Two More Examples to Drive Rack Middleware

‹prev | My Chain | next›

From yesterday, I have the skeleton of Rack middleware:

module Rack
  class ThumbNailer
    def initialize(app)
      @app = app
    end
    def call(env)
      @app.call(env)
    end
  end
end

Right now, that doesn't do much—it simply passes thru the results of the target Rack application (my Sinatra application in this case). This is exactly what I described in RSpec examples—for most resources the middleware should just pass-thru the application results.

Even for most images in the application, the image should be passed thru with no modifications:

  context "Accessing an image" do
    context "without thumbnail param" do
      it "should return image directly from the target app" do
        get "/foo.jpg"
        last_response.body.should contain("Target app")
      end
    end
  end

This new example still passes since the ThumbNailer middleware only passes-thru.

But, if a :thumbnail parameter is supplied, I want a thumbnail image. From my prototype, I know that ImageScience needs to save this to the file system. To return that image, I will have to create a new File instance and read from it:

  context "Accessing an image" do
    context "with thumbnail param" do
      before(:each) do
        file = mock("File", :read => "Thumbnail")

        File.
          stub!(:new).
          and_return(file)
      end
    end

In real life, the read will lift image data, for the mock, I read the string "Thumbnail" for testing. Specifically:

      it "should return a thumbnail" do
        get "/foo.jpg", :thumbnail => 1
        last_response.body.should contain("Thumbnail")
      end

To make that pass, I need access to the parameters. The easiest way to accomplish that is via Rack::Request object. With that I can create a conditional based on the presence of the :thumbnail parameter:

    def call(env)
      req = Rack::Request.new(env)
      if !req.params['thumbnail'].blank?
        filename = "foo"
        thumbnail = File.new(filename).read
        [200, { }, thumbnail]
      else
        @app.call(env)
      end
    end

I am using a dummy value for the filename here—a future example will drive the actual storage location. This example is only driving that a file is read and that is the simplest thing that can possibly work. Except:

1)
NoMethodError in 'ThumbNailer Accessing an image with thumbnail param should return a thumbnail'
undefined method `read' for #
./thumbnailer.rb:12:in `call'
/home/cstrom/.gem/ruby/1.8/gems/rack-test-0.5.0/lib/rack/mock_session.rb:30:in `request'
/home/cstrom/.gem/ruby/1.8/gems/rack-test-0.5.0/lib/rack/test.rb:207:in `process_request'
/home/cstrom/.gem/ruby/1.8/gems/rack-test-0.5.0/lib/rack/test.rb:57:in `get'
./thumbnailer_spec.rb:46:

Ack! I forgot that I am working inside a module. I really want to access the top-level File class, not something encapsulated in the Rack:: namespace. Leading double-colons to the rescue:

    def call(env)
      req = Rack::Request.new(env)
      if !req.params['thumbnail'].blank?
        filename = "foo"
        thumbnail = ::File.new(filename).read
        [200, { }, thumbnail]
      else
        @app.call(env)
      end
    end

With that example passing, I call it a night. Tomorrow I will pick back up with BDDing this bit of Rack middleware into submission.