[Compiler] Rewrite of the watch command that adds the following improvements:

1. For robustness, recompile is now based on comparison of corresponding css/sass file timestamps.
2. If a sass file is removed, the corresponding css file is automatically deleted.
3. CSS files will be automatically recompiled if removed. It is no longer necessary to resave the sass file.
4. First time compile is not performed if not necessary.

lib/compass/actions.rb

@@ -73,6 +73,13 @@ module Compass
       end
     end
 
+    def remove(file_name)
+      if File.exists?(file_name)
+        File.unlink file_name
+        logger.record :remove, basename(file_name)
+      end
+    end
+
     def basename(file)
       relativize(file) {|f| File.basename(file)}
     end

lib/compass/commands/update_project.rb

@@ -21,12 +21,12 @@ module Compass
         end
       end
 
-      def new_compiler_instance(options = {})
+      def new_compiler_instance(additional_options = {})
         Compass::Compiler.new(working_path,
           projectize(Compass.configuration.sass_dir),
           projectize(Compass.configuration.css_dir),
           Compass.sass_engine_options.merge(:quiet => options[:quiet],
-                                            :force => options[:force]).merge(options))
+                                            :force => options[:force]).merge(additional_options))
       end
 
     end

lib/compass/commands/watch_project.rb

@@ -7,45 +7,44 @@ module Compass
   module Commands
     class WatchProject < UpdateProject
 
-      attr_accessor :last_update_time
+      attr_accessor :last_update_time, :last_sass_files
 
       def perform
-        puts ">>> Compiling all stylesheets."
-        super
-        self.last_update_time = most_recent_update_time
-        puts ">>> Compass is now watching for changes. Press Ctrl-C to Stop."
-        loop do
-          # TODO: Make this efficient by using filesystem monitoring.
-          begin
-            sleep 1
-          rescue Interrupt
+        Signal.trap("INT") do
           puts ""
           exit 0
         end
-          file, t = should_update?
-          if t
+        puts ">>> Compass is watching for changes. Press Ctrl-C to Stop."
+        loop do
+          # TODO: Make this efficient by using filesystem monitoring.
+          compiler = new_compiler_instance(:quiet => true)
+          remove_obsolete_css(compiler)
+          recompile(compiler)
+          sleep 1
+        end
+      end
+
+      def remove_obsolete_css(compiler)
+        sass_files = compiler.sass_files
+        deleted_sass_files = (last_sass_files || []) - sass_files
+        deleted_sass_files.each do |deleted_sass_file|
+          css_file = compiler.corresponding_css_file(deleted_sass_file)
+          remove(css_file) if File.exists?(css_file)
+        end
+        self.last_sass_files = sass_files
+      end
+
+      def recompile(compiler)
+        if file = compiler.out_of_date?
           begin
             puts ">>> Change detected to: #{file}"
-              super
+            compiler.run
           rescue StandardError => e
             ::Compass::Exec.report_error(e, options)
           end
-            self.last_update_time = t
-          end
         end
       end
 
-      def most_recent_update_time
-        Dir.glob(separate("#{projectize(Compass.configuration.sass_dir)}/**/*.sass")).map {|sass_file| File.stat(sass_file).mtime}.max
-      end
-
-      def should_update?
-        t = most_recent_update_time
-        if t > last_update_time
-          file = Dir.glob(separate("#{projectize(Compass.configuration.sass_dir)}/**/*.sass")).detect {|sass_file| File.stat(sass_file).mtime >= t}
-          [file, t]
-        end
-      end
     end
   end
 end

lib/compass/compiler.rb

@@ -22,13 +22,25 @@ module Compass
     end
 
     def css_files
-      @css_files ||= sass_files.map{|sass_file| "#{to}/#{stylesheet_name(sass_file)}.css"}
+      @css_files ||= sass_files.map{|sass_file| corresponding_css_file(sass_file)}
+    end
+
+    def corresponding_css_file(sass_file)
+      "#{to}/#{stylesheet_name(sass_file)}.css"
     end
 
     def target_directories
       css_files.map{|css_file| File.dirname(css_file)}.uniq.sort.sort_by{|d| d.length }
     end
 
+    def out_of_date?
+      sass_files.zip(css_files).each do |sass_filename, css_filename|
+        return sass_filename unless File.exists?(css_filename)
+        return sass_filename if File.stat(sass_filename).mtime > File.stat(css_filename).mtime
+      end
+      false
+    end
+
     def run
       Compass.configure_sass_plugin!
       target_directories.each do |dir|