Fixing sprockets 3 deprecation in a gem (method register_engine is deprecated)

[Schwad]

Versions

• Ruby 2.4.3
• Rails 5.1.3
• sprockets 3.7.3
• jquery-tmpl-rails 1.1.0

Issue:

The latest version of jquery-tmpl-rails triggers a sprockets deprecation warning of:

    DEPRECATION WARNING: Sprockets method `register_engine` is deprecated.
    Please register a mime type using `register_mime_type` then
    use `register_compressor` or `register_transformer`.

    https://github.com/rails/sprockets/blob/master/guides/extending_sprockets.md#supporting-all-versions-of-sprockets-in-processors 
    (called from block (2 levels) in <class:Railtie> at /my/path/to/gems/jquery-tmpl-rails-ef6f58001d41/lib/jquery-tmpl-rails/engine.rb:10)

Attempts at resolution:

Following the deprecation warning, the problem within the gem simply resides here: https://github.com/jimmycuadra/jquery-tmpl-rails/blob/master/lib/jquery-tmpl-rails/engine.rb#L10

Which is currently:

app.assets.register_engine(".tmpl", JqueryTemplate)

I then visited the sprockets guide for handling deprecations, extensibility and compliance with sprockets 2,3,4. https://github.com/rails/sprockets/blob/master/guides/extending_sprockets.md#supporting-all-versions-of-sprockets-in-processors

I made my best attempt at rectifying the problem with a commit on this fork here: oceanshq/jquery-tmpl-rails@de7a598

Which was:

    asset.register_mime_type 'application/javascript', extensions: ['.tmpl'], charset: :unicode
    asset.register_preprocessor 'application/javascript', JqueryTemplate

However, this attempt simply deals out breakages when trying to run the web application at all or test it on the CI. So obviously I've missed something. I am not the gem maintainer but am trying to put this PR together myself as we rely on this gem.

Questions

• Is there a simple syntactical approach I am missing here?
• It was unclear to me in the docs, although register_engine is deprecated in Sprockets 3, is it actually possible to migrate from register_engine while staying in Sprockets 3?
• If we are remaining on Sprockets 3 for the foreseeable future, is it kosher to silence this? Generally I work viewing deprecations as 'bugs that need fixing' and would rather not.

[Schwad]

I am trying to see how it has been done elsewhere and I think I may have found a commit for js-routes that implemented these changes along with support for 2 + 4.... railsware/js-routes@2c6c227#diff-1eb2fc1e42a530917862e373dd71049f