Why Function Return Nil Firebase Swift

Firebase for SWIFT iOS: returning nil value in function

You need to use closures as getDocument() does not return synchronously.

func display(userid: String, handler: (String) -> Void) {
    let docRef = db.collection("user").document(uid)
    docRef.getDocument { (document, error) in
        if let document = document, document.exists {
            if let displayName = document.get("username") as? String {
                handler(displayName)
            }
        } else {
            print("Document does not exist")
        }
    }
}

And use it like so:

self.myLabel.text = "Loading..."
display(userid: {USER_ID}) { [weak self] displayName in
    print(displayName)
    self.myLabel.text = displayName
}

Why function return nil FireBase Swift

You're returning downloadURL before the asynchronous function within your callee function has finished execution.

At this point, it will have nil value, which is correct.

You'll want to use a completion handler to return downloadURL when its ready, like -

func storagePutData(uid: String, image: UIImage, success:@escaping (String?)->(), error: @escaping (Error?) -> Void) {
        var downloadURL: String!
        let imageName = NSUUID().uuidString
        let uploadDara = UIImageJPEGRepresentation(image, 0.1)!
        let ref = self.refStorage.child("usersProfileImage/\(uid)/\(imageName).png")

        ref.putData(uploadDara, metadata: nil) { (metaDara, error) in
            if let err = error {
                error(err)
            }
            ref.downloadURL(completion: { (url, error) in
                if let err = error {
                    error(err)
                } else {
                    downloadURL = url?.absoluteString
                    success(downloadURL)
                }
            })
        }
    }

Why does this query return nil from Firestore

The getDocuments callback gets called with two values (querySnapshot and `error), only one of which will have a value.

You're ignoring the error and assuming that querySnapshot has a value, which not only leads to the error you get, but also hides the likely cause.

I recommend following the pattern used in this example from the Firestore documentation on getting documents:

db.collection("cities").whereField("capital", isEqualTo: true)
    .getDocuments() { (querySnapshot, err) in
        if let err = err {
            print("Error getting documents: \(err)")
        } else {
            for document in querySnapshot!.documents {
                print("\(document.documentID) => \(document.data())")
            }
        }
}

By logging the error you can see what went wrong with your getDocuments() call.

Firebase returns nil when fetching data

When you observe a .value event for a list of nodes, you need to loop over snapshot.children to get the individual nodes. From the documentation on listening for value events for lists:

_commentsRef.observe(.value) { snapshot in
  for child in snapshot.children {
    ...
  }
}

The ... is where your current code inside the .childAdded listener goes.

Current Firebase User Returning Nil

This typically means that the currentUser is not yet initialized by the time that code runs. Since restoring the user's sign-in state requires a call to the server, it is an asynchronous operation.

Any code that depends on the current user state being restored should be inside (or be called from) an addStateDidChangeListener as in your first snippet. So you may need a snippet like that in your other view controller(s) too.

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