元素作为 AnyObject vs 元素 vs AnyObject？

Question

阅读 Apple 的代码我看到了以下内容（谈论 Keychain 服务）：

query[kSecAttrService as String] = service as AnyObject?

AnyObject?的目的是什么？

我认为可以简化为

query[kSecAttrService as String] = service as AnyObject

有什么线索吗？

这是来自 Apple 示例的整个 sn-p：

private static func keychainQuery(withService service: String, account: String? = nil, accessGroup: String? = nil) -> [String : AnyObject] {
    var query = [String : AnyObject]()
    query[kSecClass as String] = kSecClassGenericPassword
    query[kSecAttrService as String] = service as AnyObject?

    if let account = account {
        query[kSecAttrAccount as String] = account as AnyObject?
    }

    if let accessGroup = accessGroup {
        query[kSecAttrAccessGroup as String] = accessGroup as AnyObject?
    }

    return query
}

Answer 1

也许 sn-p 来自一些 Swift 2 代码，在 Swift 3+ 中是

private static func keychainQuery(withService service: String, account: String? = nil, accessGroup: String? = nil) -> [String : Any] {
    var query = [String : Any]()
    query[kSecClass as String] = kSecClassGenericPassword
    query[kSecAttrService as String] = service

    if let account = account {
        query[kSecAttrAccount as String] = account
    }

    if let accessGroup = accessGroup {
        query[kSecAttrAccessGroup as String] = accessGroup
    }

    return query
}