Implementing Codable for UIColor

Question

Is it possible to implement the Encodable and Decodable properties for UIColor

When I try to add a Decodable extension I get an error

extension UIColor : Decodable {
    public required init(from decoder: Decoder) throws {
        self.init(red: 1, green: 1, blue: 1, alpha: 1)
    }
}

error: ColorStuff.playground:98:21: error: initializer requirement 'init(from:)' can only be satisfied by a required initializer in the definition of non-final class 'UIColor' public required init(from decoder: Decoder) throws {

Am I missing something obvious here?

I have no issues with the Encodable extension - it seems its a Decodable issue.

The error message implies to me that I cannot do this due to not having access to the UIColor class definition

Compare https://stackoverflow.com/questions/46522572/initializer-requirement-initjson-can-only-be-satisfied-by-a-required-init. — Martin R, Feb 01 '18 at 15:40
You're missing this obvious error: ```'required' initializer must be declared directly in class 'UIColor' (not in an extension)``` — user28434'mstep, Feb 01 '18 at 15:41
Yea thats what i was afraid of - as said by last sentence. So we'll call it a resounding NO WAY JOSE — Jeef, Feb 01 '18 at 15:45
You can always attempt to [subclass](https://stackoverflow.com/questions/21212513/how-to-inherit-from-uicolor-correctly-or-how-to-access-uicolor-subclass-property) it and have the subclass implement `Codable`. Or perhaps write a wrapper for it. — , Feb 01 '18 at 16:06

score 15 · Accepted Answer · answered Feb 01 '18 at 16:41

You cannot make UIColor conform to Decodable in an extension because of the error given by the compiler.

One solution is to make a Codable wrapper type and use that instead.

Since UIColor already conforms to NSCoding, let's just write a generic type so we can encode and decode anything that conforms to NSCoding.

import UIKit

struct WrapperOfNSCoding<Wrapped>: Codable where Wrapped: NSCoding {
    var wrapped: Wrapped

    init(_ wrapped: Wrapped) { self.wrapped = wrapped }

    init(from decoder: Decoder) throws {
        let container = try decoder.singleValueContainer()
        let data = try container.decode(Data.self)
        guard let object = NSKeyedUnarchiver.unarchiveObject(with: data) else {
            throw DecodingError.dataCorruptedError(in: container, debugDescription: "failed to unarchive an object")
        }
        guard let wrapped = object as? Wrapped else {
            throw DecodingError.typeMismatch(Wrapped.self, DecodingError.Context(codingPath: container.codingPath, debugDescription: "unarchived object type was \(type(of: object))"))
        }
        self.wrapped = wrapped
    }

    func encode(to encoder: Encoder) throws {
        let data = NSKeyedArchiver.archivedData(withRootObject: wrapped)
        var container = try encoder.singleValueContainer()
        try container.encode(data)
    }
}

let colors = [UIColor.red, UIColor.brown]
print(colors)
let jsonData = try! JSONEncoder().encode(colors.map({ WrapperOfNSCoding($0) }))
let colors2 = try! JSONDecoder().decode([WrapperOfNSCoding<UIColor>].self, from: jsonData).map({ $0.wrapped })
print(colors2)

If I try to use your (excellent) wrapper with a single value it throws an error when encoding - ie - jsonData = try! JSONEncoder().encode([w]) succeeds let jsonData = try! JSONEncoder().encode(w) fails with let w = WrapperOfNSCoding(colors[0]) error is Fatal error: 'try!' expression unexpectedly raised an error: Swift.EncodingError.invalidValue(WrapperOfNSCoding #1... Why would that be — daven11, Mar 16 '18 at 23:08
I will be happy to help you if you post your own top-level question. Feel free to add a comment here linking to it if you do. — rob mayoff, Mar 16 '18 at 23:09
Thanks very much - https://stackoverflow.com/questions/49330725/wrapperofnscoding-fails-with-single-value — daven11, Mar 16 '18 at 23:15

Paul B · Answer 2 · 2022-04-20T09:39:51.390

There is as way to make UIColor Codable despite that required init. You can extend Codable itself, so that UIColor starts conforming it automatically.

import UIKit

extension Decodable where Self: NSSecureCoding {

    public init(from decoder: Decoder) throws {
        let container = try decoder.singleValueContainer()
        let data = try container.decode(Data.self)
        guard let object = try NSKeyedUnarchiver.unarchiveTopLevelObjectWithData(data) as? Self else {
            throw DecodingError.dataCorruptedError(
                in: container,
                debugDescription: "Invalid object"
            )
        }
        self = object
    }
}

extension Encodable where Self: NSSecureCoding {
    public func encode(to encoder: Encoder) throws {
        var container = encoder.singleValueContainer()
        let data = try NSKeyedArchiver.archivedData(withRootObject: self, requiringSecureCoding: true)
        try container.encode(data)
    }
    
}

extension UIColor: Codable { }

Check it

import XCTest
class CodingTextCase: XCTestCase {
    let encoder = JSONEncoder()
    let decoder = JSONDecoder()
    
    func testUIColor() throws {
        let colorAsJSON = try encoder.encode(UIColor.red)
        print(String(data: colorAsJSON, encoding: .utf8)!)
        let uiColor = try? decoder.decode(UIColor.self, from: colorAsJSON)
        XCTAssertEqual(uiColor!, UIColor.red)
    }
}
CodingTextCase.defaultTestSuite.run()

But please note that in this case data for instance of UIColor will take about 5 hundred bytes and storing it as RGBA can be more compact.

Implementing Codable for UIColor

2 Answers2

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