A DataSnapshot contains data from a Firebase Database location. Any time you read Firebase data, you receive the data as a DataSnapshot.

DataSnapshots are passed to the blocks you attach with observe(_:with:) or observeSingleEvent(of:with:). They are efficiently-generated immutable copies of the data at a Firebase Database location. They can’t be modified and will never change. To modify data at a location, use a DatabaseReference (e.g. with setValue(_:)).

The entry point for accessing a Firebase Database. You can get an instance by calling Database.database(). To access a location in the database and read or write data, use FIRDatabase.reference().

A DatabaseQuery instance represents a query over the data at a particular location.

You create one by calling one of the query methods (queryOrdered(byChild:), queryStarting(atValue:), etc.) on a DatabaseReference. The query methods can be chained to further specify the data you are interested in observing.

A FIRDatabaseReference represents a particular location in your Firebase Database and can be used for reading or writing data to that Firebase Database location.

This class is the starting point for all Firebase Database operations. After you’ve obtained your first FIRDatabaseReference via [FIRDatabase reference], you can use it to read data (ie. observeEventType:withBlock:), write data (ie. setValue:), and to create new FIRDatabaseReferences (ie. child:).

A MutableData instance is populated with data from a Firebase Database location. When you are using runTransactionBlock(_:), you will be given an instance containing the current data at that location. Your block will be responsible for updating that instance to the data you wish to save at that location, and then returning using TransactionResult.success(withValue:).

To modify the data, set its value property to any of the Objective-C types supported by Firebase Database, or any equivalent natively bridgeable Swift type:

• NSNumber (includes booleans)
• NSDictionary
• NSArray
• NSString
• nil / NSNull to remove the data

Note that changes made to a child MutableData instance will be visible to the parent.

Placeholder values you may write into Firebase Database as a value or priority that will automatically be populated by the Firebase Database server.

Used for runTransactionBlock(_:). A TransactionResult instance is a container for the results of the transaction.