Smalltalk and Object Orientation: an Introduction - Free

As the framework takes shape you should be continually attempting to refine it by adding more
default behavior, more hooks into it as well as additional ways for users to interact with it. You will also
need to produce concrete instantiations of the framework to test its functionality and to customizability.
These example instantiations should be included with the framework as part of the deliver ed product
(whether it is delivered internally or externally to your organization).
The whole development process is extremely iterative but is by necessity subject to the strictest of
software engineering practice. In addition suitable documentation of the framework is essential (and this
is where design patterns come into their own) from design, implementation and user perspectives.
Finally, some useful general comments about framework development include:
• Develop frameworks by examining existing applications.
• Develop small focused frameworks (breaking down larger frameworks when the opportunity to
do so is identified).
• Build frameworks using an iterative process prototyping how the frameworks will be used all the
time.
• Always look out for additional fun ctionality for the framework. For such functionality,
determine whether it should be a default behavior of the framework or whether a hook should be
provided to allow a user to implement it.
• Good documentation is essential and should include sample instant iations of the framework,
descriptions of the framework architecture, descriptions of the framework and its intent,
guidelines on using the framework and cookbook examples for particular operations.
21.2.3 What is a design pattern?
Design patterns capture expertise in building object oriented software [Gamma et al 1993; Johnson
1992; Beck and Johnson 1994]. A design pattern describes a solution to a recurring design problem. It
also contains information on the applicabili ty of a pattern, the trade offs which must be made and any
consequences of the solution. Books are now appearing which present such design patterns for a range
of applications. For example, [Gamma et al, 1995] is a widely cited book which presents a cat alog of
23 design patterns. Design patterns are extremely useful for both novice and experienced object
oriented designers. This is because they encapsulate extensive design knowledge and proven design
solutions with guidance on how to use them. Reusing common patterns opens up an additional level of
design reuse, where the implementations vary, but the micro -architectures represented by the patterns
still apply.
There are potentially very many design patterns available to a designer. A number of these patterns
may superficially appear to suit their requirements, even if the design patterns are available on -line (via
some hyper text style browser) it is still necessary for the designer to search through them manually,
attempting to identify the design which best matches their requirements.
In addition, once they have found the design which they feel best matches their needs, they must
then consider how to apply it to their application. This is because a design pattern describes a solution
to a particular design problem. This solution may include multiple trade offs which are contradictory
and which the designer must choose between, although some aspects of the system structure can be
varied independently (although some attempts have been made to automate this process for example
[Budinsky et al 1996]).
Patterns seem to be exceptionally well suited for documenting frameworks. They certainly provide
guidance on how to use a framework, the purpose of the framework, they can also include concrete
examples and can be used to document the design of the framework.
21.2.4 Pattern templates
The pattern template used in [Gamma et al , 1995] provides a standard structure for the information
which comprises a design pattern. This makes it easier to comprehend a design pattern as well as
providing a concrete structure for those defining new patterns. Gamma’s book [Gamma et al , 1995]
provides a detailed description of the template; only a summary of it is presented in Table 21.1.
Patterns are best suited for teaching how to use a framework, but with care they can meet all three
purposes identified earlier. By describing the design of a framework in terms of patterns, you describe
both the design and the rationale behind the design. As patterns also show how to use the framework for
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