Template engine

Since Louis factored out all HTML from qanda and added a configuration file to the templates, it's a small task to add a template engine, replace the PHP views with e.g. Twig templates.

= Use case =


 * Template providers don't have to worry about core changes in the question view files
 * Template providers can apply any CSS framework specific HTML they want, like Foundation, Bootstrap etc.
 * User can show some question the way they want (with some html ability)
 * User can show all questions as a table

NB: A lot of usage for template editor is minor changes to the CSS. In that case, you do not want to be independent of core changes.

= Requirements =

Must have

 * Template files in template zip packages can override question view files Not possible anymore, see question templates.
 * A custom template should be possible to be completely independent of core, but not required to As above.
 * Template zip package can upload template files in template editor
 * Actual views used in public part must use the template engine

Should have

 * Possible to edit Twig templates in template editor
 * Use same tags as Backbone template engine (Twig can configure this)

Could have

 * Include another file : the actual view use different files (see Talk)
 * Add Twig filter to process EM
 * Idea :
 * User can only add some css/update template.css : use case : bigger button
 * User want only a simple HTML update : some real forum example : help above answers, show text at right of answers, multiple movenext/moveprev (in 3.0 : we can put any button/index we want : no id for this button : don't break HTML)
 * User choose another 'theme' for his template : use case : theming (lot of color, font-style ....)
 * in 3.0 replace boostrap.css but this create a new complete template (with views and other): a GUI and a theme system ? Need some pstpl too i think
 * pstpl can be part of theming too : using panel or well class at different position (wquestion-wrapper / form-wrapper etc ...)
 * User choose another framework : @todo : included view use 'framework class' in core
 * User want to completely use his own views

Will not have

 * No additional templates will be added to the template engine (like token form)
 * See discussion
 * example why must able https://www.limesurvey.org/forum/can-i-do-this-with-limesurvey/107945-modify-label#146221
 * layout must not be updated ?
 * See https://github.com/LimeSurvey/LimeSurvey/blob/develop/application/views/layouts/public.php

= Twig =

http://twig.sensiolabs.org/

= Security =

Longer article about template injection: http://blog.portswigger.net/2015/08/server-side-template-injection.html

"The lowest risk approach is to simply use a trivial template engine such as Mustache"

About CPU load: It wold be easy for a template designer to make a loop within a loop within a loop etc, to exhaust the CPU power of the server. This vulnerability is in all template engines where you can loop an array (Mustache, Twig). To protect against this, the server should have a max script runtime set.
 * BUT: Rendering PDF takes a long time, how to know the right limit?

= Test instructions =

How to write good bug reports and feature requests:

https://www.limesurvey.org/community-services/bug-tracker

https://www.limesurvey.org/community-services/feature-requests

= Manual construction =

Template_config.xml