元素是AnyObject还是元素对AnyObject？

Question

阅读Apple的代码，我看到了以下内容(谈论密钥链服务)：

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

有线索吗？

以下是苹果样本中的全部片段：

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

也许这段代码来自某些Swift 2代码，在Swift 3+中它是

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
}