Firebase检查空值（Swift） [英] Firebase checking for null value (Swift)

问题描述

我正在运行下面的代码，看看是否有用户打开应用程序已经登录，然后检查他们是否已经建立了一个配置文件。我无法检查从配置文件检查返回的空值的值。

$ p $覆盖func viewDidLoad（）{
super.viewDidLoad（）

//检查用户是否已经登录
//通过检查Firebase来查看authData是否不为
如果ref.authData！=无{

//如果用户已经登录，请获取他们的uid。
//然后检查Firebase以查看uid是否已经创建了配置文件
let uid = ref.authData.uid
ref.queryOrderedByChild（uid）。q​​ueryEqualToValue（uid）。 observeSingleEventOfType（.Value，withBlock：{snapshot in
let profile = snapshot.value
print（profile）
}）

 
 
 
 < null> 
  

如何检查这个值？

< pre $ 如果profile == nil


不起作用

p>

如果我这样做

$ $ p $ $ code>让profile = snapshot.value as？字符串

首先，它总是返回nil，即使有snapshot.value

解决方案

利用exists（）方法来确定快照是否包含值。使用你的例子：

  let uid = ref.authData.uid 
 ref.queryOrderedByChild（uid）。 queryEqualToValue（uid）
 .observeSingleEventOfType（.Value，withBlock：{snapshot in 
 $ b $ guard snapshot.exists（）else {
 print（User does not exist）
返回
} 
 
 print（User \（snapshot.value）exists）
}）
  pre> 
 
 
。 
 
 
 
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 below：Bool = snapshot.exists（）
 
 icon = yesWeAreFollowing？ tick：cross
} 
  
 
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
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 
 <null>
how do i check for this value?
 if profile == nil 
does not work

If I do
let profile = snapshot.value as? String
first, then it always returns nil even if there is a snapshot.value
解决方案
Take advantage of the exists() method to determine if the snapshot contains a value. To use your example:
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
    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"
        }


                        
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