如何在不将变量转换为字符串的情况下判断变量在 Swift 中是否为 Optional(nil)

isNil最好基于展平包含值的可选性.(如果您将isNil直接合并到您的下标中，则可能没有实际用途.)

如果您不关心解包失败的详细信息:

public extension Any? {
  /// Represent an `Optional` with `Any?` instead of `Any`.
  ///
  /// If `any` is an optional, this instance will copy it.
  /// Otherwise, this instance will wrap it.
  ///
  /// - Note: Use this to avoid an `Any?` actually representing an `Any??`.
  init(flattening any: Any) {
    switch any {
    case let optional as Self:
      self = optional
    }
  }

  var isNil: Bool { flatMap(Self.init) == nil }
}

subscript(key: String) -> Any? {
  ( Mirror(reflecting: self).children
    .first { $0.label == key }?
    .value
  ).flatMap(_?.init)
}

如果您这样做了:

public extension Optional {
  /// Represents that an `Optional` was `nil`.
  struct UnwrapError: Error & Equatable {
    public init() { }
  }
}

public extension Any? {
  /// The wrapped value, whether `Wrapped` is an `Optional` or not.
  /// - Throws: `Any?.UnwrapError` when `nil`,
  ///   or  `Any??.UnwrapError` when wrapping another `Optional` that is `nil`.
  var doublyUnwrapped: Wrapped {
    get throws {
      switch self {
      case let doubleWrapped?? as Self?:
        return doubleWrapped
      case _?:
        throw Self?.UnwrapError()
      case nil:
        throw UnwrapError()
      }
    }
  }

  var isNil: Bool { (try? doublyUnwrapped) == nil }
}

subscript(key: String) -> Any {
  get throws {
    try (
      Mirror(reflecting: self).children
        .first { $0.label == key }?
        .value
    )
    .doublyUnwrapped
  }
}