Telling not-asking
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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.
![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](https://image.slidesharecdn.com/telling-not-asking-130514023713-phpapp02/85/Telling-not-asking-8-320.jpg)
![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](https://image.slidesharecdn.com/telling-not-asking-130514023713-phpapp02/85/Telling-not-asking-9-320.jpg)
![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](https://image.slidesharecdn.com/telling-not-asking-130514023713-phpapp02/85/Telling-not-asking-10-320.jpg)
![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](https://image.slidesharecdn.com/telling-not-asking-130514023713-phpapp02/85/Telling-not-asking-11-320.jpg)
![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](https://image.slidesharecdn.com/telling-not-asking-130514023713-phpapp02/85/Telling-not-asking-12-320.jpg)
![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](https://image.slidesharecdn.com/telling-not-asking-130514023713-phpapp02/85/Telling-not-asking-17-320.jpg)
![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](https://image.slidesharecdn.com/telling-not-asking-130514023713-phpapp02/85/Telling-not-asking-18-320.jpg)
![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](https://image.slidesharecdn.com/telling-not-asking-130514023713-phpapp02/85/Telling-not-asking-19-320.jpg)
![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](https://image.slidesharecdn.com/telling-not-asking-130514023713-phpapp02/85/Telling-not-asking-20-320.jpg)
Telling not-asking
- 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