Definition
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A Design Pattern is
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Design Patterns
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a description of a standard solution for
a standard problem in design.
Goal: Reuse of design information
A pattern must not be “new”!
Uwe Assmann
RISE
(Slides derived from Walter F. Tichy, Universität
Karlsruhe, Germany, Used by permission)
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Introduction
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What is a Design Pattern?
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Christopher Alexander “The timeless way of building”
For what can they be used?
How does it look like?
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Hunting for the “Quality without a name”
Patterns grasp centers of beauty
You have a language for beauty, consisting of patterns
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Literature:
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Alexander’s Laws on Beauty
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„Design Patterns”, Gamma et al, Addison Wesley,1995
„A System of Patterns”, Buschmann et al, Wiley, 1995. Part II
from 2000
„Pattern Languages of Program Design”, Conferences, AddisonWesley, 1995,...
„Software Architecture”, Shaw & Garlan, Prentice Hall, 1996.
“Object-oriented Design and Patterns” W. Pree, 1995, AddisionWesley
TDDB84 “Design Patterns” (running in period I, 2003)
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a book of the century: A MUST
dependent on culture
Beauty cannot be invented but must be
combined/generated by patterns from a pattern
language
Alexander means “composition”
Example: Model/View/Controller (MVC)
Model/View/Controller
Relation between model and views is grasped by the
design pattern Observer.
T Views may register at the model (observers).
T They are notified if the model changes. Then, every view
updates itself by accessing the data of the model.
T Views are independent of each other. The model does
not know how views visualize it.
T Observer decouples strongly.
MVC is a agglomeration of classes to control a user interface
and a data structure (from Smalltalk)
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Model:
View:
Controller:
Data structure or object, invisible
Representation(s) on the screen
Encapsulates reactions on inputs of
users, couples model and views
MVC is a complex design pattern and combines the simpler
ones compositum, strategy, observer.
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Views as Observer
Window
x
y
z
a
60
50
80
b
30
30
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Pattern 2: Compositum
Window
Views may be nested. (Compositum)
T Example: object inspector in debuggers
T Compositum requires that
Window
c
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20
10
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a
b
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c
a=50%
b=30%
c=20%
Model
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The class CompositeView is a subclass of View
Can be used in the same way as View
Pattern 3: Strategy
Purpose Design Pattern
The relation between controller and view is a Strategy
pattern.
T There may be different control strategies
1)
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Lazy or eager update of views
Menu or keyboard input
A view may select subclasses of Controller, even
dynamically
No other class changes
Improve communication in teams
1)
2)
2)
Abstracts design concepts
1)
2)
3)
3)
2)
3)
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Patterns are “mini-frameworks”
Documentation
May be used to capture information in reverse engineering
Improvement of the state of the art of software
engineering
1)
4)
Between clients and programmers
Between designers, implementers and testers
A “software engineer” without the knowledge of patterns is a
programmer
Fix a glossary for software engineering
Prevent re-invention of well-known solutions
Improve code quality and structure
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Structure for Design Pattern
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There are several formats
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Name (incl. Synonyms)
Task and Context
Solution
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Gof (Gamma Gang of Four)
Alexandrian format
Structure (Components, Relations): UML class or object diagram
Interactions und Consequences: interaction diagrams
Implementation (code examples)
Design Pattern Catalogue
Overview Patterns
Design Pattern Categories
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M
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several are around
Basic patterns characterize simple collaboration scenarios in class
graphs, sequence diagrams, or other diagrams.
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Basic patterns
TemplateMethod: Vary the
algoritm in a subclass
Objectifier: Delegate behavior
to special object
Mediator: Object to
encapsulate communication
Decorator: Dynamic
Extension of classes
Proxy: Representant for
remote objects
Compositum: Part/Whole
hierarchies
Adapter: Adapt an interface to
another
Data patterns
Singleton: globally unique
objects
Flyweight: Space
optimizations by
representants
Memento: Snapshot of state
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A classification is hard
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Are used to form larger patterns
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Data patterns characterize state of a set of objects and their
responsibilities
Creational Patterns determine how objects and object families are
allocated
Coupling patterns (Structural patterns) divide systems into
subsystems such that these can be exchanged independently
Commonality patterns condense common parts of classes into
new classes
Control flow patterns (Behavioral patterns) describe common
control flow situations
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•
•
•
•
•
•
•
•
•
Coupling Patterns
Repository: central
scratchpad
Facade: encapsulating class
for subsystem
Layers: layered facades
Bridge: decouple abstraction
from implementation
Observer: event based
communication, dynamic
Event channel: channel of
observers
Pipe: stream oriented
communication
Framework:
•
Control flow (behavioral)
patterns
•
Visitor: Decouple iteration
over tree from actions
ChainOfResponsibility:
cascaded delegation
Commad: reified command
procedure
Blackboard: organized
repository
Strategy: algorithm family
Iterator: iteration without
knowing internals
Interpreter
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•
•
•
•
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Creational Patterns
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FactoryMethod: Vary allocation in subclass
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Abstract Factory: Allocation of object family
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Builder:Incremental allocation of complex structured
object
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Prototype: Clone an object
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Relations between Patterns [Zimmer PLOP 1]
Creation patterns
Builder
Prototype
Coupling patterns
Control flow patterns
Interpreter
Layers
Strategy
Basic Patterns
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Visitor
Facade
Abstract Factory
T
Iterator
ChainOf
Responsibility
Observer
T
T
Bridge
FactoryMethod
According to [W. Zimmer: Frameworks und Design
Pattern, PhD 1997 Universität Karlsruhe] Design
Patterns can be build from the following basic ones:
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Comman
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T
T
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Template
Method
Objectifier
Proxy
Compositum
Singleton
Memento
Adapter
Mediator
Glue
Basic patterns
Decorator
Flyweight
Data patterns
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TemplateMethod
Objectifier (Delegation)
Mediator
Decorator
Proxy
Compositum
Adapter
Data Patterns
T Singleton (singleton)
T Flyweight
T Memento
Template Method
Objectifier (Delegation Object)
T
AbstractClass
TemplateMethod()
primitiveOperation1()
primitiveOperation2()
...
primitiveOperation1();
...
primitiveOperation2();
...
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T
T
T
T
ConcreteClass
Reify a method by an object
Delegate the behavior of the method to the object
Vary the delegatee
Factorization
Varying behavior
Used with: Bridge, Builder, Command, Iterator,
Observer, State, Strategy, Visitor.
primitiveOperation1()
primitiveOperation2()
Define the skeleton of an algorithm (template method)
Delegate parts to hook methods that are filled by subclasses
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Template Method
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Allows for varying behavior.
Separate invariant from variant parts of an algorithm
Factor out common code into super classes
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Control extensions by only allowing “hook methods”
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Avoid code replacement
Mediator
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T
T
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Mediators encapsulate communication
Partners can register with the mediator and do not know
with whom they talk
Communication can be varied dynamically
Mediator
Example Mediator
Mediator
Mediator
Decoupling windows from text fields
Colleague
Mediator
DialogueMediator
ConcreteMediator
ConcreteColleague1
showDialog()
createWidgets()
widgetChanged(Widget)
ConcreteColleague2
Widget
changed()
Mediator.widgetChanged(this)
Typical Object Structure:
AColleague
AConcreteMediator
AColleague
Mediator
AColleague
Mediator
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T
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T
createWidgets()
widgetChanged(Widget)
InputField
select()
field
Mediator
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Example Mediator
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liste
TextDialogueMediator
AColleague
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T
ListBox
Mediator
Proxy
Many dependencies between parts of a window
Different widgets have different dependencies
Encapsulate behavior in a mediator that mediates
between widgets
Spare class factorization
Widgets can be registered/unregistered dynamically
Flexible, but anonymous communication
Subject
Client
operation()
...
realSubject
RealSubject
Proxy
operation()
...
operation()
...
...
realSubject.operation()
Hide the access to a real subject by a representant
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Proxy
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References instead of values
Protocol proxy: counts references (reference-counting
garbage collection
Indirection proxy: assembles all references to an
object to make it replaceable
Remote proxy: representant of a remote object
Caching proxy: caches values which had been loaded
from the subject
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Adapter (2)
Client
AdaptierteClass
operation()
spezificOperation()
adapted
Object
Adapter
Remote
Loading lazy on demand
operation()
adaptedObject.spezificOperation()
Protection proxy
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Goal
Without Multiple Inheritance, but object adapter
Firewall
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Adapter (Wrapper)
Example: Use of an External Class Library
External Library
Client
GoalClass
AdaptedClass
operation()
specificOperation()
DrawingEditor
GraficObject
framew()
createManipulator()
Can also be
interface
TextDisplay
largeness()
(Implementation)
Adapter
operation()
specificOperation()
Adapts an interface to a client
May use multiple inheritance, but need not
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Linie
Text
frame()
createManipulator()
frame()
createManipulator()
return text.largeness()
return new TextManipulator
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Compositum
Compositum
Part/Whole hierarchies, e.g., nested graphic objects
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Class Compositum contains a container for the child
objects
aPicture
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Pattern is hard to employ when it sits on top of a
complex inheritance hierarchy
T
aPicture
aPicture
aLine
aLine
For iterations on hierarchies
aRectangle
aRectangle
common operations: draw(), move(), delete(), scale()
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Structure Compositum
Client
Data Patterns
Component
commonOperation()
add(Component)
remove(Component)
getType(int)
Memento
}
Pseudo implementations
childObjects
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Leaf
Compositum
commonOperation()
commonOperation()
add(Component)
remove(Component)
getType(int)
for all g in childObjects
g.operation()
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Flyweight
Singleton
Structure for Singleton
Structure for Flyweight
Flyweighte
FlyweightFactory
Singleton
Flyweight
getFlyweight(key)
static Singleton()
private Singleton (first: bool)
operation()
getDaten()
if uniqueInstance == null
then uniqueInstance = new Singleton (true)
return uniqueInstance
operation(extrinsicState)
if (Flyweighte[key] exists){
return Flyweight[key];
} else {
create new Flyweight;
add to Flyweight;
return it;
}
static uniqueInstance
data
Assure one globally unique object of a class
ConcreteFlyweight
SeparatelyUsed
ConcreteFlyweight
operation(extrinsicState))
operation(extrinsicState))
intrinsicState
gesamtState
Client
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Flyweight
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T
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Representants with minimal space
Example Flyweight
Object modeling for characters in editor
Set of objects is very large
State can be moved from object to external memory
(extrinsic state)
Identity of objects can be neglected
Introduce flyweight objects
Column
Row
e
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Row
x
a
m
Row
p
Set of Flyweights
e
l
a
b
c
d
e
f
g
h
i
j
k
l
m
n
o
p
q
r
s
t
u
v
w
x
y
z
Memento
Coupling (Structural) Patterns
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Extensionalize state of object
T
Client
Memento
restore(Memento m)
createMemento()
getState()
setState()
State
State
return new Memento(State)
memento
T
Container
T
Observer
Bridge
(Adapter)
Facade
State = m.getState()
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Example Memento
Observer (Publisher/Subscriber)
Observer
Breakpoints and Undo in editors, debuggers:
Window
x
y
z
a
60
50
80
b
30
30
10
Window
a
Rectangles should stay linked
If they are moved.
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Window
c
10
20
10
b
c
a=50%
b=30%
c=20%
Undo might be wrong if state
Is not sufficiently memorized
Subject
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Notify on change
Queries
Structure Observer
Observer
Subject
register(Observer)
unregister(Observer)
notify()
Observer
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Loose coupling in communication
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Dynamic change of communication
Anonymous communication
Multi-cast and broadcast communication
Cascading effects (wavefront..)
If an abstraction has two aspects
Observer
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update ()
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for all b in Observer {
b.update ()
}
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T
ConcreteObserver
Subject
Concreteubject
getState()
setState()
T
ObserverState =
Subject.getState()
update ()
T
T
ObserverState
Observers decide what happens
And one of them depends on the other
Reuse of aspects is possbile.
return SubjectState
SubjectState
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Sequence Diagram Observer
aConcreteubject
aConcreteObserver
anotherConcreteObserver
register()
Observer Implementation
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T
setState()
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notify()
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If there is more than one subject: Send Subject as
Parameter of notify(Subject s).
Push model: subject sends data in notify()
Pull model: observer fetches data itself
Mediator between subjects and observer:
T
T
update n()
getState()
update n()
getState()
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May filter events
Stops cascaded propagations
Event Channel
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T
T
T
Bridge (Handle/Body)
Anonymous communication with events
Partners don’t know each other
Combination of Mediator and Observer
Also called software bus
Client
Decouple an abstraction from
Its implementation
Abstraction
imp
Implementation
operation()
operationsImp()
imp.operationsImp()
ConcreteImplementationA
SpezializedAbstraction
operationsImp()
ConcreteImplementationB
operationsImp()
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Structure Event Channel
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Example
An event channel is a mediator that communicates with
events, and let every partner register as subject and
observer
Factorisation of class hierarchies with facet hierarchies
Window
Window
Event channel (software bus)
XWindow
Send Event
Register
Unregister
Partner 1
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PMWindow
XWindow
PMWindow
IconWindow
receiveEvent
partner N
XIconWindow
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PMIconWindow
Example
Facade
imp
Window
drawText()
drawRectangle()
WindowImp
Clients
devicedrawText()
devicedrawLinie()
Bridge
imp.devicedrawLinie()
imp.devicedrawLinie()
imp.devicedrawLinie()
imp.devicedrawLinie()
Facade
IconWindow
TransientWindow
XWindowImp
PMWindowImp
drawRahmen()
drawCloseButton()
devicedrawText()
devicedrawLinie()
devicedrawText()
devicedrawLine()
Subsystem
drawRectangle()
drawText()
drawRectangle()
XDrawLine()
XDrawString()
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Facade
T
Encapsulate a set of classes into a subsystem, and
represent this subsystem by a façade class
Possible on object level
T
Better information hiding
T
T
Then at run time façade objects exist
Facade
T
Façade provides a view to the subsystem
T
Facades decouple, minimize dependencies
Allow to exchange and reuse subsystem easier
If classes of the subsystem are again facades, layers
result
T
T
T
T
T
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There may be several views, several facades
The reliesOn relation is acyclic
Layers are nested facades (reducible graphs!)
Façade and Layers
Control Flow Patterns
T
Strategy
Command
Visitor
T
ChainOfResponsibility
T
Clients
T
T
Iterator
Upper layer
Facade
Lower layer
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Façade and Layers
T
T
Have been used no several times
Acyclic uses/reliesOn relation
T
T
Strategy
Nested facades enforce an acyclic uses relation!
Client
Layered design with abstract machines
contextInterface()
strategy
Strategy
algorithmInterface()
ConcreteStrategyA
ConcreteStrategyB
ConcreteStrategyB
algorithmInterface()
algorithmInterface()
algorithmInterface()
Define a family of algoriths
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Example Strategy
Structure for Command
Encapsulate formatting algorithms
Caller
Client
Composition
Formatter
traverse()
repair()
Command
exec()
Formatter
format()
Receiver
Formatter.format()
Receiver
action()
SimpleFormatter
TeXFormatter
ArrayFormatter
format()
format()
format()
ConcreteCommand
exec()
State
Receiver.action()
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Command (Command Object)
T
T
Represent (reify) a command as an object
Object
T
T
T
T
T
T
T
Example for Command
Can be undone
Can be memoized
Waiting queues
Multi-user management
Parameterization of commands
Protocol of commands
Application
Menu
MenuEntry
Command
add
(Document)
add(MenuEntry)
clicked()
exec()
Command.exec()
Dokument
open()
close()
cut()
copy()
paste()
Also know as Action, Transaction
document
PasteCommand
exec()
document.paste()
A command has a method exec which executes the operation
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Command for Opening Document
Visitor
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Command
T
exec()
T
T
application
Application
add
(Document)
OpenCommand
exec()
queryUser()
Document
open()
close()
cut()
copy()
paste()
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Encapsulate an operation on a collection (tree) of
objects as an object.
Separate tree inheritance hierarchy from command
hierarchy
Simple extensibility of both hierarchies
Simpler inheritance structures, otherwise multiplication
of classes
Abbreviation for multi methods
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T
name = queryUserr()
dok = new Documnt(name)
application.add(dok)
dok.open()
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dispatch/polymorphism on two arguments, not only the first
First dispatch on tree object (method accept), then operation
(method visit) objects
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MacroCommand With Compositum
Structure for Visitor
Client
Visitor
visitConcreteElemA(ConcreteElementA)
visitConcreteElemB(ConcreteElementB)
Command
exec()
MakroCommand
ConcreteVisitor1
ConcreteVisitor2
visitConcreteElemA(ConcreteElementA)
visitConcreteElemB(ConcreteElementB)
visitConcreteElemA(ConcreteElementA)
visitConcreteElemB(ConcreteElementB)
Commande
ObjectStructure
exec()
Element
accept(Visitor)
ConcreteElementA
for all b in Commande
b.exec()
ConcreteElementB
accept(Visitor b)
operationA()
accept(Visitor)
operationB()
b.visitConcreteElementA
(this)
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b.visit...B
(this)
Example Compiler Abstract Syntax Trees
Relations between Patterns [Zimmer PLOP 1]
Creation patterns
Builder
Prototype
Coupling patterns
Control flow patterns
Interpreter
Layers
Strategy
Visitor
Node
Facade
typeCheck()
generateCode()
prettyPrint()
The operations are distributed
Over the classes.
In case of extensions,
All classes must be extended.
VariableRefNode
AssignmentNode
typeCheck()
generateCode()
prettyPrint()
typeCheck()
generateCode()
prettyPrint()
Abstract Factory
Iterator
ChainOf
Responsibility
Observer
Bridge
FactoryMethod
Command
Template
Method
Objectifier
Proxy
Compositum
Singleton
Memento
Adapter
Mediator
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Basic patterns
Abstract Syntax Trees with Visitors
The End
NodeVisitor
visitAssignment(AssignmentNode)
visitVariableRef(VariableRefNode)
TypeCheckingVisitor
CodeGenerationVisitor
visitAssignment(AssignmentNode)
visitVariableRef(VariableRefNode)
visitAssignment(AssignmentNode)
visitVariableRef(VariableRefNode)
Program
Node
accept(NodeVisitor)
AssignmentNode
VariableRefNode
accept(NodeVisitor b)
accept(NodeVisitor)
b.visitAssignment (this)
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Glue
b.visitVariableRef (this)
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Decorator
Flyweight
Data patterns