Rack Middleware
Rack is a system of classes that can modify content on-the-fly and intercept requests before they are processed by the server (Middleman).
Middleman has full access to Rack Middleware which opens up an expansive universe of libraries which work with Middleman.
Example: Syntax Highlighting
Note: The official middleman-syntax extension is a better way to do syntax highlighting - what follows is just an example of how to use Rack Middleware.
This site is written in Middleman and features many code blocks which have
syntax highlighting. This syntax highlighting is accomplished outside the scope
of Middleman. This site renders <code> blocks and then Rack Middleware takes
over and enhances those blocks with syntax highlight. The Middleware in use is
called Rack::Codehighlighter. Here's how it can be used in your config.rb:
require "rack/codehighlighter"
require "pygments"
use Rack::Codehighlighter,
:pygments,
:element => "pre>code",
:pattern => /\A:::([-_+\w]+)\s*\n/,
:markdown => true
Make sure you add the right dependencies to your Gemfile to make those work:
gem "rack-codehighlighter", :git => "git://github.com/wbzyl/rack-codehighlighter.git"
gem "pygments.rb"
The above block required the rack/codehighlighter and pygments.rb libraries.
Then the use command tells Middleman to use this Middleware. The rest is
standard Rack Middleware setup, passing some variables to the Middleware itself
instructing the syntax highlighter on how to locate code blocks and which
backend to use for parsing the syntax.
Build Cycle
The Rack Middleware is run on all requests, including those done during the build cycle. This means anything the Rack Middleware effects during preview will be present in the built files. However, be aware that once the project is built, it is a static site. Rack Middleware which does processing on requests, expecting things like cookies, sessions or variables, won't work once the site is built.