The document discusses an object-oriented approach called "tell, don't ask" where objects publish events to notify other objects of state changes instead of directly querying objects for information. It provides an example of a CreateAction use case object that tells an EventBus of action creations instead of coordinating notifications itself. Various listener objects then receive notifications to perform tasks like speaking on Campfire or sending emails. This allows separating notification logic from the use cases.

1. Telling, not asking
@kevinrutherford

2. “Tell, don't ask”
… means WHAT, exactly … ?

3. “Tell, don't ask”
"Procedural code gets information
then makes decisions. Object-oriented
code tells objects to do things."
— Alec Sharp, Smalltalk by Example

4. How can I do that?

5. How can I do that?
… without violating the SRP?

6. How can I do that?
… without violating the SRP?
… without having DoubleDispatch everywhere?

7. controllers,
views
A simple to-do list app
rake tasks,
test scripts
etc
use case
objects

8. A use case object
class CreateAction
def initialize(user)
@user = user
end
def execute(options)
project = @user.open_project(options[:project_id])
action = project.create_action(options[:attrs])
unless action.errors.any?
event = Event.create!(summary: "##{action.id} created", project_id: project.id)
msg = "#{@user.name} created #{action.title}"
project.campfire_room.speak(msg)
PushMessage.instance.publish("action-do.#{action.project.id}.action_item",
{:action_item_id => action})
action.project.users.reject {|u| u == @user}.each do |u|
ActionItemMailer.action_item_created(action, u).deliver
end
end
action
end
end

9. A use case object
class CreateAction
def initialize(user)
@user = user
end
def execute(options)
project = @user.open_project(options[:project_id])
action = project.create_action(options[:attrs])
unless action.errors.any?
event = Event.create!(summary: "##{action.id} created", project_id: project.id)
msg = "#{@user.name} created #{action.title}"
project.campfire_room.speak(msg)
PushMessage.instance.publish("action-do.#{action.project.id}.action_item",
{:action_item_id => action})
action.project.users.reject {|u| u == @user}.each do |u|
ActionItemMailer.action_item_created(action, u).deliver
end
end
action
end
end

10. A use case object
class CreateAction
def initialize(user)
@user = user
end
def execute(options)
project = @user.open_project(options[:project_id])
action = project.create_action(options[:attrs])
unless action.errors.any?
event = Event.create!(summary: "##{action.id} created", project_id: project.id)
msg = "#{@user.name} created #{action.title}"
project.campfire_room.speak(msg)
PushMessage.instance.publish("action-do.#{action.project.id}.action_item",
{:action_item_id => action})
action.project.users.reject {|u| u == @user}.each do |u|
ActionItemMailer.action_item_created(action, u).deliver
end
end
action
end
end

11. A use case object
class CreateAction
def initialize(user)
@user = user
end
def execute(options)
project = @user.open_project(options[:project_id])
action = project.create_action(options[:attrs])
unless action.errors.any?
event = Event.create!(summary: "##{action.id} created", project_id: project.id)
msg = "#{@user.name} created #{action.title}"
project.campfire_room.speak(msg)
PushMessage.instance.publish("action-do.#{action.project.id}.action_item",
{:action_item_id => action})
action.project.users.reject {|u| u == @user}.each do |u|
ActionItemMailer.action_item_created(action, u).deliver
end
end
action
end
end

12. A use case object
class CreateAction
def initialize(user)
@user = user
end
def execute(options)
project = @user.open_project(options[:project_id])
action = project.create_action(options[:attrs])
unless action.errors.any?
event = Event.create!(summary: "##{action.id} created", project_id: project.id)
msg = "#{@user.name} created #{action.title}"
project.campfire_room.speak(msg)
PushMessage.instance.publish("action-do.#{action.project.id}.action_item",
{:action_item_id => action})
action.project.users.reject {|u| u == @user}.each do |u|
ActionItemMailer.action_item_created(action, u).deliver
end
end
action
end
end

13. A simple to-do list app

14. “Tell, don't ask”
"Procedural code gets information
then makes decisions. Object-oriented
code tells objects to do things."
— Alec Sharp, Smalltalk by Example

15. “Tell, don't ask”
"Procedural code gets information
then makes decisions. Object-oriented
code tells objects to do things."
— Alec Sharp, Smalltalk by Example

16. “Tell, don't ask”
"Sometimes an object will change
its state, and on those occasions
it might be expected to report
that change to others."
— David West, Object Thinking

17. A use case object
class CreateAction
def initialize(user)
@user = user
end
def execute(options)
project = @user.open_project(options[:project_id])
action = project.create_action(options[:attrs])
unless action.errors.any?
event = Event.create!(summary: "##{action.id} created", project_id: project.id)
msg = "#{@user.name} created #{action.title}"
project.campfire_room.speak(msg)
PushMessage.instance.publish("action-do.#{action.project.id}.action_item",
{:action_item_id => action})
action.project.users.reject {|u| u == @user}.each do |u|
ActionItemMailer.action_item_created(action, u).deliver
end
end
action
end
end

18. A use case object
class CreateAction
def initialize(user)
@user = user
end
def execute(options)
project = @user.open_project(options[:project_id])
action = project.create_action(options[:attrs])
unless action.errors.any?
event = Event.create!(summary: "##{action.id} created", project_id: project.id)
msg = "#{@user.name} created #{action.title}"
project.campfire_room.speak(msg)
PushMessage.instance.publish("action-do.#{action.project.id}.action_item",
{:action_item_id => action})
action.project.users.reject {|u| u == @user}.each do |u|
ActionItemMailer.action_item_created(action, u).deliver
end
end
action
end
end

19. A use case object : after
class CreateAction
def initialize(user)
@user = user
end
def execute(options)
project = @user.open_project(options[:project_id])
action = project.create_action(options[:attrs])
unless action.errors.any?
EventBus.announce('action-created', action: action, actor: @user, publisher: self)
end
action
end
end
https://rubygems.org/gems/event_bus

20. The Campfire listener
class CampfireAnnouncer
def action_created(payload)
actor = payload[:actor]
action = payload[:action]
project = action.project
msg = "#{actor.name} created #{action.title}"
project.campfire_room.speak(msg)
end
def action_deleted(payload)
#...
end
#...
end

21. Listener registration
class ApplicationController < ActionController::Base
before_filter do
EventBus.clear
EventBus.register(CampfireAnnouncer.new)
EventBus.register(EventLogger.new)
EventBus.register(EmailNotifier.new)
EventBus.register(PushNotifier.new)
end
#...
end

22. A simple to-do list app

23. A simple to-do list app
listeners
event bus

24. “Tell, don't ask”
Tell, don't ...

25. “Tell, don't ask”
Tell, don't wait to be asked

26. “Tell, don't ask”
Tell, don't wait to be asked
@kevinrutherford
http://silkandspinach.net
http://refactoringinruby.info
https://rubygems.org/gems/event_bus