Add Url helpers to previews

[reeganviljoen]

closes #1764

What are you trying to accomplish?

Add Rails Url helpers to previews, for instance, if resources :items is defined in routes.rb, I may want to use items_path in the component preview method

What approach did you choose and why?

I added include Rails.application.routes.url_helpers to preview.rb to add the helpers to the previews class

A similar way to achieve this is by using the preview template which will have the URL helpers loaded but that seems like overkill just to get access to a helper

Anything you want to highlight for special attention from reviewers?

[joelhawksley]

👏🏻

[joelhawksley]

Suggested change

	* Add URL helpers to previews
	* Add support for URL helpers to previews.

[joelhawksley]

Brilliant! Thanks for the clean PR ❤️

[peteryates]

This change is causing me a bit of a headache when trying to use ViewComponent outside of Rails, specifically for building the guide for GOV.UK Components.

See this PR.

[Spone]

I think we can rework this to only include the helpers if Rails is defined. Can you have a look @reeganviljoen?

[peteryates]

Actually the fix isn't too bad - less involved than I thought half an hour ago! I totally realise we're using ViewComponent in a really unintended way to build the our guide so am happy to work around this on our end.

Simply initialising a fake Rails app before kicking off the Nanoc build works for us.

Consider this not a problem, thank you @Spone and @reeganviljoen :)

[rmacklin]

This seems to have broken the docs:build rake task:

$ bundle exec rake docs:build
Building YARD documentation.
[warn]: in YARD::Handlers::Ruby::MixinHandler: Undocumentable mixin: YARD::Parser::UndocumentableError for class ViewComponent::Preview
	in file 'lib/view_component/preview.rb':7:

	7: include Rails.application.routes.url_helpers

Files:          45
Modules:        42 (   33 undocumented)
Classes:        55 (   51 undocumented)
Constants:      39 (   38 undocumented)
Attributes:     31 (    2 undocumented)
Methods:       171 (   81 undocumented)
 39.35% documented
Converting YARD documentation to Markdown files.
rake aborted!
NoMethodError: undefined method `routes' for nil:NilClass
/Users/rmacklin/src/github.com/ViewComponent/view_component/lib/view_component/preview.rb:7:in `<class:Preview>'
/Users/rmacklin/src/github.com/ViewComponent/view_component/lib/view_component/preview.rb:6:in `<module:ViewComponent>'
/Users/rmacklin/src/github.com/ViewComponent/view_component/lib/view_component/preview.rb:5:in `<top (required)>'
/Users/rmacklin/src/github.com/ViewComponent/view_component/lib/view_component/base.rb:11:in `require'
/Users/rmacklin/src/github.com/ViewComponent/view_component/lib/view_component/base.rb:11:in `<top (required)>'
/Users/rmacklin/src/github.com/ViewComponent/view_component/Rakefile:81:in `require'
/Users/rmacklin/src/github.com/ViewComponent/view_component/Rakefile:81:in `block (2 levels) in <top (required)>'
/Users/rmacklin/.asdf/installs/ruby/2.7.5/bin/bundle:23:in `load'
/Users/rmacklin/.asdf/installs/ruby/2.7.5/bin/bundle:23:in `<main>'
Tasks: TOP => docs:build
(See full trace by running task with --trace)

I had to comment out the include Rails.application.routes.url_helpers line from preview.rb in order to successfully run the rake task.

We could change the task to work around this, but I think @Spone's suggested approach of only including the helpers when inside a Rails application makes sense.

[rmacklin]

I've opened a PR to do so here: #1795