Understanding Frameworks

When designing a framework F , the designer abstracts away certain key methods of certain key classes of the framework leaving them as virtual functions (in C++ terminology). One of the most important contributions that F makes to the final application built on F is the flow of control implemented in the non-virtual functions of F ; this relieves the application builder from having to worry about this important and difficult issue. An application builder who builds a complete application A using F refines the abstract base classes of F by providing specific bodies for the virtual functions, thus implementing specific behaviors. We use the term behavioral refinement to characterize this type of refinement. The important questions we address in this chapter are the following: how do we understand the behavior –specifically the flow-of-control behavior– implemented by the framework code, how do we understand the behavior implemented by the method bodies defined in the application code, and how do we combine these to obtain an understanding of the behavior of the entire application? We propose a trace based approach for specifying the behavior of the framework, in particular the control flow. The particular refinement that an application builder implements is captured in an appropriate refined specification of the respective virtual functions of F . We then show how this refined specification can be combined with the specification of the framework to arrive at the behavior of the entire application. We illustrate our approach on a simple diagram editor framework.