Swift Codable: Cannot decode dictionary of type [String: Any] or [String: Decodable]

Question

In my custom initializer I'd like to decode a dictionary from JSON and then assign its values to properties in the class. To my surprise, compiler does not allow me to decode the dictionary, I get the error:

Value of protocol type 'Any' cannot conform to 'Decodable'; only struct/enum/class types can conform to protocols

If I try to decode dictionary of type [String: Decodable] the error message says:

Value of protocol type 'Decodable' cannot conform to 'Decodable'; only struct/enum/class types can conform to protocols

My initializer looks like this:

public let total: Int

required init(from decoder: Decoder) throws {
    let container = try decoder.container(keyedBy: CodingKeys.self)
    ...
    if let dict = try container.decodeIfPresent([String: Any].self, forKey: .tracks),
           let value = dict["total"] as? Int { // Error is displayed at this line
        total = value
    } else {
        total = 0
    }
    ...
}

When I looked for the answer I found this answer and according to it the code above should not cause any problems.

Answer 1

What you are looking for is nestedContainer. It helps you "skip" a hierarchy level in your JSON. Ie: let's imagine that in your code, it's all in the same level (one struct), but in the JSON, it's not, there is a sub dictionary.

For test purpose, your JSON might look like:

{
    "title": "normal",
    "tracks": {
                   "name": "myName",
                   "total": 3
              }
}

If we want in our model:

struct TestStruct: Codable {
    let title: String
    let name: String
    let total: Int
}

We need to use nestedContainer(keyedBy: forKey:):

extension TestStruct {
    enum TopLevelKeys: String, CodingKey {
        case title
        case tracks
    }
    
    enum NestedLevelCodingKeys: String, CodingKey {
        case name
        case total
    }
    
    init(from decoder: Decoder) throws {
        let container = try decoder.container(keyedBy: TopLevelKeys.self)
        self.title = try container.decode(String.self, forKey: .title)
        let subcontainer = try container.nestedContainer(keyedBy: NestedLevelCodingKeys.self, forKey: TopLevelKeys.tracks)
        self.name = try subcontainer.decode(String.self, forKey: .name)
        self.total = try subcontainer.decode(Int.self, forKey: .total) //You can use here a `decodeIfPresent()` if needed, use default values, etc.

    }
}

In your sample, you used decodeIfPresent() for the subdictionary. It's unclear if it was for test purpose, or if the sub dictionary was sometimes not present. If that's the case and you can have a JSON like this:

{
    "title": "normal"
} 

Then, before calling nestedContainer(keyedBy: forKey:), use if container.contains(TopLevelKeys.tracks) {} and set default values if needed in the else case.