无法将“ nsnull”类型的价值投射为“ nsdictionary”

Question

我在行中遇到一个错误，" self.email =（snapshot.value as！nsdictionary）[" email"] as！string"，说它不能抛出它的价值键入" nsnull"。如何解决错误？我以为，由于我有一个函数，可以将所有内容转换为字符串。

struct User {
    var email: String!
    var uid: String!
    var ref: DatabaseReference!
    var key: String = ""
    init(snapshot: DataSnapshot) {
        self.email = (snapshot.value as! NSDictionary)["email"] as! String
        self.uid = (snapshot.value as! NSDictionary)["uid"] as! String
        self.ref = snapshot.ref
        self.key = snapshot.key
    }
    init(email: String, uid: String) {
        self.email = email
        self.uid = uid
        self.ref = Database.database().reference()
    }
    func toAnyObject() -> [String: Any] {
        return ["email":self.email, "uid":self.uid]
    }
}

Answer 1

正如El Captain v2.0指出的那样，不要强制解开值，因为它可能是nil，您会崩溃。改用if let：

// if snap has a value, then you can use the value. Otherwise not, you can call an else if you want to.
if let snap = snapshot.value as? [String:Any] { 
    self.email = snap["email"] as! String 
}

，我也会跳过as! String，而是这样做的，您的Incase不会获得email值。

if let snap = snapshot.value as? [String:Any], let email = snap["email"] as? String  { 
    // Use email in here now
    print(email)
}

因此，如果您有其他要获得的字段，只需将它们添加到if-statement中。

Answer 2

    struct User {
    var email: String!
    var uid: String!
    var ref: DatabaseReference!
    var key: String = ""
    init(snapshot: DataSnapshot){
        self.email = (snapshot.value as! NSDictionary)["email"] as! String
        self.uid = (snapshot.value as! NSDictionary)["uid"] as! String
        self.ref = snapshot.ref
        self.key = snapshot.key
    }
    init(email: String, uid: String){
        if let snap = snapshot.value as? [String:Any] {
        // Do not unwrap forcefully, instead use as mentioned below 
        // so that in case snap["email"] gives null that could be 
        //handled. 
        //Also please check that whether you have "firstname" var 
        //present in not in your code and dictionary both. 
        self.email = snap["email"] as? String ?? "" //Provide some default value or empty string
        }
        self.uid = uid
        self.ref = Database.database().reference()
    }
    func toAnyObject() -> [String: Any]{
        return ["email":self.email, "uid":self.uid]
    }
    }