1. DCI Paradigm
ANUG, 24. november 2010

2. Agenda
• Intro to the DCI paradigm
• Real OO
• Real separation of concerns
• Readable business logic
• Testable
• Break
• Demo

3. Who Am I?
• Comp. Sci. master from University of Aarhus 2001
• Developer and Architect at Mjølner Informatics
• Blog: horsdal.blogspot.com

4. 4
Domain models at a glance
Wikipedia:
a conceptual model of a
system which describes the
various entities involved in
that system and their
relationships
“Basic domain concepts change
slowly; it’s the features and
technology that change
rapidly”
-James O. Coplien
“a conceptual model of a system
which describes the various
entities involved in that system
and their relationships”
-Wikipedia

5. Domain Model at a Glance
• About vocabulary
• UL
• About objects first
• Classes a distant second
• The very core of OO architectures
• Captures the essence
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6. 6
MVC at a glance
Problem
Develop UI that:
• Supports change w/o affecting core
functionality
• Displays current state and responds to
state changes immediately
Solution
Divide into three loosely coupled parts:
• Model objects encapsulate a core entities
• Views present data and information to the
user.
• Controllers are associated with views and
allow manipulation of the presented data
and information.
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Direct manipulation metaphor

8. MVC at a glance
….again
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Originally about the end user

9. MVC at a glance
….again
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Great for simple apps

10. But where is the behavior?
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11. Moving Into Use Case Territory
• Varied behavior
• Complex behavior
• Domain knowledge mandated
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Moving Into Use Case Territory

13. Where are the Objects Here?
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Which accounts?
Classes?
Roles!
Objects

14. Where is the use case implemented?
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15. Where is the use case implemented?
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The conflict
“Basic domain concepts change
slowly; it’s the features and
technology that change
rapidly”
-James O. Coplien
“[…] algorithms are, or should be,
first class citizens in most
programs”
-James O. Coplien
Traditional OO spreads
algorithms across objects
Features are often in service
objects…these tend to break the
connection between domain
model and UI

17. “Is” vs “Does”
17
Domain model
Database schema
Architecture
Stable
End user interaction
User experiences
Design
Ever changing

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DCI: Roles
• In the implementation of use cases
• Objects are manipulated
• Objects play roles
• Objects can take on several roles during runtime
• Roles can be played by several types of objects
• Actions are associated with roles
• Where are actions associated with roles implemented?

19. 19
DCI: Interaction
• Involves several domain objects selected dynamically
• …based on context
• Implemented in the roles
SavingAccount as MoneySource CheckinngAccount as MoneySink TransactionManager

20. 20
DCI: Context
• Encapsulates runtime object graph
• Assign roles to objects
• not to classes
MoneyTransferContex
MoneySource MoneySink TransactionManager

21. 21
DCI: Data
• Concrete domain objects
• Dumb or barely smart
SavingAccount
Balance
Account number
Owner
CheckingAccount
Balance
Account number
Owner

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DCI and MVC in Concert

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The conflict - Solved
“Basic domain concepts change
slowly; it’s the features and
technology that change
rapidly”
-James O. Coplien
“[…] algorithms are, or should be,
first class citizens in most
programs”
-James O. Coplien
Roles separate features from
core domain concepts
Context or roles contain the
algorithms directly

24. Where are actions associated with roles implemented?
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Traits at a glance
Traits are a simple composition mechanism for structuring
object-oriented programs. A Trait is essentially a
parameterized set of methods; it serves as a behavioral
building block for classes and is the primitive unit of code
reuse. With Traits, classes are still organized in a single
inheritance hierarchy, but they can make use of Traits to
specify the incremental difference in behavior with respect
to their superclasses.
-http://scg.unibe.ch/research/traits

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SavingsAccount CheckingAccount
<<Abstract>>
Account
<<interface>>
MoneySource
<<interface>>
MoneySink
<<trait>>
TransferMoneySource
TransferMoneyContext
Methodless roles
Model
Methodfull roles
Context

27. Where are actions associated with roles implemented?
• In traits!
• Composition of domain objects and role traits
• Hard in many languages
• But geeky tricks follow
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28. Background - DCI
• Invented by Trygve Reenskaug
• Who also invented MVC
• James O. Coplien recently published a book about DCI, agile
and lean
• Nice implementation in Scala, Ruby and Smalltalk
• Acceptable implementations in Python, C++, C# and others

29. Acknowlegdements
• Direct manipulation metaphor and DCI overview drawings:
Trygve Reenskaug in the common sense article
• Is vs. Does, MVC+DCI figure and quotes: James O. Copliens
book

30. References
• The book:
http://www.amazon.co.uk/Lean-Architecture-Agile-Software-
Development/dp/0470684208/ref=sr_1_1?ie=UTF8&s=books&qid=128
2367651&sr=8-1
• Introductory article: http://www.artima.com/articles/dci_vision.html
• Data, Context, and Interaction, an architectural pattern in computer
software development http://en.wikipedia.org/wiki/DCI
• DCI home:
http://heim.ifi.uio.no/~trygver/themes/babyide/babyide-index.html
• My C# DCI sample: http://horsdal.blogspot.com/2009/05/dci-in-c.html
• Google group:
http://groups.google.com/group/object-composition?hl=en