DCI
Data Context Interaction
About me:
Dawid Sklodowski
Works at @HouseTrip (we are hiring)

DCI: Why?
Separating behaviour from data model
Making behaviour first-class citizen
Supporting style of thinking closer to
people’s mental model

DCI: Who?
Invented by Trygve Reenskaug
Current formulation mostly by Trygve
Reenskaug and Jim Coplien
In 2009

Example
Ninja Elf kills Pirate Dwarf

Elf object would...
Reduce victim’s health points to 0.
Add some experience to itself.
Log frag on leader board.
Actually it should check who gets killed.
Or... maybe we should have fight before
deciding who dies.

Lets leave the elf
object alone
He doesn’t need to know how killing is
implemented in system.
Lets implement activities (behaviours) for our
characters.

Data

Data
Data is “what system is”.
Objects which contain methods to change
their state.
No interaction with other objects

Context
Entry point
Injects roles
Knows who does what

Interaction
Interaction is “what system does”.
Implemented as roles which are played by
objects in run-time

MVC and DCI
(how can it play with Rails)
DCI can be complementary to MVC
Controller loads adequate objects and
initiates context with them
Context injects roles to objects and let them
to play

MVC and DCI
ControllerUser
Context
Model
View
Rolesdata object

Use Case: Context

Sample Role

How to implement?
with Ruby
Problem: Role Injection

How to implement
1. Decoration: #extend
Cons:
There is no
#unextend method
Tends to be slow
on some Rubies

How to implement
2. Presenter
Pros:
Object encapsulation
Cons:
Object encapsulation
(however object still
accessible via
#object method)

How to implement
3. Delegation
Cons:
Uses
#method_missing to
implement delegation

How to implement
4. Delegate Class
It creates delegation methods for instance methods of delegated
class.
Cons:
Static delegation
relationship
Role is a class
instead of module

How to implement
Dynamic DelegateClass
Dynamic delegation relationship, but isn’t that slow?

How to implement
Any pattern can be implemented in dynamic way

Performance comparison
Implementation

Performance
comparison
Results

How to implement
DelegateClass: Performance Wise
Caching anonymous role classes

Other pros
We can chain roles on object
We don’t pollute original object with role
leftovers (unlike #extend)

Summary
Separates behaviour from data model
Can play nicely with Rails and MVC
No straightforward way to implement with
Ruby
But it is doable

Thank You
github.com/dawid-sklodowski
onceuponaline.info
@DawidSklodowski