getUnicorns.swift

override func viewWillAppear(_ animated: Bool) {
        super.viewWillAppear(animated)
        ref.child("unicorns").observe(.value) { snapshot in
            var unicorns = [Unicorn]()
            for unicornSnapshot in snapshot.children {
                let unicorn = Unicorn(snapshot: unicornSnapshot as! DataSnapshot)
                unicorns.append(unicorn)
            }
            self.unicorns = unicorns
        }
    }