custom_file_matchers.rb

module CustomFileMatchers
  #
  # １つのマッチャを処理するためのクラス
  #
  class MoveFile
    def initialize(src, dest)
      @src  = src
      @dest = dest
    end

    #
    # xxx.should ...としたときのxxxがtrialとして与えられるので、
    # これがテストに通るか否かをbooleanで返す
    #
    def matches?(trial)
      src_digest = digest_of(@src)

      unless FileTest.exist?(@src)
        @error = "source file didn't exist"
        return false
      end

      trial.call

      unless !FileTest.exist?(@src)
        @error = "source file still exists"
        return false
      end

      unless FileTest.exist?(@dest)
        @error = "destination file doesn't exist"
        return false
      end

      unless src_digest == digest_of(@dest)
        @error = "destination file's MD5 checksum doesn't match with source's one"
        return false
      end

      true
    end

    #
    # テストに通らなかったときにエラーメッセージを
    # 取得するために呼ばれる
    #
    def failure_message_for_should
      "expected #{@src} to move to #{@dest} (#{@error})"
    end

    private

    def digest_of(path)
      str = ""
      File.open(path, "rb") do |f|
        buf = ""
        while f.read(256, buf)
          str << buf
        end
      end

      Digest::MD5.hexdigest(str)
    end
  end
  
  #
  # xxx.should ...の...の部分で、
  # マッチャを処理するクラスをインスタンス化して返す
  #
  def move_file(src, dest)
    MoveFile.new(src, dest)
  end
end

custom_file_matchers_spec.rb

describe CustomFileMatchers do
  include CustomFileMatchers

  describe "::MoveFile" do
    it "should pass when a file moved" do
      src  = "#{RAILS_ROOT}/tmp/a.txt"
      dest = "#{RAILS_ROOT}/tmp/b.txt"

      FileUtils.touch src
      lambda { FileUtils.mv src, dest }.should move_file(src, dest)
    end

    it "should NOT pass when a file didn't move" do
      src  = "#{RAILS_ROOT}/tmp/a.txt"
      dest = "#{RAILS_ROOT}/tmp/b.txt"

      FileUtils.touch src
      lambda {}.should_not move_file(src, dest)
    end
  end
end