Question

我正在运行下面的代码，看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个人资料。我无法检查从配置文件检查返回的空值值

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)
        })

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>

如何检查此值？

 if profile == nil

不起作用

如果我这样做

let profile = snapshot.value as? String

首先，即使存在snapshot.value

，它也总是返回nil

Answer 1

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")
})

。

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"
        }

Answer 2

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。在您知道路径的情况下，查询会增加不必要的开销。

例如

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

最好通过值观察有问题的节点，如果它不存在则返回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")
    }
})

如果您以其他方式存储它;也许

random_node_id
   uid: their_uid
   name: "some name"

然后查询将按顺序排列，如此

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

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

   });

Firebase检查空值（Swift）

2 个答案: