Firebase检查空值（Swift）

I'm running the code below to see if a user that opens the app is already logged in, and then to check if they've set up a profile. I'm having trouble checking for the null value value returned from the profile check

我正在运行下面的代码，以查看打开应用程序的用户是否已登录，然后检查他们是否已设置配置文件。我在检查配置文件检查返回的空值时遇到问题

override func viewDidLoad() {
    super.viewDidLoad()

    //Check to see if user is already logged in
    //by checking Firebase to see if authData is not nil
    if ref.authData != nil {

        //If user is already logged in, get their uid.
        //Then check Firebase to see if the uid already has a profile set up
        let uid = ref.authData.uid
        ref.queryOrderedByChild("uid").queryEqualToValue(uid).observeSingleEventOfType(.Value, withBlock: { snapshot in
                let profile = snapshot.value
                print(profile)
        })

In the last line when I print(profile), I either get the profile information, or

在我打印（个人资料）的最后一行，我要么获取个人资料信息，要么

 <null>

how do i check for this value?

我如何检查这个值？

 if profile == nil

does not work

不起作用

If I do

如果我做

let profile = snapshot.value as? String

first, then it always returns nil even if there is a snapshot.value

首先，即使有snapshot.value，它总是返回nil

2 个解决方案

#1

Take advantage of the exists() method to determine if the snapshot contains a value. To use your example:

利用exists（）方法确定快照是否包含值。要使用您的示例：

let uid = ref.authData.uid
ref.queryOrderedByChild("uid").queryEqualToValue(uid)
         .observeSingleEventOfType(.Value, withBlock: { snapshot in

    guard snapshot.exists() else{
        print("User doesn't exist")
        return
    }

    print("User \(snapshot.value) exists")
})

。

Here's another handy example, Swift4

这是另一个方便的例子，Swift4

    let path = "userInfo/" + id + "/followers/" + rowId

    let ref = Database.database().reference(withPath: path)

    ref.observe(.value) { (snapshot) in

            let following: Bool = snapshot.exists()

            icon = yesWeAreFollowing ? "tick" : "cross"
        }

#2

You may want to explore another option: since you know the uid of the user and therefore the path to that user, there's no reason to query. Queries add unnecessary overhead in situations where you know the path.

您可能想要探索另一个选项：因为您知道用户的uid以及该用户的路径，所以没有理由进行查询。查询会在您知道路径的情况下增加不必要的开销。

For example

例如

users
  uid_0
    name: "some name"
    address: "some address"

You are much better off observing the node in question by value, which will return null if it doesn't exist

通过值观察有问题的节点要好得多，如果不存在则返回null

ref = "your-app/users/uid_0"

ref.observeEventType(.Value, withBlock: { snapshot in
    if snapshot.value is NSNull {
        print("This path was null!")
    } else {
        print("This path exists")
    }
})

in the event you are storing it some other way; perhaps

如果你以其他方式存储它;也许

random_node_id
   uid: their_uid
   name: "some name"

then a query would be in order, like this

然后查询将按顺序排列，就像这样

ref.queryOrderedByChild("uid").queryEqualToValue(their_uid)
   .observeEventType(.Value, withBlock: { snapshot in

       if snapshot.exists() {
           print("you found it!")
       }

   });

#1