module AbstractController::Callbacks::ClassMethods

def _insert_callbacks(callbacks, block = nil)

* options - A hash of options to be used when adding the callback.
* name - The callback to be added.
==== Block Parameters

* block - A proc that should be added to the callbacks.
options hash as the last parameter.
* callbacks - An array of callbacks, with an optional
==== Parameters

the normalization across several methods that use it.
then call the block with each callback. This allows us to abstract
Take callback names and an optional callback proc, normalize them, 
def _insert_callbacks(callbacks, block = nil)
  options = callbacks.extract_options!
  callbacks.push(block) if block
  options[:filters] = callbacks
  _normalize_callback_options(options)
  options.delete(:filters)
  callbacks.each do |callback|
    yield callback, options
  end
end

def _normalize_callback_option(options, from, to) # :nodoc:

:nodoc: 
def _normalize_callback_option(options, from, to) # :nodoc:
  if from_value = options.delete(from)
    filters = options[:filters]
    from_value = ActionFilter.new(filters, from, from_value)
    options[to] = Array(options[to]).unshift(from_value)
  end
end

def _normalize_callback_options(options)

* except - The callback should be run for all actions except this action.
* only - The callback should be run only for this action.
==== Options

except: :index, if: -> { true } # the :except option will be ignored.

are used together.
Note that :if has priority over :except in case they

only: :index, if: -> { true } # the :if option will be ignored.

are used together.
Note that :only has priority over :if in case they

:if => proc {|c| c.action_name == "index" }.
The basic idea is that :only => :index gets converted to

+:if+ and +:unless+ options of ActiveSupport::Callbacks.
If +:only+ or +:except+ are used, convert the options into the 
def _normalize_callback_options(options)
  _normalize_callback_option(options, :only, :if)
  _normalize_callback_option(options, :except, :unless)
end