对于TestStruct，需要自定义init(from:)初始值设定项。对于TestEnum，不需要自定义init(from:)初始值设定项。

struct TestStruct {
    let testEnum: TestEnum
    let failDecode: Bool
}

extension TestStruct {
    enum TestEnum: String, Codable {
        case val1 = "VAL1"
        case val2 = "VAL2"
    }
}

extension TestStruct: Decodable {
    public init(from decoder: Decoder) throws {
        do {
            testEnum = try TestEnum(from: decoder)
            failDecode = false
        } catch {
            failDecode = true

            // You must initialize testEnum to some valid value,
            // even though you couldn't decode it!
            testEnum = .val1
        }
    }
}

如果你想要Codable一致性，而不仅仅是Decodable一致性，那么你还需要提供一个匹配的encode(to:)方法。

extension TestStruct: Encodable {
    func encode(to encoder: Encoder) throws {
        try testEnum.encode(to: encoder)
    }
}

(Note Codable是Decodable & Encodable的别名，因此提供单独的Decodable和Encodable一致性与提供单个Codable一致性相同。
我忽略了encode(to:)实现中的failDecode标志，但是如果设置了failDecode，您可能希望编码一个非法值，这个版本在failDecode为真时编码nil：

extension TestStruct: Encodable {
    func encode(to encoder: Encoder) throws {
        if failDecode {
            try Optional<TestEnum>.none.encode(to: encoder)
        } else {
            try testEnum.encode(to: encoder)
        }
    }
}