swift - 条件协议(protocol)一致性 : Cannot convert value of type 'Array<_>' to specified type '[UInt8]'

Question

尝试使用条件一致性在 Swift 4.1 中扩展数组、字符串和字典，但在尝试使用符合 Decodable 的 Element 初始化数组时遇到了死胡同/Encodable 通过名为 BinaryCodable 的自定义协议(protocol)。

以下摘自https://github.com/mikeash/BinaryCoder但已进行调整以使用 Swift 的新条件一致性以使其能够编译。

extension Array: BinaryCodable where Element: BinaryDecodable, Element: BinaryEncodable {
    public func binaryEncode(to encoder: BinaryEncoder) throws {
        try encoder.encode(self.count)
        for element in self {
            try element.encode(to: encoder)
        }
    }

    public init(fromBinary decoder: BinaryDecoder) throws {
        let binaryElement = Element.self

        let count = try decoder.decode(Int.self)
        self.init()
        self.reserveCapacity(count)
        for _ in 0 ..< count {
            let decoded = try binaryElement.init(from: decoder)
            self.append(decoded)
        }
    }
}

extension String: BinaryCodable {
    public func binaryEncode(to encoder: BinaryEncoder) throws {
        try (Array(self.utf8) as! BinaryCodable).binaryEncode(to: encoder)
    }

    public init(fromBinary decoder: BinaryDecoder) throws {
        let utf8: [UInt8] = try Array(fromBinary: decoder)
        if let str = String(bytes: utf8, encoding: .utf8) {
            self = str
        } else {
            throw BinaryDecoder.Error.invalidUTF8(utf8)
        }
    }
}

但是，我得到:

Cannot convert value of type 'Array<_>' to specified type '[UInt8]'

对于这一行:

let utf8: [UInt8] = try Array(fromBinary: decoder)

如有任何帮助，我们将不胜感激。

Answer 1

为了Array<UInt8>成为BinaryCodable , 它的元素类型 UInt8必须是 BinaryCodable ，事实并非如此。该协议(protocol)有其所需方法的默认实现，但一致性必须仍然明确声明:

extension UInt8: BinaryCodable {}

然后是你的extension String编译，你甚至可以摆脱强制转换as! BinaryCodable在里面编码方法(并使用 guard 允许节省一行):

extension String: BinaryCodable {
    public func binaryEncode(to encoder: BinaryEncoder) throws {
        try Array(self.utf8).binaryEncode(to: encoder)
    }

    public init(fromBinary decoder: BinaryDecoder) throws {
        let utf8: [UInt8] = try Array(fromBinary: decoder)
        guard let str = String(bytes: utf8, encoding: .utf8) else {
            throw BinaryDecoder.Error.invalidUTF8(utf8)
        }
        self = str
    }
}