ios - 读取 firebase 数据库并存储在字符串变量中失败

Question

我正在尝试从 Firebase 读取用户数据，但用户名变量始终为 nil。

let storage = Storage.storage()
let storageRef = storage.reference()
let userID = Auth.auth().currentUser?.uid
let canvasName = NSUUID().uuidString
var username: String? = nil

//get username
self.ref.child("Users").observeSingleEvent(of: .value, with: { (snapshot) in
    let usersProfile = snapshot.value as? NSDictionary
    let userProfile = usersProfile![userID!] as? NSDictionary
    username = userProfile!["Username"] as? String            
}) { (error) in
    print(error.localizedDescription)
}

//create new canvas
let canvasValue:[String:Any] = ["UserID":userID!,
                                "Username": username!,//always nil 
                                "TimeCreated": NSDate().timeIntervalSince1970,
                               ]
let canvasAutoID = self.ref.child("Users").child(userID!).child("Canvas").childByAutoId().key
    self.ref.child("Canvas").child(canvasAutoID!).setValue(canvasValue)

这是 Firebase 数据库的图像。

Answer 1

observeSingleEvent 异步工作。它可能必须发出网络请求才能获取该数据，因此闭包将在函数调用后的某个不确定时间运行。它之后的代码，即您尝试访问 username 的代码，首先运行，这意味着 username 仍然具有其初始值 nil。您可以通过将代码移动到闭包内来解决此问题，如下所示，或者将其放入您从闭包内调用的另一个函数中。

//get username
self.ref.child("Users").observeSingleEvent(of: .value, with: { (snapshot) in
    let usersProfile = snapshot.value as? NSDictionary
    let userProfile = usersProfile![userID!] as? NSDictionary
    username = userProfile!["Username"] as? String 
    //create new canvas
    let canvasValue:[String:Any] = ["UserID":userID!,
                                    "Username": username!, 
                                    "TimeCreated": NSDate().timeIntervalSince1970,
                                   ]
    let canvasAutoID = self.ref.child("Users").child(userID!).child("Canvas").childByAutoId().key
    self.ref.child("Canvas").child(canvasAutoID!).setValue(canvasValue)          
}) { (error) in
        print(error.localizedDescription)
}