Checking if Firebase snapshot is equal to nil in Swift

问题

I am trying to query Firebase to check if any user that has waiting: "1" and then when the snapshot is returned I want to see whether it is equal to nil. I have attempted to do this but the method I have used does not work and I only have some sort of out put if the snapshot is not equal to nil. I have added the code I currently have and the JSON text from Firebase.

import UIKit
import Firebase
import Spring

class GamesViewController: UIViewController {

let ref = Firebase(url: "https://123test123.firebaseio.com")
var activityIndicator: UIActivityIndicatorView = UIActivityIndicatorView()   

@IBAction func StartGamePressed(sender: AnyObject) {
    print("test1")
    var peopleWaiting: [String] = []

    let userRef = Firebase(url:"https://123test123.firebaseio.com/users")
    userRef.queryOrderedByChild("waiting").queryEqualToValue("1")
        .observeEventType(.ChildAdded, withBlock: { snapshot in
            print(snapshot.key)
            if snapshot.key == nil {
                print("test2")
                let userData = ["waiting": "1"]
                let usersRef = self.ref.childByAppendingPath("users")
                let hopperRef = usersRef.childByAppendingPath("\(self.ref.authData.uid)")

                hopperRef.updateChildValues(userData, withCompletionBlock: {
                    (error:NSError?, ref:Firebase!) in
                    if (error != nil) {
                        print("Data could not be saved.")
                        self.displayAlert("Oops!", message: "We have been unable to get you into a game, check you have an internet conection. If this problem carries on contect support")
                    } else {
                        print("Data saved successfully!")
                        let storyboard = UIStoryboard(name: "Main", bundle: nil)
                        let Home : UIViewController = storyboard.instantiateViewControllerWithIdentifier("continueToGame")
                        self.presentViewController(Home, animated: true, completion: nil)

                    }

                })


            } else {
                var randomUID: String
                peopleWaiting.append(snapshot.key)
                let randomIndex = Int(arc4random_uniform(UInt32(peopleWaiting.count)))
                randomUID = peopleWaiting[randomIndex]
                print(randomUID)
                let storyboard = UIStoryboard(name: "Main", bundle: nil)
                let Home : UIViewController = storyboard.instantiateViewControllerWithIdentifier("continueToGame")
                self.presentViewController(Home, animated: true, completion: nil)

            }
        })
}

func displayAlert(title: String, message: String){

    let formEmpty = UIAlertController(title: title, message: message, preferredStyle: UIAlertControllerStyle.Alert)
    formEmpty.addAction((UIAlertAction(title: "Ok", style: .Default, handler: { (action) -> Void in

    })))

    self.presentViewController(formEmpty, animated: true, completion: nil)
}

func activityIndicatorFunction(){

    activityIndicator = UIActivityIndicatorView(frame: CGRectMake(0, 0, 100, 100))
    activityIndicator.backgroundColor = UIColor(red:0.16, green:0.17, blue:0.21, alpha:1)
    activityIndicator.layer.cornerRadius = 6
    activityIndicator.center = self.view.center
    activityIndicator.hidesWhenStopped = true
    activityIndicator.activityIndicatorViewStyle = UIActivityIndicatorViewStyle.WhiteLarge
    view.addSubview(activityIndicator)
    activityIndicator.startAnimating()
    UIApplication.sharedApplication().beginIgnoringInteractionEvents()

}


}

JSON Data:

{
"68e42b7f-aea5-4c3f-b655-51a99cb05bb0" : {
  "email" : "test1@test1.com",
  "username" : "test1",
  "waiting" : "0"
},
"8503d5a8-fc4a-492b-9883-ec3664898b4f" : {
  "email" : "test2@test2.com",
  "username" : "test2",
  "waiting" : "0"
}
}

回答1:

There are a few things going on here, but the most important one is that you cannot test for the existence of children with .ChildAdded. That makes sense if you think about it: the .ChildAdded event is raised when a child is added to the location. If no child is added, the event won't be raised.

So if you want to test if a child exists at a location, you need to use .Value. Once you do that, there are various way to detect existence. Here's one:

ref.queryOrderedByChild("waiting").queryEqualToValue("1")
   .observeEventType(.Value, withBlock: { snapshot in
       print(snapshot.value)
       if !snapshot.exists() {
           print("test2")
       }
   });

回答2:

Check for NSNull. This is the code for observing a node. Queries work much the same way.

Here's a complete and tested app. To use, change the string 'existing' to some path you know exists, like your users path and the 'notexisting' to some path that does not exist

    let myRootRef = Firebase(url:"https://your-app.firebaseio.com")
    let existingRef = myRootRef.childByAppendingPath("existing")
    let notExistingRef = myRootRef.childByAppendingPath("notexisting")

    existingRef.observeEventType(.Value, withBlock: { snapshot in

        if snapshot.value is NSNull {
            print("This path was null!")
        } else {
            print("This path exists")
        }

    })

    notExistingRef.observeEventType(.Value, withBlock: { snapshot in

        if snapshot.value is NSNull {
            print("This path was null!")
        } else {
            print("This path exists")
        }

    })

Please note that by using .Value, there will be a guaranteed a return result and the block will always fire. If your code used .ChildAdded then the block will only fire when a child exists.

Also, check to make sure of how your data appears in firebase.