Class Graph Views

An application typically has many conceptually separate pieces of behavior written over a common class graph. Each behavior makes different assumptions about the form of the class graph and the methods provided by the classes. The behaviors’ overlapping assumptions make it difficult to modify the class graph in mid-development; each modification can break assumptions that must be tracked down and mended. We propose a flexible system for providing separate views of the class graph to each behavior. Each view specifies only the details that the particular behavior makes assumptions about. By programming to the simplified view of the class graph, each behavior becomes more robust to modifications to the class graph. Inevitably, modifications to the class graph will require that broken assumptions be patched up, but these modifications need only be performed in the view specification, simplifying maintenance. A view can be applied simultaneously to several parts of the class graph, promoting code reuse. A key innovation is that the same view is exported regardless of how is it applied. This is vital to being able to transparently layer views and thus minimize maintenance costs. A common organization is to have a general purpose lower level view provide a stable interface to a changing class graph and to build high level views tailored to specific behavior on top of that. This approach offers minimized maintenance costs (by virtue of all high-level views sharing the provided low-level interface), and simplified behavior (as each behavior sees a class graph tailored to its needs).