RealFile = File
RealFileUtils = FileUtils
RealDir = Dir
RealFileUtils::Dir = RealDir
RealFileUtils::File = RealFile

module FakeFS
  def self.activate!
    Object.class_eval do
      remove_const(:Dir)
      remove_const(:File)
      remove_const(:FileUtils)
      const_set(:Dir, FakeFS::Dir)
      const_set(:File, FakeFS::File)
      const_set(:FileUtils, FakeFS::FileUtils)
    end
  end

  def self.deactivate!
    Object.class_eval do
      remove_const(:Dir)
      remove_const(:File)
      remove_const(:FileUtils)
      const_set(:Dir, RealDir)
      const_set(:File, RealFile)
      const_set(:FileUtils, RealFileUtils)
    end
  end
end

def FakeFS
  return ::FakeFS unless block_given?
  ::FakeFS.activate!
  yield
ensure
::FakeFS.deactivate!
end