async_helpers.rb

module Sinatra
  module AsyncHelpers
    def async_start
      env['async.callback'].call [response.status, response.headers, dbody] 
      throw :async
    end
    
    def async_write data
      dbody << data
    end
  
    def async_finish
      dbody.succeed
    end
  
    def on_async_interrupt &block
      dbody.callback { block.call }
      dbody.errback { block.call }
    end
  
    def dbody
      @dbody ||= DeferrableBody.new
    end
    
    protected
  
      class DeferrableBody
        include EventMachine::Deferrable

        def << body
          body.each do |chunk|
            @body_callback.call(chunk)
          end
        end
        alias_method :call, :<<

        def each &blk
          @body_callback = blk
        end
      end
  
  end
end

example.rb

  get '/delay/:n' do |n|
    EM.add_timer(n) do
      async_write ["{a:'delayed for #{n} seconds'}"]
      async_finish
    end
    
    response.headers['Content-Type'] = 'application/x-javascript'
    async_start
  end

  get '/timer/:n' do |n|
    cancel = false
    writer = proc {
      unless cancel
        async_write ["time: " + Time.now.to_s, "<br />"]
        EM.add_timer(n, writer)
      end
    }
    EM.add_timer(n, writer)
    
    on_async_interrupt do
      cancel = true
    end
    async_start
  end