Represents the fundamental data structures of the user-defined property and the interface of UI listener.

## Package Specification

The core package represents the key data structures of the user-defined property. Other packages build on this class to add, remove, replace a user-defined property. UI can add listener to the design element to accept the notification event sent each time the operation is done, undone or redone.

The key classes in this package include:

- {@link org.eclipse.birt.report.model.api.core.Listener} is the interface class to accept the notification event sent by all report elements. See the extensive documentation in this class for information about the services common to all report elements.
- {@link org.eclipse.birt.report.model.api.core.UserPropertyDefn} provides generic access to a user-defined property. The class provides some getter/setters to the name, type, choices, and so on.