How to get list of variables referenced in a liquid template

Question

I'm working on a project which uses the Liquid template language. I have a page with iframes showing some rendered liquid templates and an editor section. In the editor section the user can change the values of some of the variables referred to in the liquid templates. At this point some of the templates need to be re-rendered but I only want to re-render those templates which refer to the variable which has been changed.

The point I'm stuck on is how best to get a list of the variables referred to in a liquid template.

So if I do this:

templates=[]
templates[0] = Liquid::Template.parse("hi {{name}}") 
templates[1] = Liquid::Template.parse("my cat is called {{cat_name}}") 

I want to know that 'name' is referenced but that (e.g.) 'cat' is not in template 0. Then when name is changed I can re-render template[0] and when cat is changed I can re-render template[1].

I've found that I can get a list of nodes and check each one to see if it is a variable, e.g. templates[0].root.nodelist[1] has a type of 'Liquid::Variable' but the 'name' method of the variable returns an object of type 'Liquid::VariableLookup' which in turn doesn't seem to have a method for getting a name out of it and this feels like I might be going about it the wrong way.

Answer 1

Following https://github.com/Shopify/liquid/issues/685#issuecomment-471499796 you can use FileTreeVisitor class for that. Namely:

template = Liquid::Template.parse("hi {{name}}")

Liquid::ParseTreeVisitor.for(template.root)
  .add_callback_for(Liquid::VariableLookup) do |node|
  [node.name, *node.lookups].join('.')
end.visit.flatten.compact # => ["name"]