Preserve params when push_patch from handle_event

I have 2 live components on the same page, one for pagination and another for search. Both write their state into URL query params which are later handled by the live controller.

Pagination component:

def handle_event("pagination", %{"page" => _} = params, socket) do
  path = ITJWeb.Router.Helpers.live_path(socket, ITJWeb.OffersLive, params)
  socket = push_patch(socket, to: path)
  {:noreply, socket}

Search component:

def handle_event("offer_search", %{"search" => search}, socket) do
  path = ITJWeb.Router.Helpers.live_path(socket, ITJWeb.OffersLive, search)
  socket = push_patch(socket, to: path)
  {:noreply, socket}


def mount(params, _session, socket) do
  {:noreply, socket} = handle_params(params, nil, socket)
  {:ok, socket}

def handle_params(params, _uri, socket) do
  {:noreply, socket}

The issue is that each component resets all existing query params. In other words, when you change the page, the search gets reset. There is a live demo:

The issue is that live_path requires all query params to be explicitly passed as a third argument but the existing query params (if I understand correctly) are not passed into handle_event.

  1. Is there a way to get current URL query params inside handle_event?
  2. Is there a way to do live_path + push_patch with preserving the current URL query params?
  3. Should I approach the whole problem differently? Is handle_event of a live component a good place to modify URL or should I delegate it to the controller somehow?

URLs are essentially global/shared state for all liveviews/components on a single page, so my suggestion is to consolidate modification of that state in a single place. A.k.a instead of your search or pagination component doing a push_patch let them trigger something on the parent liveview, which is aware of the current state of the URL and can “add” the changes on top.

What’s with calling handle_params from within mount? handle_params is called by the system. If you add some IO.inspect or Logger.debug commands in mount and handle_params you’ll see when and in what order it all happens.

Thank you for your reply! By sending messages as described in LiveView as the source of truth? It seems like a good idea and matching what docs say. They also explicitly say I can do live patches there, though, which confused me.

Thank you, good catch. Somehow I thought handle_params is only executed when you update params, not when you load the page. Now I see my assumption was wrong. I’ll update the code.

Or writing them as Function Components. Then you handle the event in the liveview. None of your components are managing their own state so could be Function Components.

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That also might be a good idea. My motivation was to encapsulate into components handling of events generated by the components and live to the controller only dealing with URL queries. It seems like a good separation but now I’m not sure if it’s worth it. There are just a few lines of code in components anyway, not a big deal. I’m not sure if it will solve the issue, though, since LiveView.handle_event accepts all the same arguments as LiveComponent.handle_event and so also can’t preserve query params on URL patches.

I still can’t wrap my head around how to update the existing URL query.

  1. LiveView.handle_event accepts all the same params as LiveComponent.handle_event
  2. LiveView.handle_info (which I can trigger with send from LiveComponent) also doesn’t know anything about the current URL query.

The best I have in mind is to pass the query params in assigns of all components but then, I suppose, it will re-render the whole page, and then what’s even the point of having LiveView. And also it’s messy and easy to get out of sync. I’m wondering why the current URL isn’t in the socket in the first place.

UPD: what I mean is that I see many ways how to update the LiveView state for different kinds of events but I can’t see a clean way to keep the URL query in sync with these changes.

That’s certainly not the case. I have a LiveView that I set myriad URL query params on and they don’t get overridden. Though I keep the params in assigns so maybe I’m telling a fib :slight_smile:

When you’re using the URL query params, you use handle_params to handle assigning anything that is associated with them. You don’t assign or update them elsewhere (i.e. in handle_event or handle_info). If you want to change them, you use push_patch.

You might find Sophie’s article on the topic useful, though she doesn’t seem to use push_patch.

  def self_path(socket, extra \\ %{}) do
    Routes.live_path(socket, __MODULE__, Enum.into(extra, socket.assigns.params))

  defp patch(socket, extra) do
    push_patch(socket, to: self_path(socket, extra))

  def handle_params(params, _uri, socket) do
      |> assign_params(params)
      |> assign_page(params)
      |> assign_search(params)
      |> assign_results()

  defp assign_params(socket, params) do
    assign(socket, :params, Map.take(params, ["page", "search"]))

  defp assign_page(socket, %{"page" => page}) do
    assign(socket, :page, String.to_integer(page))

  defp assign_page(socket, _params) do
    assign(socket, :page, 1)

  def handle_event("submit", %{"search" => search_string}, socket) do
    {:noreply, patch(socket, search: search_string, page: 1)}

  def handle_event("goto_page", %{"number" => number}, %{assigns: %{page: page}} = socket) do
    number = String.to_integer(number)

    if number == page do
      {:noreply, socket}
      {:noreply, patch(socket, page: number)}

Just the friendly reminder to use mix phx.routes to figure out the Routes. function for your path; instead of Routes.live_path.


You’d probably use verified routes once you hit phoenix 1.7.

I did hit this, so now just use:

<.simple_form for={@form} action={~p"/"} >

instead of

<.form for={@form} action={Routes.home_path(@conn, :action, params)} >

and the :action from verified_route is whatever you defined in the router for that path

I see that people resurrected this thread with the release of Phoenix 1.7. So, probably, I should provide a final solution to it.

What I ended up doing is explicitly re-add search params to the new path when updating the page number:

And I do not re-add the page number when the search is updated because I want to get the user to the first page when they change the parameters anyway.

So, this solution is a bit messy (because the pagination component must know about all other components that change the page parameters, which is a good example of bad coupling) and you should avoid doing that in big projects. That was good enough for a small pet project, though.

People here (@cmo and @Sleepful) say that in Phoenix 1.7 verified routes should preserve GET parameters. I can’t check it right now (the project is still on Phoenix 1.6), so I’ll mark it as the answer. Feel free to drop a message in the thread if it doesn’t work for you.