Add initial FileWatcher implementation. The Backend is just an abstract implementation, which will be inherited by backends that do the heavy lifting.

activesupport/lib/active_support.rb

@@ -42,6 +42,7 @@ module ActiveSupport
 
   autoload :DescendantsTracker
   autoload :FileUpdateChecker
+  autoload :FileWatcher
   autoload :LogSubscriber
   autoload :Notifications
 

activesupport/lib/active_support/file_watcher.rb

@@ -0,0 +1,41 @@
+module ActiveSupport
+  class FileWatcher
+    class Backend
+      def initialize(path, watcher)
+        @watcher = watcher
+        @path    = path
+      end
+
+      def trigger(files)
+        @watcher.trigger(files)
+      end
+    end
+
+    def initialize
+      @regex_matchers = {}
+    end
+
+    def watch(path, &block)
+      return watch_regex(path, &block) if path.is_a?(Regexp)
+      raise "Paths must be regular expressions. #{path.inspect} is a #{path.class}"
+    end
+
+    def watch_regex(regex, &block)
+      @regex_matchers[regex] = block
+    end
+
+    def trigger(files)
+      trigger_files = Hash.new { |h,k| h[k] = Hash.new { |h2,k2| h2[k2] = [] } }
+
+      files.each do |file, state|
+        @regex_matchers.each do |regex, block|
+          trigger_files[block][state] << file if file =~ regex
+        end
+      end
+
+      trigger_files.each do |block, payload|
+        block.call payload
+      end
+    end
+  end
+end

activesupport/test/file_watcher_test.rb

@@ -0,0 +1,65 @@
+require 'abstract_unit'
+
+class FileWatcherTest < ActiveSupport::TestCase
+  class DumbBackend < ActiveSupport::FileWatcher::Backend
+  end
+
+  def setup
+    @watcher = ActiveSupport::FileWatcher.new
+
+    # In real life, the backend would take the path and use it to observe the file
+    # system. In our case, we will manually trigger the events for unit testing,
+    # so we can pass any path.
+    @backend = DumbBackend.new("RAILS_WOOT", @watcher)
+
+    @payload = []
+    @watcher.watch %r{^app/assets/.*\.scss$} do |pay|
+      pay.each do |status, files|
+        files.sort!
+      end
+      @payload << pay
+    end
+  end
+
+  def test_one_change
+    @backend.trigger("app/assets/main.scss" => :changed)
+    assert_equal({:changed => ["app/assets/main.scss"]}, @payload.first)
+  end
+
+  def test_multiple_changes
+    @backend.trigger("app/assets/main.scss" => :changed, "app/assets/javascripts/foo.coffee" => :changed)
+    assert_equal([{:changed => ["app/assets/main.scss"]}], @payload)
+  end
+
+  def test_multiple_changes_match
+    @backend.trigger("app/assets/main.scss" => :changed, "app/assets/print.scss" => :changed, "app/assets/javascripts/foo.coffee" => :changed)
+    assert_equal([{:changed => ["app/assets/main.scss", "app/assets/print.scss"]}], @payload)
+  end
+
+  def test_multiple_state_changes
+    @backend.trigger("app/assets/main.scss" => :created, "app/assets/print.scss" => :changed)
+    assert_equal([{:changed => ["app/assets/print.scss"], :created => ["app/assets/main.scss"]}], @payload)
+  end
+
+  def test_more_blocks
+    payload = []
+    @watcher.watch %r{^config/routes\.rb$} do |pay|
+      payload << pay
+    end
+
+    @backend.trigger "config/routes.rb" => :changed
+    assert_equal [:changed => ["config/routes.rb"]], payload
+    assert_equal [], @payload
+  end
+
+  def test_overlapping_watchers
+    payload = []
+    @watcher.watch %r{^app/assets/main\.scss$} do |pay|
+      payload << pay
+    end
+
+    @backend.trigger "app/assets/print.scss" => :changed, "app/assets/main.scss" => :changed
+    assert_equal [:changed => ["app/assets/main.scss"]], payload
+    assert_equal [:changed => ["app/assets/main.scss", "app/assets/print.scss"]], @payload
+  end
+end