Create a custom tag

Custom tags, internally called scomps, are module-defined tags, which are used when the logic is too complex to be executed in templates.

Custom tags add logic to templates or generate HTML/Javascript constructs that are too difficult for templates. A good example is the menu scomp which implements the menu, including sub menus -and highlighting of the current menu item.

Implementing a custom tag

A scomp module lives in a module under the scomps/ folder, and has the following naming convention: scomp_modulename_scompname.

So a fictitiuos scomp called likebutton in the mod_facebook module would defined in the Erlang module called scomp_facebook_likebutton.erl.

You can implement scomps by using the scomp behaviours. An example scomp that does absolutely nothing is implemented as follows:

-export([vary/2, render/3]).

vary(_Params, _Context) -> nocache.
render(Params, _Vars, Context) ->
    %% FIXME: render like button here
    {ok, Context}.

Scomp internals

During the evaluation of scomps we are able to:

  • wire actions
  • add scripts
  • add validators
  • add context variables (accessible only by other scomps)

After rendering a template it is inspected to find all scripts, actions and validators. They are placed as #context records in the resulting iolist, the #contexts should be replaced by the ‘render’ record-attribute. There will be a special ‘script’ atom to signify the place where all collected scripts will be inserted. When the script atom is not found, then the result of the template is a context, unless there are no collected scripts.

Process state diagram, how all processes work together to render a scomp:

Controller        Template                        Scomp server        Scomp       Page/Session/User
  |------------>  render(Template,ReqContext)
                      | ------------------------------- lookup missing var ---------->|
                      |<------------------------------------- value ------------------|
                      |------------------> render(Scomp,ReqContext)
                                                      |---------> render(ReqContext)
                                                      :               |
                                                  (if cached)         |
                                                      :               |
                      |<------------ Output ----------|
                  Filter scripts
  |<---- Output ------|
reply user agent

The scripts/actions/validators are similar to the ones defined with Nitrogen, though the record structure is redefined to accomodate easy construction by the template compiler.