON‐encoded object as a string (e.g.
"value": "{"adblock":false,...}"), that string still has JSON syntax (with escaped quotes). If you try to decode it directly as a string into a property that you later expect to decode as JSON, the decoder isn’t automatically “unwrapping” and parsing that inner JSON. In other words, Swift’s JSONDecoder sees the raw text with escape characters; if you try to decode this “value” as if it were already an object, you get an error like “dataCorrupted” because the inner text isn’t valid JSON at the outer level.

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How to Decode the Value Field Properly

One common approach is a two‑step process:

1. Decode the outer response normally.
   In your model, let the value property come in as a normal String.

2. Re-decode the inner JSON string.
   Once you have the raw string (which is actually JSON text), convert it to a Data object (for example, using data(using: .utf8)) and then run it through a JSONDecoder (or JSONSerialization) to obtain the inner object.

For example, suppose you have a model for the inner token information. You might define it like this:

SWIFTCopied
struct PlaybackTokenDetails: Decodable {
    let adblock: Bool
    let authorization: Authorization
    // ... additional fields
}

struct Authorization: Decodable {
    let forbidden: Bool
    let reason: String
}

Then, adjust your Auth_Data model to do a second-pass decode:

SWIFTCopied
struct Auth_Data: Decodable {
    // The raw value from the outer JSON
    let rawValue: String
    let signature: String

    // A computed property or an extra initializer can do the inner decoding:
    var tokenDetails: PlaybackTokenDetails? {
        guard let data = rawValue.data(using: .utf8) else { return nil }
        return try? JSONDecoder().decode(PlaybackTokenDetails.self, from: data)
    }

    private enum CodingKeys: String, CodingKey {
        case rawValue = "value"
        case signature
    }
}

With this approach, your outer decoding decodes value as a string. Then, if you need to work with the JSON object inside, you can call the computed property tokenDetails to get the decoded inner structure.

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Alternative: Custom Decoding Directly in Auth_Data

If you want to automatically check whether the value is a plain string or a JSON object encoded as a string, you can write a custom initializer for Auth_Data that tries to decode the inner JSON. For example:

SWIFTCopied
struct Auth_Data: Decodable {
    let tokenDetails: PlaybackTokenDetails
    let signature: String

    private enum CodingKeys: String, CodingKey {
        case value, signature
    }

    init(from decoder: Decoder) throws {
        let container = try decoder.container(keyedBy: CodingKeys.self)
        // First, grab the raw string
        let rawValue = try container.decode(String.self, forKey: .value)
        guard let innerData = rawValue.data(using: .utf8) else {
            throw DecodingError.dataCorruptedError(forKey: .value,
                                                   in: container,
                                                   debugDescription: "Invalid encoding for JSON object in value")
        }
        do {
            self.tokenDetails = try JSONDecoder().decode(PlaybackTokenDetails.self, from: innerData)
        } catch {
            throw DecodingError.dataCorruptedError(forKey: .value,
                                                   in: container,
                                                   debugDescription: "Failed to decode inner JSON: \(error.localizedDescription)")
        }
        self.signature = try container.decode(String.self, forKey: .signature)
    }
}

In this example the custom initializer for Auth_Data takes the string value, converts it to Data, and then decodes it into a PlaybackTokenDetails structure. Adjust the inner model (PlaybackTokenDetails) to include all fields you need.

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Summary

• Why Swift fails: When the value field contains an escaped JSON object (i.e. a string that itself is valid JSON) the standard decoding path expects a plain string. Trying to decode it directly (as if it were already a decoded JSON object) leads to a parsing error.

• How to fix it: First decode the field as a string, then convert that string to a Data object and decode it into your desired inner model. This can be done with a computed property or within a custom initializer for your authentication model.

This solution lets your code gracefully handle both plain strings and JSON-encoded strings. Happy coding!