Expanding the ViewKit Framework Through Reusable Classes

The rolodex program described above provides one example. This simple program could be written in any number of ways. The object-oriented design described above may actually take longer to develop and require more code to implement than some other approaches, although the ViewKit library will provide a substantial amount of support, which tends to reduce the amount of code to be written. The additional work is a result of the emphasis on clean, abstract interfaces and a well-defined separation of tasks. In return for the additional time spent designing for an object- oriented model, the rolodex example has the potential to yield some new classes that may be reusable in some situations. The Rolodex class is rather specialized, but it could be useful in some applications. The LabeledText class is very simple, but is quite general purpose and could easily be useful elsewhere. The Address class is less task-specific than the Rolodex class, and might also be useful in other situations. Thus, the design of this simple application has resulted in three new, potentially reusable classes, which could be added to the pool of ViewKit-compatible components available to future applications.

The ViewKit framework supports the creation of such components in several ways. First, the VkComponent class defines a common interface to be supported by all components. As long as user interface components are implemented as subclasses of VkComponent, the ViewKit library provides additional services that can operate on and work with these classes. The VkComponent class also supports C++ member function callbacks, which provides a way for classes to interact with one another without requiring hard-coded dependencies between classes. (Hard-coded dependencies decrease reusability.) Finally, the ViewKit framework provides a core set of services that all components can depend on. Classes can readily use dialogs, the undo facility, and so on, knowing that these core ViewKit facilities will always be available in any ViewKit application.

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