Christopher Alexander
Design Patterns
Jens Gustavsson
TAPESTRY OF LIGHT AND DARK
Create alternating areas of light and dark throughout
the building, in such a way that people naturally walk
toward the light, whenever they are going to
important places: seats, entrances, stairs passages,
places of special beauty, and make other areas
darker, to increase the contrast.
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Software Design Patterns
What is good software design?
A Design Pattern is a standard solution for a
standard design problem in a certain context.
Goal: resuse design information
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Functionality
Runtime qualities
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Other qualities
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Example: Facade
Performance
Safety
Security
Reliability
Usability
Portability
Maintainability
Testability
Example: Facade
Facade
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How to describe design patterns?
Facade
Intent
Provide a unified interface to a set of interfaces in a subsystem. Facade defines
a higher-level interface that makes the subsystem easier to use.
Motivation
Structuring a system into subsystems helps reduce complexity. A common
design goal is to minimize the communication and dependencies between
subsystems. … example …
Facade
Facade
Applicability
Consequences
Use the Facade pattern when:
z you want to provide a simple interface to a complex subsystem. This makes
subsystems more reusable and easier to customize.
z there are many dependencies between clients and the implementation
classes of an abstraction. Introduce a facade to decouple the subsystem
from other subsystems, thereby promoting subsystem independence and
portability.
z you want to layer your subsystems. Use a facade to define an entry point to
each subsystem level.
The Facade pattern offers the following benefits:
1.
It shields clients from subsystem components, thereby reducing the
number of objects that clients deal with and making subsystem easier
to use.
2.
It promotes weak coupling between subsystem and its clients. Weak
coupling lets you vary the components of the subsystem without
affecting its clients.
3.
It doesn't prevent applications from using subsystem classes if they
need to.
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Facade
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Structure
Participants
Collaborations
Implementation
Sample Code
Known Uses
Related Patterns
Patterns do not contain new ideas!
Composite, motivation
Composite, motivation
aPicture
Text
aRectangle
aLine
aText
aCircle
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Composite, motivation
Composite, motivation
aPicture
Text
aRectangle
aLine
aText
aCircle
Composite, applicability
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Represent hierarchies of objects
Ignore difference between compositions of
objects and individual objects
aCircle
aLine
aLine
aPicture
aLine
aLine
Composite, structure
Component
Client
*
operation()
add(Component)
remove(Component)
getChild(int)
Leaf
operation()
Composite
operation()
add(Component)
remove(Component)
getChild(int)
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Composite, our example
Composite, consequences
VisualComponent
Client
move()
setColor()
move()
setColor()
Rectangle
move()
setColor()
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Makes the client simple
Easy to add new kinds of components
Can make design too general
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Line
Circle
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move()
setColor()
*
add(VisualComponent)
remove(VisualComponent)
getChild(int)
Picture
move()
setColor()
add(VisualComponent)
remove(VisualComponent)
getChild(int)
State, motivation
DrawingController
Client
Text
tool
mouseClicked() {
if (tool == zoom) {
...
} else if (tool == select) {
...
}
}
keyPressed() {
if (tool == zoom) {
...
} else ...
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State, example
DrawingController
Client
mouseClicked()
keyPressed()
CreationTool
mouseClicked()
keyPressed()
SelectionTool
mouseClicked()
keyPressed()
State, structure
Tool
mouseClicked()
keyPressed()
ZoomTool
mouseClicked()
keyPressed()
Some thoughts
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Context
Many patterns are simple
Common vocabulary
A language facilitates thought
If design patterns are the building elements
for systems, shouldn't they be the primitives
in programming languages?
request()
ConcreteStateA
handle()
State
handle()
ConcreteStateB
handle()
ConcreteStateC
handle()
Anti Patterns
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Poltergeist
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Copy and Paste Programming
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Shortlived, stateless, forwards responsibility
Typical names: manager, controller, startProcess
Several copies of almost identical code
Boat Anchor
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Something without value for a project that is used
anyway
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