Take into account when a sprite image changes and is recompiled

[agrobbin]

Before, the sprite wasn't correctly recompiled, but the CSS was updated, causing all kinds of hell.

[santry]

This is still mostly black magic to me, but it seems like reasonable black magic.

[agrobbin]

For posterity, a few things need to happen for sprockets to notice that the generated sprite image has changed in the middle of a compilation.

First is removing the asset from the environment's cache:

sprockets_environment.instance_variable_get(:@assets).delete_if { |_, asset| asset.pathname == sprite_map.filename }

This removes the BundledAsset object from the @assets Hash when its pathname is the same as the sprite_map.filename.

Next is tapping into the environment's trail object, which is a Hike::Index. This object is responsible for keeping a cache of what is currently on the filesystem, including stats. This is used to determine whether an asset is fresh?, basically if the cached mtime is older than the filesystem's mtime for a particular asset. We have to clear out 2 things from the trail:

sprockets_environment.send(:trail).tap do |trail|
  trail.instance_variable_get(:@entries).delete(Spritely.directory.to_s)
  trail.instance_variable_get(:@stats).delete(sprite_map.filename.to_s)
end

We remove the entire directory from the cached set of Dir.entries as well as the sprite file itself from the cached set of stat results.