And now, a friendly message from your local Tell, don’t ask Department.
Example 1
Not so good:
<% if current_user.admin? %>
<%= current_user.admin_welcome_message %>
<% else %>
<%= current_user.user_welcome_message %>
<% end %>
Better:
<%= current_user.welcome_message %>
Example 2
Not so good:
def check_for_overheating(system_monitor)
if system_monitor.temperature > 100
system_monitor.sound_alarms
end
end
Better:
system_monitor.check_for_overheating
class SystemMonitor
def check_for_overheating
if temperature > 100
sound_alarms
end
end
end
Example 3
Not so good:
class Post
def send_to_feed
if user.is_a?(TwitterUser)
user.send_to_feed(contents)
end
end
end
Better:
class Post
def send_to_feed
user.send_to_feed(contents)
end
end
class TwitterUser
def send_to_feed(contents)
twitter_client.post_to_feed(contents)
end
end
class EmailUser
def send_to_feed(contents)
# no-op.
end
end
Example 4
Not so good:
def street_name(user)
if user.address
user.address.street_name
else
'No street name on file'
end
end
Better:
def street_name(user)
user.address.street_name
end
class User
def address
@address || NullAddress.new
end
end
class NullAddress
def street_name
'No street name on file'
end
end
Good OOP is about telling objects what you want done, not querying an object and acting on its behalf. Data and operations that depend on that data belong in the same object.
Tell, don’t ask!
Next Steps & Related Reading
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