class Roda::RodaPlugins::Middleware::Forwarder

Forwarder instances are what is actually used as middleware.

def call(env)

pass handling of the request to the next middleware.
If this returns a result, return that result directly. Otherwise,
When calling the middleware, first call the current middleware. 
def call(env)
  res = nil
  call_next = catch(:next) do
    env[@mid.opts[:middleware_env_var]] = true
    res = @mid.call(env)
    false
  end
  if call_next
    res = @app.call(env)
  end
  if handle_result = @mid.opts[:middleware_handle_result]
    handle_result.call(env, res)
  end
  res
end

def initialize(mid, app, *args, &block)

and store +app+ as the next middleware to call.
Make a subclass of +mid+ to use as the current middleware, 
def initialize(mid, app, *args, &block)
  @mid = Class.new(mid)
  if configure = @mid.opts[:middleware_configure]
    configure.call(@mid, *args, &block)
  elsif block || !args.empty?
    raise RodaError, "cannot provide middleware args or block unless loading middleware plugin with a block"
  end
  @app = app
end