Using Object-Oriented Frameworks

Often it is di cult to reuse a software component outside of its original context. Object-oriented frameworks can provide the context in which the component is meant to be reused and thus allow for a signi cant amount of reuse. An object-oriented framework is the reusable design of a system or subsystem implemented through a collection of concrete and abstract classes and their collaborations [Beck and Johnson, 1994]. The concrete classes provide the reusable components, while the design provides the context in which they are used. A framework is more than a collection of reusable components. It provides a generic solution to a set of similar problems within an application domain. The framework itself is incomplete and provides places called hooks at which users can add their own components speci c to a particular application. Developing an application from a framework di ers from developing an application on its own. The framework already supplies the architecture of the application, and users ll in the parts left incomplete by the framework. A framework typically includes the main control loop and calls application extensions to perform speci c tasks. Unlike the reuse of pure function libraries, framework users give up control of the design. In return, users are able to develop applications much more quickly, and a single framework can form the basis for a whole family of related applications. Since frameworks can be complex and di cult to learn, documenting all aspects of the framework is important to aid in the user's understanding of the framework and to make the framework easier to use. In this chapter we discuss techniques and concepts related to using frameworks. Many users will use a framework as it was meant to be used, but others will want to use the framework in new, non-standard ways. Still others will want to evolve the framework to incorporate new capabilities. Although the focus of the paper is mainly on using the framework as it was meant to be used, but we discuss issues related to many other aspects of use.