Smalltalk and Object Orientation: an Introduction - Free

21. Frameworks and Patterns for Object Oriented
Design
21.1 Introduction
Designing complex software systems is hard. It is a great deal easier to reuse an existing software
system, merely modifying it where necessary, than to build it from scratch. These two facts have led to
a great deal of interest in what has become termed software frameworks in the object oriented
community. A software framework is “the reusable design of a system or a part of a system expressed
as a set of abstract classes [and concrete classes] and the way instances of (subclasses of) those classes
collaborate” [Beck and Johnson 1994]. However such frameworks are notoriously difficult to document.
For example the Model View Controller (MVC) framewo rk in Smalltalk is very powerful, but it has
proved difficult to explain in a clear and simple manner how the MVC should be used [Krasner and
Pope 1988].
In the object oriented community a number of researchers have explored the work of an architect
who d esigned a language for encoding knowledge of the design and construction of buildings
[Alexander et al 1977, Alexander 1979]. The knowledge is described in terms of patterns which capture
both a recurring architectural arrangement and a rule for how and wh en to apply this knowledge. That
is, they incorporate knowledge about the design as well as the basic design relations.
The result is that there is now a growing community exploring how software frameworks can be
documented using (software) design patterns (for example, [Johnson 1992] and [Birrer and
Eggenschmiler 1993]). Johnson’s paper describes the form that these design patterns take and the
problems encountered in applying them.
The remainder of the chapter is structured in the following manner: Sectio n two considers what a
framework is and the role of patterns in documenting frameworks. Section three introduces the
HotDraw framework and Section four presents a very simple pattern documenting how you can use
HotDraw to construct a simple editor.
21.2 Patterns and frameworks
21.2.1 What is a framework?
A framework is a reusable design of a program or a part of a program expressed as a set of classes. That
is, a framework is a set of prefabricated software building blocks that programmers can use, extend, or
customize for specific computing solutions. With frameworks, software developers don’t have to start
from scratch each time they write an application. Frameworks are built from a collection of objects, so
both the design and code of a framework m ay be reused. However frameworks are not necessarily easy
to design or implement. Questions such as “how much will it cost” to produce a framework and “how
much will we benefit” from the network are difficult to answer [Moser and Nierstrasz 1996]. In addition
reusing frameworks instead of libraries can cause subtle architectural changes in an application calling
for innovative management [Sparks, Benner and Faris, 1996].
Frameworks can provide solutions to different types of problem domain. These include a pplication
frameworks, domain frameworks and support frameworks:
• Application frameworks encapsulate expertise applicable to a wide variety of programs. These
frameworks encompass a horizontal slice of functionality that can be applied across client
domains. An application framework might provide the basic facilities of a payroll system, or a
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