To test the events you need to be logged in. If you are not registered, try registering in this link
Firebase Experiments : Authentication.
Cleanups : Once testing is completed in authentication page there is a signout button to signout or delete user whichever you prefer.
This method is triggered once when the listener is attached and again every time the data, including children, changes.
You can use the once() method to simplify this scenario: it triggers once and then does not trigger again. This is useful for data that only needs to be loaded once and isn't expected to change frequently or require active listening.
Child event : child_added
Retrieve lists of items or listen for additions to a list of items. This event is triggered once for each existing child and then again every time a new child is added to the specified path. The listener is passed a snapshot containing the new child's data.
Child event : child_changed
Listen for changes to the items in a list. This event is triggered any time a child node is modified. This includes any modifications to descendants of the child node. The snapshot passed to the event listener contains the updated data for the child.
Child event : child_removed
Listen for items being removed from a list. This event is triggered when an immediate child is removed.The snapshot passed to the callback block contains the data for the removed child.
Child event : child_moved
Listen for changes to the order of items in an ordered list. child_moved events always follow the child_changed event that caused the item's order to change (based on your current order-by method).