//Firebase RealtimeDB Class
_ users
  |
  |_ a_uid
     |
     |_ email: "some email"
     |
     |_ groups
       |    |_ group_1: true
     |      |_ group_2: true
     |
     |_ location
     |  |_ coords: "[52.5, 67.1]"
     |  |_ name: "Oyebanjo Solarin Street, Lagos"
     |  |_ visibility: true
     |
     |_ username: "some username"

//1st
struct LocationStruct {
        var coords: String?
        var name: String?
        var visibility: Bool?
    }

//2nd
class UserClass {
    var email = ""
    var username = ""
    var groupsDict: [String: Any]
    var loc = LocationStruct()

    init(snap: FIRDataSnapshot) {

        let userDict = snap.value as! [String: Any]

        self.email = userDict["email"] as! String
        self.username = userDict["username"] as! String

        self.groupsDict = userDict["groups"] as! [String: Any]

        let locationDict = userDict["location"] as! [String: Any]
        self.loc.coords = locationDict["coords"] as? String
        self.loc.name = locationDict["name"] as? String
        self.loc.visibility = locationDict["visibility"] as? Bool
    }
}

//3rd
ref.child("users").child("a_uid")
                      .observeSingleEvent(of: .value, with: { snapshot in

        let user = UserClass(snap: snapshot)

        //this code is just to show the UserClass was populated.
        print(user.email)
        print(user.username)

        for group in user.groupsDict { //iterate over groups
            print(group)  //and print each one
        }

        print(user.loc.coords!) //print the location data
        print(user.loc.name!)
        print(user.loc.visibility!)
    })