V2 Extensions as Middleware

[jhollinger]

A greatly simplified extension structure based around middleware instead of hooks. The extension system is comprised of multiple "stacks" of middlewares. A "render" fires off a nested call structure of these stacks:

around_result
  around_blueprint_init # only on first pass
    around_serialize_object | around_serialize_collection
      around_blueprint
        around_field_value | around_object_value | around_collection_value

Much like in Rack, each middleware receives a "context" and may choose to modify it before yielding to the next middleware. The yield returns the response, which the middleware may modify before returning. A middleware may choose not to yield at all and return a response of its choosing (e.g. a caching layer).

# Runs every time an object is serialized
def around_blueprint(ctx)
  # replace or modify the object being serialized
  ctx.object = modify ctx.object

  # yield to the next middleware or Blueprinter core
  result = yield ctx

  # Wrap the result before handing it to the previous middleware's yield or Blueprinter core
  { data: result }
end

Instead of specialized hooks like field_value, exclude_field?, and field_result, a single around_field_value hook can be used for anything relating to a field's value:

def around_field_value(ctx)
  val = yield ctx
  if ctx.options[:skip_on] && val == ctx.options[:skip_on]
    # calling skip will abort the whole stack and cause the field to be ignored
    # this is how if/unless/etc are now implemented
    skip
  else
    val
  end
end