Creating Codable object with Codable objects of any type?

Question

I wish to create a class which will store a Date and any object that conforms to the Codable protocol. I would like this class to also conform to the Codable protocol itself.

I can do this for one object as follows:

class CodableContainerA: NSObject, Codable {
    var date: Date?
    var item: CodableTypeA?
}

I'd rather not have to create a separate CodableContainerX for each CodableTypeX I have.

My current workaround is to create a class called CodableBaseClass that conforms to Codable, derive every CodableType from it and define my class as follows:

class CodableContainer: NSObject, Codable {
    var date: Date?
    var item: CodableBaseClass?
}

This seems a long way around and feels like something I should be able to do by making the CodableTypes conform to a protocol, but I'm not sure how. If I define item to be of type Codable? or (Any & Codable)? I get an error stating

Type 'CodableContainer' does not conform to protocol 'Decodable'

I'm using Swift 4.

Any help or advice would be gratefully accepted. Many thanks,

Code Different · Accepted Answer · 2018-01-05 14:48:45Z

5

This is what generic classes are for:

class CodableContainer<T: Codable>: NSObject, Codable {
    var date: Date?
    var item: T?
}

answered Jan 5, 2018 at 14:48

Code Different

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Comments

Rob · Accepted Answer · 2018-01-05 16:01:55Z

4

Whenever you find yourself wondering about a separate class for each underlying type, think "generics". E.g.

struct CodableContainer<T: Codable>: Codable { 
    let date: Date
    let item: T
}

I'd use struct unless you needed class for some specific reason.
I'd make the properties immutable unless you needed them mutable for some reason.
I wouldn't subclass NSObject unless there was some specific reason that this particular container needed to be NSObject subclass.
I'd make the properties non-optional unless you absolutely needed them to be optional.

But edit this as you see fit. The key point is the generic pattern.

You might want a decoding function that decodes using some consistent DateDecodingStrategy, e.g.

extension CodableContainer {
    static func decodeJSON(_ data: Data) throws -> CodableContainer {
        let decoder = JSONDecoder()
        decoder.dateDecodingStrategy = .iso8601
        return try decoder.decode(self, from: data)
    }
}

Obviously, use whatever dateDecodingStrategy you want. But then you can decode with:

let result = try CodableContainer<Foo>.decodeJSON(data)

edited Jan 5, 2018 at 16:01

answered Jan 5, 2018 at 14:54

Rob

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1 Comment

Josh Paradroid Over a year ago

I've marked Code Different's answer as the accepted answer as it was first and more concise but I do really appreciate your answer and suggestions and I have incorporated them into my code. These are all things I've wondered about today but I figured I'd get it working before I made it work well. Hopefully my code is now both! Many thanks.

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