ios - Swift 中的 Firebase。无法将类型 'NSNull' 的值转换为 'NSDictionary'

Question

每次打开我的应用程序并连接时，都会导致错误。我认为我将值正确地转换为 [String: String] 但不是。解决这个问题的正确方法是什么？

 class func info(forUserID: String, completion: @escaping (User) -> Swift.Void) {
    FIRDatabase.database().reference().child("users").child(forUserID).child("credentials").observeSingleEvent(of: .value, with: { (snapshot) in

        //This line is the reason of the problem. 
        let data = snapshot.value as! [String: String]
        let name = data["name"]!
        let email = data["email"]!
        let link = URL.init(string: data["profile"]!)
        URLSession.shared.dataTask(with: link!, completionHandler: { (data, response, error) in
            if error == nil {
                let profilePic = UIImage.init(data: data!)
                let user = User.init(name: name, email: email, id: forUserID, profilePic: profilePic!)
                completion(user)
            }
        }).resume()
    })
}

错误说

Could not cast value of type 'NSNull' (0x1ae148588) to 'NSDictionary' (0x1ae148128).

Answer 1

当网络服务返回值 <null> 时, 它被表示为 NSNull目的。那是一个实际的对象并将其与 nil 进行比较将返回 false .

这是我的做法:

if let json = snapshot.value as? [String: String] {
    //if it's possible to cast snapshot.value to type [String: String]
    //this will execute
}