How can I run custom code before any asset precompile in Rails?

Question

Something like (in an initializer):

Sprockets.before_precompile do
  # some custom stuff that preps or autogenerates some asset files
  # that should then be considered in the asset pipeline as if they were checked in
end

Specifically, I'd like to run a gulp task to bundle some Javascript with some special preprocessors, and I'd rather not rewrite my gulpfile to get the asset pipeline to handle everything... and I also want this to work on Heroku without needing a custom buildpack if at all possible. Any thoughts? Presumably Sprockets has these types of hooks.

Answer 1

As I can see from the source, Sprockets does not have such a hook, but you could use rake task hooks. For example, you would create a rake task that starts all the preprocessors, gulp, etc, so this task could be put before precompilation.

# lib/tasks/before_assets_precompile.rake

task :before_assets_precompile do
  # run a command which starts your packaging
  system('gulp production')
end

# every time you execute 'rake assets:precompile'
# run 'before_assets_precompile' first    
Rake::Task['assets:precompile'].enhance ['before_assets_precompile']

Then you just run rake assets:precompile, and as a result the task before_assets_precompile will be executed right before it.

Also make sure to use system instead of exec, because exec will exit the process on a stage of running this pre-task and will not run assets:precompile after itself as it was expected.

Sources:

1. Rake before task hook
2. http://www.dan-manges.com/blog/modifying-rake-tasks