How do I generate a sprited image file with Compass? - ruby

Question

Background
I have a directory of images, app/assets/images/sprites/flags/*.png that will be sprited. Compass, Sass, and Sprockets work together such that I can put
#import "sprites/flags/*.png";
...
.flag.en {
#include flags-sprite(en)
}
in my .scss file. By doing so, Compass generates the file app/assets/images/sprites/flags-abc123.png when I compile SCSS files.
Problem
I would like to precompile that flags-abc123.png file via a Rake task or shell command without compiling my .scss file. Is there a way to do so?
Rationale
Compiling all of the .scss files in the application is a lengthy part of the deploy process. We would like to speed up deploys by generating them on one machine and distributing them. Unfortunately, the other machines generate bad cache-busting URLs because they are missing the compiled sprite files. If we can precompile the sprite files, we can speed up deploys dramatically.
What I tried
I tried running bundle exec compass sprite app/assets/images/sprites/flags/*.png. That generates app/assets/stylesheets/_flags.scss, but doesn't generate app/assets/images/sprites/flags-abc123.png.

Answer 1

I ended up making a Rake task:
class SpriteTask
include Rake::DSL
attr_reader :file_task
def initialize(glob)
#glob = glob
build_compass_sprite_map
define_task
end
private
attr_reader :glob, :map
def build_compass_sprite_map
uri = Sass::Script::String.new(glob)
context = Object.new
kwargs = {}
kwargs.extend Compass::SassExtensions::Functions::Sprites::VariableReader
#map = Compass::SassExtensions::Sprites::SpriteMap.from_uri uri, context, kwargs
#map.options = {}
end
def define_task
#file_task = file(map.filename => map.image_filenames) do |task|
map.generate
end
end
end
and using it like so:
require 'sprite_task'
namespace :sprites do
task :generate do
SpriteTask.new('foo/*.png').file_task.invoke
SpriteTask.new('bar/*.png').file_task.invoke
end
end