Design Patterns
Ric Holt & Sarah Nadi
U Waterloo, March 2010
What is a Design Pattern
• A Design Pattern systematically names,
explains, and implements an important
recurring design.
• These define well-engineered design
solutions that practitioners can apply when
crafting their applications.
2
Why Design Patterns?
• Good designers do not solve every problem
from first principles. They reuse solutions.
• Practitioners do not do a good job of
recording experience in software design for
others to use. Patterns help solve this
problem.
3
Four Example Patterns
1.
2.
3.
4.
MVC (Model View Controller)
Adapter
Facade
Factory
4
1. MVC Pattern
(Model View Controller)
• There is a model (subject) which can be
thought of as the application
• There are one or more views of the object
(by observers)
• There is a controller that mediates between
the model and the view(s).
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Example MVC
Views
a = 50%
b = 30%
c = 20%
a
b
c
a
b
c
Controller
a b c
Model
503020
[Mancoridis picture]
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Example MVC
Views
a = 50%
b = 30%
c = 20%
a
b
c
a
b
c
Controller
See
a b c
Model
Interact
503020
[Mancoridis picture]
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Multiple View Problem
• Need to keep all the views consistent
• If user (or one of users) changes a view, all
other views should be updated
8
Information Hiding:
Insulating Model from Presentation
• The model is the “back end” or “business
logic”, or “application”
• To minimize complexity, the model should
know as little as possible about
– How users interact with system
– How many users are there
– Any other models
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Implementing MVC
• There are many choices
–
–
–
–
Where is list of views (observers) kept?
How is notification of change transmitted?
How manage web interface?
Should a view ask for (or should it be told of)
details about changes?
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What Kind of Pattern is MVC?
• Design pattern proper?
– Use it for structuring modest configurations
• Architectural pattern?
– Use it for overall structure of a software
system, defining interactions among services,
browsers etc.
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2. Adapter Pattern
• You have an existing client (application)
that uses an old interface to an existing
support package.
• You are given a new interface to a new
support package
• You need to produce an adapter so that:
– The client can use the new interface instead of
the old one (without changing the client)
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Illustration of Adapter Pattern
Old
interface
Old
package
Client
Old
interface
Client
Adapter
New
interface
New
package
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Implementing Adapter Pattern Using
Object Composition
Client
Old Interface
New Interface
Request()
SpecificRequest()
inherit
Adapter
Request()
adaptee
SpecificRequest()
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Advantages of Adapter Pattern
• Can change behavior of existing software
without changing its source code
• Can be used for legacy software
• Do not need access to old source code
15
3. Facade Pattern
• A facade provides a single simplified
interface to a larger part of the system (a
subsystem)
• The facade makes this subsystem more
readable and easier to understand
• Having facades minimizes communication
and dependencies between subsystems
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Structure of the Facade Pattern
Facade
Subsystem
Classes
[Mancoridis picture]
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Adapter vs. Facade
• An Adapter is used when the wrapper must
respect a particular interface and must
support a polymorphic behavior. On the
other hand, a facade is used when one wants
an easier or simpler interface to work with
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4. Factory Pattern
• A creational pattern dealing with creating
objects without knowing their concrete class
• It defines an interface for creating an object,
but let the subclasses decide which class to
instantiate
• Useful when concrete class of object should
be decided at runtime according to
parameters
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How does it work
<static>
<abstract>
Product
ProductFactory
createProduct(in param) : Product
inherits
<Implementation Class>
ProductA
<Implementation Class>
ProductB
Produces different subclasses
of Product according to
parameters
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