DB The entry point for accessing a Firebase Database.
 DBDataSnapshot A DataSnapshot instance contains data from a Firebase Database location.
 DBError Instances of DatabaseError are passed to callbacks when an operation failed.
 DBMutableData Instances of this class encapsulate the data and priority at a location.
 DBQuery The DBQuery class (and its subclass, DBReference) are used for reading data.
 DBReference A Firebase reference represents a particular location in your Database and can be used for reading or writing data to that Database location.
 DBServerValue Contains placeholder values to use when writing data to the Firebase Database.
 DBWhenDisconnect The DBWhenDisconnect class is used to manage operations that will be run on the server when this client disconnects.