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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

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DCI with Ruby and Rails