Inside Taligent Technology - Properties of frameworks

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Properties of frameworks

A framework is a collection of cooperating classes that make up a reusable design solution for a given problem domain. It typically includes objects that provide default behavior, like MacApp's menus and windows, and programmers use it by inheriting some of that default behavior and overriding other behavior so that the framework calls application code at the appropriate times.

There are three main differences between frameworks and class libraries:

Behavior versus protocol. Class libraries are essentially collections of behaviors that you can call when you want those individual behaviors in your program. A framework, on the other hand, provides not only behavior but also the protocol or set of rules that govern the ways in which behaviors can be combined, including rules for what a programmer is supposed to provide versus what the framework provides.

Don't call us, we'll call you. With a class library, the code the programmer writes instantiates objects and calls their member functions. It's possible to instantiate and call objects in the same way with a framework--that is, to treat the framework as a class library--but to take full advantage of a framework's reusable design, a programmer writes code that overrides and is called by the framework. The framework manages the flow of control among its objects. Writing a program involves dividing up responsibilities among the various pieces of software that get called by the framework rather than specifying how the different pieces should work together. This relationship is expressed by the principle: "Don't call us, we'll call you."

Implementation versus design. With class libraries programmers reuse only implementations, whereas with frameworks they reuse design. A framework embodies the way a family of related programs or pieces of software work. It represents a generic design solution that can be adapted to a variety of specific problems in a given domain. For example, a single framework can embody the way a user interface works, even though two different user interfaces created with the same framework might solve quite different interface problems.

A framework often embodies specific domain expertise. Application code customizes the framework in a way that solves a particular application problem within the general domain of the problem the framework was designed to solve. In the case of an application framework like MacApp, the expertise is in the domain of writing programs for the Mac OS that display menus and windows and perform other basic tasks consistently and reliably. In the case of a framework developed by a bank, the domain expertise embodied in the framework might be how customer accounts or certain kinds of financial transactions work. In this case, a programmer might customize the framework to create specific kinds of accounts or financial instruments.

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