Base class for custom element handlers.
To implement a new element handler, one must create a new class extending this class, override {@link #getChildHandler} if nestedelements need to be parsed, override {@link #processAttribute} ifattributes need to be parsed, and override {@link #processEndElement()} to receive the text() value and post-processthe element.
If the handler wishes to store unrecognized XML contents in an {@link XmlBlob} value, it must call {@link #initializeXmlBlob} either in theconstructor, in parent's {@link #getChildHandler}, or in {@link #processAttribute}. The resulting {@link XmlBlob} value is availablefollowing the invocation of {@link #processEndElement()}through the object passed to {@link #initializeXmlBlob}.
This class implements overridable methods to support unrecognized XML parsing if desired.