rack_tap.rb

# Middleware to tap into request so that you can provide an alternate response depending upon request conditions.
#
# Supplied block should return a Rack-compatible response, or nil/false to pass on handling of the request.
#
# Example:
# 
#   use Rack::Tap do |env|
#     if Rack::Request.new(env).host == 'admin.myapp.com'
#       [301, {'Location' => 'http://www.myapp.com/admin'}, []]
#     end
#   end
#
#   run MyApp
#
module Rack
  class Tap
    def initialize(app, &block)
      @app = app
      @block = block
    end

    def call(env)
      @block.call(env) || @app.call(env)
    end
  end
end