关联枚举的通用类型

Question

我有一个enum，如下所示：

public enum MyErrorEnum: LocalizedError {
    case FileNotFound(String = "Failed to find file.", file: String)

    public var errorDescription: String? {
        if let current = Mirror(reflecting: self).children.first {
            let mirror = Mirror(reflecting: current.value);
            // Initial error description.
            let message = mirror.children.first?.value as? String
                ?? current.label ?? "Unknown-case";
            var context = "";
            // Iterate additional context.
            var i = 0;
            for associated in mirror.children {
                if i >= 1 {
                    if let text = associated.value as? String {
                        context += "\n  ";
                        if let label: String = associated.label {
                            context += "\(label): "
                        }
                        context += text;
                    }
                }
                i += 1;
            }
            return context.isEmpty ? message : (
                message + " {" + context + "\n}"
            );
        }
        return "\(self)";
    }
}

用法：

do {
    let path = "/path/to/file.txt";
    throw MyErrorEnum.FileNotFound(
        file: path
    );
} catch {
    print(error.localizedDescription);
}

输出：

Failed to find file. {
  file: /path/to/file.txt
}

---

现在我想在任何实现LocalizedError 的enum 中重用errorDescription，只需将errorDescription 的逻辑移动到扩展中即可。

我应该如何改变下面的工作扩展（就上述而言）？

public enum MyErrorEnum: LocalizedError {
    case FileNotFound(String = "Failed to find file.", file: String)
}

extension DONT_KNOW_WHAT_GOES_HERE where Self: LocalizedError {
    public var errorDescription: String? {
        // ... imagine all above logic moved here.
    }
}

Answer 1

正如 cmets 中所建议的，Swift 似乎还没有为关联枚举提供 Generic-Type（虽然它为 Raw-Value 枚举提供了 RawRepresentable）。

解决方案；为了不影响实现LocalizedError 的所有内容，请创建您自己的协议，例如：

public protocol LocalizedErrorEnum: LocalizedError {
    var errorDescription: String? { get }
}

extension LocalizedErrorEnum {
    public var errorDescription: String? {
        if let current = Mirror(reflecting: self).children.first {
            let mirror = Mirror(reflecting: current.value);
            // Initial error description.
            let message = mirror.children.first?.value as? String
                ?? current.label ?? "Unknown-case";
            var context = "";
            // Iterate additional context.
            var i = 0;
            for associated in mirror.children {
                if i >= 1 {
                    if let text = associated.value as? String {
                        context += "\n  ";
                        if let label: String = associated.label {
                            context += "\(label): "
                        }
                        context += text;
                    }
                }
                i += 1;
            }
            return context.isEmpty ? message : (
                message + " {" + context + "\n}"
            );
        }
        return "\(self)";
    }
}

用法：

public enum MyErrorEnum: LocalizedErrorEnum {
    case FileNotFound(String = "Failed to find file.", file: String)
}