NullCodable.swift

import Foundation

/// Makes an Encodable and Decodable properties encode to `null` instead of omitting the value altogether.
@propertyWrapper
public struct NullCodable<T> {
	public var wrappedValue: T?

	public init(wrappedValue: T?){
		self.wrappedValue = wrappedValue
	}
}

extension NullCodable: Encodable where T: Encodable {
	public func encode(to encoder: Encoder) throws {
		var container = encoder.singleValueContainer()
		switch wrappedValue {
		case .some(let value):
			try container.encode(value)
		case .none:
			try container.encodeNil()
		}
	}
}

extension NullCodable: Decodable where T: Decodable {
	public init(from decoder: Decoder) throws {
		let container = try decoder.singleValueContainer()
		self.wrappedValue = try? container.decode(T.self)
	}
}


struct WithNullCodable: Codable {
	let value: String
	@NullCodable var value2: String?
}

struct WithoutNullCodable: Codable {
	let value: String
	let value2: String?
}

let withA = WithNullCodable(value: "blah", value2: "bleh")
let withB = WithNullCodable(value: "blah", value2: nil)

let withoutA = WithoutNullCodable(value: "blah", value2: "bleh")
let withoutB = WithoutNullCodable(value: "blah", value2: nil)

func jsonString<Item: Encodable>(from item: Item) -> String {
	let data = try! JSONEncoder().encode(item)
	return String(data: data, encoding: .utf8)!
}

func object<Item: Decodable>(from string: String) -> Item {
	let decoder = JSONDecoder()
	let data = string.data(using: .utf8)!
	return try! decoder.decode(Item.self, from: data)
}


let withAStr = jsonString(from: withA)
let withBStr = jsonString(from: withB)
print(withAStr) // {"value":"blah","value2":"bleh"}
print(withBStr) // {"value":"blah","value2":null}

let withoutAStr = jsonString(from: withoutA)
let withoutBStr = jsonString(from: withoutB)
print(withoutAStr) // {"value":"blah","value2":"bleh"}
print(withoutBStr) // {"value":"blah"}



let withADecoded: WithNullCodable = object(from: withAStr)
let withBDecoded: WithNullCodable = object(from: withBStr)
print(withADecoded)
print(withBDecoded)

let withoutADecoded: WithoutNullCodable = object(from: withoutAStr)
let withoutBDecoded: WithoutNullCodable = object(from: withoutBStr)
print(withoutADecoded)
print(withoutBDecoded)

Thanks for quick answer! What do you mean by flattening the wrapper of the content?

Without the singleValueContainer() it might be trying to decode a value embedded within another container, like an array or dictionary.