Interface for reading an XML document using callbacks.
XMLReader is the interface that an XML parser's SAX2 driver must
implement. This interface allows an application to set and query
features and properties in the parser, to register event handlers
for document processing, and to initiate a document parse.
All SAX interfaces are assumed to be synchronous: the parse
methods must not return until parsing is complete, and readers
must wait for an event-handler callback to return before reporting
the next event.