Codable that automatically decodes with default vals

ReflectiveDecodable.swift

import Foundation

func iterateEnum<T: Hashable>(_: T.Type) -> AnyIterator<T> {
    var i = 0
    return AnyIterator {
        let next = withUnsafeBytes(of: &i) { $0.load(as: T.self) }
        if next.hashValue != i { return nil }
        i += 1
        return next
    }
}

extension Dictionary {
    func toJSON() throws -> Data? {
        return try? JSONSerialization.data(withJSONObject: self, options: [])
    }
}

extension KeyedDecodingContainer {
    func decode<T>(_ key: K, or defaultValue: T) throws -> T where T : Decodable {
        return try decodeIfPresent(T.self, forKey: key) ?? defaultValue
    }

    func decode<T>(_ key: K) throws -> T where T : Decodable {
        return try decode(T.self, forKey: key)
    }
}

extension Decodable {
    func autoDecode<U>(_ key: U, from container: KeyedDecodingContainer<U>) throws -> Self {
        return try container.decodeIfPresent(Self.self, forKey: key) ?? self
    }
}

extension AnyIterator where Element: CodingKey {
    func caseWhere<T>(_ val: String) -> T? {
        return self.first { (key) -> Bool in
            return String(describing: key) == val
            } as? T
    }
}

func attemptCast<T>(of val: Any) -> T? where T: Codable {
    if let val = val as? Double {
        return val as? T
    } else if let val = val as? Int {
        return val as? T
    } else if let val = val as? String {
        return val as? T
    } else {
        return nil
    }
}

protocol Keyable {
    associatedtype Keys: CodingKey, Hashable
}

class ReflectiveDecodable: NSObject, Decodable {
    public init<T>(from decoder: Decoder, with keys: T.Type) throws where T: CodingKey & Hashable {
        super.init()
        let container = try decoder.container(keyedBy: T.self)

        let enumProps = iterateEnum(T.self)
        let mirroredObject = Mirror(reflecting: self)
        let props = mirroredObject.children

        for attr in props {
            let key: T? = enumProps.caseWhere(attr.label!)

            if let key = key {
                try self.setVal(forKey: key, keyString: attr.label!, attrValue: attr.value, container: container)
            }
        }
    }

    private func setVal<T>(forKey key: T, keyString: String, attrValue: Any, container: KeyedDecodingContainer<T>) throws {
        switch attrValue {
        case let intValue as Int:
            let value = try container.decode(key, or: intValue)
            self.setValue(value, forKey: keyString)
        case let stringValue as String:
            let value = try container.decode(key, or: stringValue)
            self.setValue(value, forKey: keyString)
        case let boolValue as Bool:
            let value = try container.decode(key, or: boolValue)
            self.setValue(value, forKey: keyString)
        case let doubleValue as Double:
            let value = try container.decode(key, or: doubleValue)
            self.setValue(value, forKey: keyString)
        case let dateValue as Date:
            let value = try container.decode(key, or: dateValue)
            self.setValue(value, forKey: keyString)
        default:
            self.setValue("0", forKey: keyString)
        }
    }
}

class ManyProperties: ReflectiveDecodable {
    @objc var name: String = "Alex"
    @objc var age: Int = 21
    @objc var favoriteColor: String = "green"

    enum CodingKeys: String, CodingKey {
        case name
        case age
        case favoriteColor = "favorite_color"
    }

    public required init(from decoder: Decoder) throws {
        try super.init(from: decoder, with: CodingKeys.self)
    }
}

class Pet: ReflectiveDecodable {
    @objc var name: String = "Odie"
    @objc var numberOfLegs: Int = 7
    @objc var owner: String = "John"

    enum CodingKeys: String, CodingKey {
        case name
        case numberOfLegs = "number_of_legs"
        case owner
    }

    public required init(from decoder: Decoder) throws {
        try super.init(from: decoder, with: CodingKeys.self)
    }
}

do {
    let json = try ["name":"Rex"].toJSON()

    let sut = try JSONDecoder().decode(Pet.self, from: json!)
    print(sut.numberOfLegs, sut.owner, sut.name)
} catch {
    print(error)
}