Updating a hashmap
Every client connected to a Firebase database maintains its own internal version of any active data.When data is written, it's written to this local version first.For example, attaching a listener to the root of your database is not recommended. In some cases you may want a callback to be called once and then immediately removed, such as when initializing a UI element that you don't expect to change.You can use the method to simplify this scenario: it triggers once and then does not trigger again.However, you can still update a child without rewriting the entire object.
You give this operation two arguments: an update function and an optional completion callback.
By default, read and write access to your database is restricted so only authenticated users can read or write data.
To get started without setting up Authentication, you can configure your rules for public access.
This repeats until the transaction is accepted or too many attempts have been made.
If a client loses its network connection, your app will continue functioning correctly.