What is this init.rb doing in the liquid project?

require 'liquid'
require 'extras/liquid_view'

if defined? ActionView::Template and ActionView::Template.respond_to? :register_template_handler
  ActionView::Template
else
  ActionView::Base
end.register_template_handler(:liquid, LiquidView)

What is this init.rb file run, is it a one-time thing or on application load?

Is it simply calling the regiser_template_handler method, on either the ActionView::Template or ActionView:Base class?

How 开发者_StackOverflow中文版does it work on either one?

What is this init.rb file run, is it a one-time thing or on application load?

It is run once, when the app loads and plugins/gems are loaded in.

Is it simply calling the regiser_template_handler method, on either the ActionView::Template or ActionView:Base class?

Yes. If ActionView::Template is defined and responds to register_template_handler it will call register_template_handler on it, otherwise it will call it on ActionView::Base.

How does it work on either one?

I'm not sure exactly what you mean by this? If you mean why is it like this, it's for backwards compatibility. ActionView::Template is relatively new in rails (introduced around v2.1, I think) so I guess it's so that it will gracefully fallback when used with older versions.