1 main.rb

class Main < Sinatra::Base
  register Sinatra::Context

  context "/post/:post_id" do
    before do |post_id|
      # This will be executed for all get/post/delete's in the current context.
      @post = Post[post_id]
    end

    get do |post_id|
       "You just visited /post/#{post_id} for the post {@post}"
    end

    get "/edit" do |post_id|
       "You just visited /post/#{post_id}/edit for the post {@post}"
    end

    context "/comments" do
      # Nesting contexts are okay
    end
  end
end

context.rb

module Sinatra::Context
  def self.registered(app)
    app.extend ClassMethods
  end

  module ClassMethods
    def context(prefix='', &b)
      Context.new(prefix, self).instance_eval(&b)
    end
  end

  class Context
    def initialize(prefix, klass)
      @prefix = prefix
      @klass  = klass
      @before = nil
    end

    def before(&b)
      @before = b
    end

    def method_missing(meth, *a, &b)
      @klass.send(meth, *a, &b)
    end

    [ :get, :post, :delete, :context ].each do |action|
      define_method action do |*a, &b|
        before_action = @before
        a[0] = "#{@prefix}#{a[0]}"
        @klass.send(action, *a) do |*args|
          before_action.call(*args) unless before_action.nil?
          b.call(*args)
        end
      end
    end
  end
end