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iOS json Decodable

 

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protocol DecodableDefaultSource {

    associatedtype Value: Decodable

    static var defaultValue: Value { get }

}


enum DecodableDefault {}


extension DecodableDefault {

    @propertyWrapper

    struct Wrapper<Source: DecodableDefaultSource> {

        typealias Value = Source.Value

        var wrappedValue = Source.defaultValue

    }

}


extension DecodableDefault.Wrapper: Decodable {

    init(from decoder: Decoder) throws {

        let container = try decoder.singleValueContainer()

        wrappedValue = try container.decode(Value.self)

    }

}


extension KeyedDecodingContainer {

    func decode<T>(_ type: DecodableDefault.Wrapper<T>.Type,

                   forKey key: Key) throws -> DecodableDefault.Wrapper<T> {

        try decodeIfPresent(type, forKey: key) ?? .init()

    }

}


extension DecodableDefault {

    typealias Source = DecodableDefaultSource

    typealias List = Decodable & ExpressibleByArrayLiteral

    typealias Map = Decodable & ExpressibleByDictionaryLiteral


    enum Sources {

        enum True: Source { static var defaultValue: Bool { true } }

        enum False: Source { static var defaultValue: Bool { false } }

        enum EmptyString: Source { static var defaultValue: String { "" } }

        enum EmptyList<T: List>: Source { static var defaultValue: T { [] } }

        enum EmptyMap<T: Map>: Source { static var defaultValue: T { [:] } }

    }

}


extension DecodableDefault {

    typealias True = Wrapper<Sources.True>

    typealias False = Wrapper<Sources.False>

    typealias EmptyString = Wrapper<Sources.EmptyString>

    typealias EmptyList<T: List> = Wrapper<Sources.EmptyList<T>>

    typealias EmptyMap<T: Map> = Wrapper<Sources.EmptyMap<T>>

}


extension DecodableDefault.Wrapper: Equatable where Value: Equatable {}

extension DecodableDefault.Wrapper: Hashable where Value: Hashable {}

extension DecodableDefault.Wrapper: Encodable where Value: Encodable {

    func encode(to encoder: Encoder) throws {

        var container = encoder.singleValueContainer()

        try container.encode(wrappedValue)

    }

}

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