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As The Xul Framework has progressed, the need for a clean MVC architecture underlying applications has become clear. MVC, or Model-View-Controller, provides for a clean separation between application data (models), view logic (controllers) and the user interface (views). This separation creates loosely coupled components that are easier to maintain and more importantly testable. For a more in-depth look at building Pentaho Xul application with the MVC pattern, please refer to the excellent article by Aaron Philips: MVC in Pentaho Xul Applications
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