Forwarding requests to LiveView

I’m developing a Hex package that provides a set of LiveView routes mounted from the router via live '/path', LiveViewName, session: [:blah]. This package is then meant to be included in another Phoenix app. That app’s router can then forward requests to the Hex package’s paths, including the live mounted paths.

The hex package’s router looks like this:

defmodule RailwayUiWeb.Router do
  use RailwayUiWeb, :router

  pipeline :browser do
    plug :accepts, ["html"]
    plug :fetch_session
    plug :fetch_flash
    plug Phoenix.LiveView.Flash
    plug :protect_from_forgery
    plug :put_secure_browser_headers
  end

  scope "/", RailwayUiWeb do
    pipe_through :browser

    get "/", PageController, :index
    live "/published_messages", PublishedMessageLive.Index, session: [:current_user_uuid]
    live "/consumed_messages", ConsumedMessageLive.Index, session: [:current_user_uuid]
  end
end

And the calling application’s router forwards requests like this:

  scope path: "/admin", as: :admin do
    forward("/railway-ipc", RailwayUiWeb.Router, namespace: "admin/railway-ipc")
  end

Route forwarding is working in that the pages provided by the package do render when a user visits /admin/raiwlay-ipc/published_messages, for example. But the live view then crashes with the message:

 (ArgumentError) cannot invoke handle_params nor live_redirect/live_link to \"http://localhost:4000/admin/railway-ipc/published_messages\" because it isn't defined in RailwayUiWeb.Router\n    (phoenix_live_view) lib/phoenix_live_view/utils.ex:135

When running the hex package as a standalone phoenix app, the live views render and run just fine.

Is there some reason why using forward to forward to paths that are mounted in the destination router via live '/path' wouldn’t work?

Hi @SophieDeBenedetto!

Unfortunately LiveView cannot be mounted on a separate router which is then forwarded to. The reason is that LiveView does route lookups and we currently don’t store the “forwarding path”.

There is an OK-ish solution to this problem which is to encapsulate the routes in a macro:

defmodule AdminRoutes do
  defmacro __using__(opts) do
    quote do
      scope unquote(opts) do
        live ...
      end
  end
end

And then in the router you can do:

use AdminRoutes, path: "/admin"

Another issue with mounting a LiveView in an external application is that you also need to define a socket in the endpoint. We were discussing some options to make the whole process easier but we haven’t reached a conclusion yet.

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Thanks for the response! Yep after digging into the source code I can see the problem. I’ll lay it out below from anyone else who might find this helpful:

  • phoenix_live_view.js implements the bindNav function which sends an event to LiveView’s channel with the URL from window.location.href. At this point, that href is the forward mounted route, in my case containing the path /admin/railway-ipc/published_messages
  • The channel handles this event by calling PhoenixLive.Utils.live_link_info!/3 with that URL.
  • This in turn calls Phoenix.Router.route_info/4 where the router argument is the router to which we are forwarding, in my case the router that mounts the LiveView (RailwayUiWeb.Router), and the path argument is the path from the URI, /admin/railway-ipc/published_messages. Since that router implements a route for just /published_messages, not /admin/railway-ipc/published_messages, this raises the error I reported above.

I will definitely play around with your suggestion and see where it gets me. Thanks again!

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