Implementing Codable for ARAnchor: “cannot be automatically synthesized in an extension…”

Question

The code extension ARAnchor: Codable {} produces the error:

\"Implementation of \'Decodable\' cannot be automatically synthesized in an extension in a d

Answer 1

Synthesizing conformace to Codable, Equatable and Hashable in different source files is currently not supported by the Swift compiler, AFAICT. See https://bugs.swift.org/browse/SR-6101 for the issue tracking this.

Answer 2

Since Codable is a type alias for the Encodable and Decodable protocols, when you use it as a type or a generic constraint, it matches any type that conforms to both protocols:

public typealias Codable = Decodable & Encodable

At the moment (Xcode 10.2.1 / Swift 5.0.1) Codable currently isn't supported yet if an extension in one file adds conformance in a different file. Check this out at https://bugs.swift.org/.

Hope this helps.

Answer 3

You could create a container object that implements Codable and then use that to encode and decode the anchor. I tried this code in a playground and it work for me. You'll want to adapt it for which data you want from the anchor; for example I encoded name but that might be useless to you and it might even break if your anchor was initialized without a name. You could also do the same thing with simd_float4x4.

import Foundation
import ARKit

class AnchorContainer: Codable {

    let anchor: ARAnchor

    init(anchor: ARAnchor) {
        self.anchor = anchor
    }

    required init(from decoder: Decoder) throws {
        let container = try decoder.container(keyedBy: CodingKeys.self)
        let name = try container.decode(String.self, forKey: .name)
        let transform0 = try container.decode(simd_float4.self, forKey: .transform0)
        let transform1 = try container.decode(simd_float4.self, forKey: .transform1)
        let transform2 = try container.decode(simd_float4.self, forKey: .transform2)
        let transform3 = try container.decode(simd_float4.self, forKey: .transform3)
        let matrix = simd_float4x4(columns: (transform0, transform1, transform2, transform3))
        anchor = ARAnchor(name: name, transform: matrix)
    }

    func encode(to encoder: Encoder) throws {
        var container = encoder.container(keyedBy: CodingKeys.self)
        try container.encode(anchor.name, forKey: .name) // Might want to make sure that the name is not nil here
        try container.encode(anchor.transform.columns.0, forKey: .transform0)
        try container.encode(anchor.transform.columns.1, forKey: .transform1)
        try container.encode(anchor.transform.columns.2, forKey: .transform2)
        try container.encode(anchor.transform.columns.3, forKey: .transform3)
    }

    enum CodingKeys: String, CodingKey {
        case name
        case transform0
        case transform1
        case transform2
        case transform3
    }

}

// EXAMPLE:

let anchor = ARAnchor(name: "Bill", transform: simd_float4x4(float4(repeating: 4), float4(repeating: 5), float4(repeating: 6), float4(repeating: 7))) // Make a arbitrary anchor
print(anchor) // Figure out what it's value is


do {
    let data = try JSONEncoder().encode(AnchorContainer(anchor: anchor))
    let anchorDecode = try JSONDecoder().decode(AnchorContainer.self, from: data)
    print(anchorDecode.anchor) // Print the value after decoding to make sure that the result is the same
} catch {
    print(error.localizedDescription)
}