STOP! Use case time!Or, a brief introduction to DCI.

Wednesday, January 9, 13

Who am I?André Medeiros

Rubyist for 7+ years

@superdealloc

github.com/andremedeiros

quintel.com

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You’ve probably heard of it

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You’ve probably heard of it

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You’ve probably heard of it

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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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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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DCI is great for this approachInvented 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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D is for DataWhat the object is

Persistence

Structure

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C is for Cookie ContextClass 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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I is for InteractionWhat 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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How it worksController Contextstarts use case

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How it worksController Contextstarts use case

Context Object(s)finds or creates theparticipating objects

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How it worksController Contextstarts use case

Context Object(s)

mixes in all the roles neededRole

RoleRole

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How it worksController Contextstarts use case

Context Object(s)

invokes the role methodsRole

RoleRole

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Code Samples

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class TransfersController def create @source = Account.find(params[:source_id]) @destination = Account.find(params[:dest_id])

@source.balance -= amount @destination.balance += amount endend

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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 endend

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AdvantagesKeeps the model slim (fat != obese)

Business logic is easy to find and track down

Facilitates reusability

Highly testable

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FUDObject#extend kills kittens

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FUDConcerns do the exact same thing

It’s not real OO

Makes you write more code

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Questions?

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