A brief introduction to DCI

1. STOP! Use case time!
Or, a brief introduction to DCI.
Wednesday, January 9, 13

2. Who am I?
André Medeiros
Rubyist for 7+ years
@superdealloc
github.com/andremedeiros
quintel.com
Wednesday, January 9, 13

3. You’ve probably heard of it
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4. You’ve probably heard of it
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5. You’ve probably heard of it
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6. You’ve probably heard of it
“DCI in Ruby is completely broken”
- Tony Arcieri (@bascule)
“Rails Developers Should Take DCI Seriously”
- Giles Bowkett (@gilesgoatboy)
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7. Wednesday, January 9, 13

8. How I develop apps
• UI first approach
• UI directly maps to operations within the code
• MVC doesn’t always conform to this standard of
development
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9. DCI is great for this approach
Invented by Trygve Reenskaug
Comes in where MVC fails: capturing behavior
Places interaction in obvious places
Models represent entities, not behavior
Splits what an object is from what it does
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10. D is for Data
What the object is
Persistence
Structure
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11. C is for Cookie Context
Class which enacts one or more use cases
Instantiated by a user action
Mixes in participating objects with Roles
Responsible for acting out the use case
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12. I is for Interaction
What the system does
Logic is contained in Role modules
Implemented by mixing in objects with Roles in a given
Context or use case
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13. How it works
Controller starts use case Context
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14. How it works
Controller starts use case Context
finds or creates the
Context participating objects Object(s)
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15. How it works
Controller starts use case Context
Context Object(s)
mixes in all the roles needed
Role
Role
Role
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16. How it works
Controller starts use case Context
Context Object(s)
invokes the role methods
Role
Role
Role
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17. Code Samples
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18. class TransfersController
def create
@source = Account.find(params[:source_id])
@destination = Account.find(params[:dest_id])
@source.balance -= amount
@destination.balance += amount
end
end
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19. class MoneyTransfer
def initialize(source, destination)
@source = source
@destination = destination
assign_transferrer(@source)
end
def execute(amount)
@source.transfer_to(@destination, amount)
end
private
def assign_transferrer(account)
account.extend(Transferrer)
end
module Transferrer
def transfer_to(destination, amount)
self.balance -= amount
destination.balance += amount
end
end
end
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20. Advantages
Keeps the model slim (fat != obese)
Business logic is easy to find and track down
Facilitates reusability
Highly testable
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21. FUD
Object#extend kills kittens
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22. FUD
Concerns do the exact same thing
It’s not real OO
Makes you write more code
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23. Questions?
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