This document discusses finding unused code in a Rails application. It provides two approaches: using Active Support Instrumentation to track which view templates and controller actions are rendered, and Coverband, a Rack middleware that measures production code usage. The Active Support approach subscribes to render events to record used templates and controller methods, while Coverband can directly measure unused lines of code in production.

1. Finding unused code
in your Rails app

2. The problem

3. Views
find app/views -type f | wc -l
1060!!

4. Controller methods
ag 'def ' app/controllers | wc -l
1231!!

5. How?

7. Active Support Instrumentation
Finding views
render_template.action_view
render_partial.action_view
=>
{
identifier: "/Users/adam/projects/notifications/app/views/posts/index.html.erb",
layout: "layouts/application"
}

8. Collecting information
config/initializers/trace_views.rb
def metric_name(path)
path.sub(/A#{Rails.root}//, '')
end
%w(render_template.action_view render_partial.action_view).map do |event_name|
ActiveSupport::Notifications.subscribe(event_name) do |*data|
event = ActiveSupport::Notifications::Event.new(*data)
template_name = metric_name(event.payload[:identifier])
UsedView.find_or_create_by_template_name(template_name)
end
end

9. Processing information
Dir.glob("app/views/**/*.haml").each do |view|
puts view unless UsedView.find_by_template_name(view)
end

10. Another approach...

11. Coverband
Rack middleware to help measure production code line
of code usage
https://github.com/danmayer/coverband