method for don't throw Swift.Error and keep default value if key is not present in json.

decode_into_sample.swift

extension KeyedDecodingContainerProtocol {
    func decode<T: Decodable>(into: inout T, forKey key: Self.Key) {
        guard let tmp = try? self.decode(T.self, forKey: key) else { return }
        into = tmp
    }
}

class A: Decodable {
    var hoge = "1"
    enum CodingKeys: CodingKey {
        case hoge
    }
    required init(from decoder: Decoder) throws {
        let container = try decoder.container(keyedBy: CodingKeys.self)
        container.decode(into: &hoge, forKey: .hoge)
    }
}

let str = "{}"
let data = str.data(using: .utf8)!
let d = JSONDecoder()
let a2 = try! d.decode(A.self, from: data)
print(a2.hoge) // 1