design patterns - formal.iti.kit.edubeckert/teaching/praktikum-formale... · design patterns:...

Entwic klung objektorientier ter Software mit formalen Methoden

Design Patterns

Bernhar d Becker t

UNIVERSITÄT KOBLENZ-LANDAU

KeY-Praktikum WS03/04 � ��

Design Patterns: Libraries for design

Collection of solutions for common software design problems

Make experiences of designer s availab le to other s

better education

Prevent software engineer s from searching for alread y found solutions

increase of effectiveness

Suppor t comm unication between developer s

easier orientation

Document design decisions of a software system

increase of maintenance





� better education


increase of effectiveness


easier orientation









� increase of effectiveness


easier orientation











� easier orientation











� easier orientation


� increase of maintenance


Description scheme of design patterns

Motiv ation

Where does the need for this pattern come from?

Structure

What is the concrete (class) structure of the pattern?

Applicability

Under whic h cir cumstances is it applicab le?

Whic h are the forces it obeys?

Consequences

What are the effects when using this pattern?


Impr oving Consistence and Reusability

Model View Contr oller

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Month View

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Day View

Organizer Data

Database

Keeping Model, View and Contr oller separate allo ws

� diff erent consistence views

� reuse of views in other conte xts

� dynamic chosen response


The »Obser ver«: Separates model and view

Purpose: Defines a 1-to-n dependance relationship between objects, so

that the state chang e of one object causes a notification of the

dependant objects.

Applicability:

If the state chang e of one object, requires to update several other

objects.

If an object has to inf orm several other unkno wn objects.

Consequence:

Independant reuse of subject and obser ver.

Obser vers can be added without changing the obser ver or subject.





dependant objects.

Applicability:

� If the state chang e of one object, requires to update several other

objects.

� If an object has to inf orm several other unkno wn objects.

Consequence:

Independant reuse of subject and obser ver.

Obser vers can be added without changing the obser ver or subject.





dependant objects.

Applicability:

� If the state chang e of one object, requires to update several other

objects.

� If an object has to inf orm several other unkno wn objects.

Consequence:

� Independant reuse of subject and obser ver.

� Obser vers can be added without changing the obser ver or subject.


The class structure of the obser ver pattern

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Two modes:

pull : the obser ver has to ask for the new subject state ( )

push : chang e inf ormation is handed over to the obser ver when

notifying

KeY-Praktikum WS03/04 � �� I


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push : chang e inf ormation is handed over to the obser ver when

notifying

KeY-Praktikum WS03/04 � �� I


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Two modes:

� pull : the obser ver has to ask for the new subject state ( �� )

� push : chang e inf ormation is handed over to the obser ver when

notifyingKeY-Praktikum WS03/04 � �� I

Update of views implementing an obser ver

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Example: Views of an organiz er

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Month View

Mo. 10. Nov.8

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Day View

Organizer Data

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KeY-Praktikum WS03/04 � �� ®

Updating the organiz er’s views

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The »Command«: Actions as objects

Purpose: Encapsulate a command as an object. Allo ws to keep trac k of

actions (transactions).

Applicability:

parameterise clients,

log commands,

realise undo functionality or

cache commands in a queue in order to execute them at a cer tain

time in the future

KeY-Praktikum WS03/04 � �� ï




Applicability:

� parameterise clients,

log commands,



time in the future





Applicability:


� log commands,



time in the future





Applicability:


� log commands,

� realise undo functionality or


time in the future





Applicability:


� log commands,

� realise undo functionality or

� cache commands in a queue in order to execute them at a cer tain

time in the future


The class structure of the command pattern

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Call of on invokes the receiver’ s operation.

Not necessar y each subc lass invokes same operation.

How could the pattern be altered to avoid encoding the receiver associ-

ation as attrib ute?



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operation.

Call of on invokes the receiver’ s operation.






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subc lasses may invokes diff erent TK M W Q U NO

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Execution of a command

General

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KeY-Praktikum WS03/04 � �� «

Example: Timer trig gered actions

One feature of the organiz er in the former examples is to execute

actions, when a cer tain date (time) has been reached.

For example:

¬ beep 5 min utes before a meeting

¬ send to all par ticipants of a meeting an invitation e-mail a week

before it is scheduled

¬ recor d a TV film

KeY-Praktikum WS03/04 ®�¯ °±

Example (cont’ d): Timer trig gered actions

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KeY-Praktikum WS03/04 ®�¯ °Ô

Literature

Õ »Design Patterns: Elements of Reusab le Object-Oriented Software«,by E. Gamma, R. Helm, R. Johnson and J. Vlissides,Addison-W esley (1995)

Õ Lecture Notes »Softwaretec hnik«,W.F. Tichy and G. Goos (WS 1998/99)

Õ Lecture Notes »Design Patterns Overview«,Rob Kremer (2002),Ö× × ØÙ Ú ÚÜÛ ÝÞ ßáà â ãä åçæ ä Þ è à ãä Ú ãé âÞ Û ÝÛ Úê ëì í Úî î ï Úð ñò Ú Øä × × Ý Þ ßÛ Úó ß ô Ýõ à Ö× ö å

KeY-Praktikum WS03/04 ®�¯ °÷

http://sern.ucalgary.ca/courses/SENG/443/W02/patterns/index.html

Design Patterns ...

Libraries For Designing System

document design solutions on an abstract level

prevent reinvention of the wheel

suppor t comm unication between developer s

make exper t kno wledg e usab le for beginner s

but:

They don’t make your design decisions.

KeY-Praktikum WS03/04 ®�¯ °ø

Design Patterns ...


ù document design solutions on an abstract level




but:



Design Patterns ...



ù




but:



Design Patterns ...



ù


ù suppor t comm unication between developer s


but:



Design Patterns ...



ù


ù suppor t comm unication between developer s

ù make exper t kno wledg e usab le for beginner s

but:



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