Autoreload gems in development

development.rb

# Inside config/environments/development.rb

# Do the following after every request to the server in development mode
ActionDispatch::Callbacks.to_cleanup do

  # If the gem's top level module is currently loaded, unload it
  if Object.const_defined?(:MyCoolGem)
    Object.send(:remove_const, :MyCoolGem)
  end

  # Instruct ruby to "unrequire" all of the gems files.
  # CAREFUL: make sure this only matches your gems files.
  $".delete_if {|s| s.include? "my_cool_gem"}

  # Re-require your gem 
  # Note: because we removed all files previously required they will be reloaded
  # even if you didn't use load/autoload in your gem.
  require "my_cool_gem"

end

it does not work for me. I am sure I put the code in the development.rb, and I puts "something" inside this part of code, I found all of these code does not execute.

It does not work for me neither. Same as mark-secondbureau

try to use ActionDispatch::Callbacks.before instead of ActionDispatch::Callbacks.to_cleanup

ActionDispatch::Callbacks.to_cleanup is delegated to ActionDispatch::Reloader and is run after each request.

module ActionDispatch
  class Reloader

    include ActiveSupport::Callbacks
    ...
    # Add a cleanup callback. Cleanup callbacks are run after each request is
    # complete (after #close is called on the response body).
    def self.to_cleanup(*args, &block)
      set_callback(:cleanup, *args, &block)
    end
    ...

module ActionDispatch
  # Provide callbacks to be executed before and after the request dispatch.
  class Callbacks

    include ActiveSupport::Callbacks

    define_callbacks :call

    class << self
      delegate :to_prepare, :to_cleanup, :to => "ActionDispatch::Reloader"
    end
    ...