Cannot conform to Decodable when using Mirror(reflecting:)

Question

I'm using Decodable in order to decode some JSON, all properties on this struct have the same structure it's just the JSON contains them in multiple arrays. The code works until I add another property let all = Mirror(reflecting: Sizes.self).children. I'm using this in order to get all the properties so I can loop over the struct to actually injest the data.

The issue I'm having is that when I store all as a constant, the code will not compile and gives me the following error:

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

struct SizeJSON: Decodable {
    var name: String
    var width: Double
    var height: Double
}

struct Sizes: Decodable {
    let small: [SizeJSON]
    let medium: [SizeJSON]
    let large: [SizeJSON]
    let all = Mirror(reflecting: Sizes.self).children
}

Answer 1

Mirror's init(reflecting:) initializer needs to pass an instance of your type to work.

You can accomplish what you are trying using a lazy variable, like this:

struct Sizes: Decodable {
    let small: [SizeJSON]
    let medium: [SizeJSON]
    let large: [SizeJSON]
    lazy var all: [SizeJSON] = {
        Mirror(reflecting: self).children
            .compactMap { $0.value as? [SizeJSON] }
            .flatMap { $0 }
    }()
}

That way you can both have access to self instance and avoid for all property to be decoded.