元素作为AnyObject vs元素vs AnyObject?

阅读苹果的代码,我看到了以下内容(谈论钥匙链服务):

query[kSecAttrService as String] = service as AnyObject?

你的目的是什么 AnyObject? ?

我认为可以简化为

query[kSecAttrService as String] = service as AnyObject

以下是苹果示例的完整片段:

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
}