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192 Chapter 7 Design and implementation

Subject Observer
Attach(Observer) Update( )
Detach(Observer)
Notify( ) for All o in observers
o -> Update( )

ConcreteSubject ConcreteObserver
observerState =
GetState( ) return subjectState Update( ) subject -> GetState( )
subjectState observerState

detail to the solution description. Figure 7.12 is the representation in UML of the
Figure 7.12 A UML
model of the Observer Observer pattern.
pattern To use patterns in your design, you need to recognize that any design problem
you are facing may have an associated pattern that can be applied. Examples of such
problems, documented in the ‘Gang of Four’s original patterns book, include:

1. Tell several objects that the state of some other object has changed (Observer
pattern).
2. Tidy up the interfaces to a number of related objects that have often been devel-
oped incrementally (Façade pattern).

3. Provide a standard way of accessing the elements in a collection, irrespective of
how that collection is implemented (Iterator pattern).
4. Allow for the possibility of extending the functionality of an existing class at
run-time (Decorator pattern).

Patterns support high-level, concept reuse. When you try to reuse executable
components you are inevitably constrained by detailed design decisions that have
been made by the implementers of these components. These range from the
particular algorithms that have been used to implement the components to the
objects and types in the component interfaces. When these design decisions con-
flict with your particular requirements, reusing the component is either
impossible or introduces inefficiencies into your system. Using patterns means
that you reuse the ideas but can adapt the implementation to suit the system that
you are developing.
When you start designing a system, it can be difficult to know, in advance, if you
will need a particular pattern. Therefore, using patterns in a design process often
involves developing a design, experiencing a problem, and then recognizing that a
pattern can be used. This is certainly possible if you focus on the 23 general-purpose