Have you seen Add bulk actions in Phoenix LiveView · FullstackPhoenix ?

Here’s a snippet of what I have done in a multi-step form some years ago. Probably there is a more elegant solution for this, but this was done “in a rush” about 4 years ago, anyway, it might help you.

The amount of possible options here was like 4 and static, maybe if you have dynamic options it might not work that well with this approach.

defmodule Web.JobTypeStepComponentLive do

  @primary_key false
  embedded_schema do
    field(:job_types, {:array, :string})
  end

  @impl true
  def mount(socket) do
    socket = assign(socket, all_checked?: false)
    {:ok, socket}
  end

  @impl true
  def update(assigns, socket) do
    socket =
      socket
      |> assign(assigns)
      |> assign_job_types()
      |> assign_changeset()
      |> assign_job_type_options()

    {
      :ok,
      socket
      |> assign(all_checked?: all_checked?(socket))
    }
  end

  @impl true
  def handle_event("validate", params, socket) do
    params = Map.get(params, "job_type_step_component", %{"job_types" => []})
    all_checked? = all_checked?(socket, params["job_types"])

    changeset =
      %__MODULE__{}
      |> changeset(params)
      |> Map.put(:action, :validate)

    {:noreply, assign(socket, changeset: changeset, all_checked?: all_checked?)}
  end

  @impl true
  def handle_event("toggle_all", _payload, socket) do
    all_ids =
      socket.assigns.job_type_options
      |> Enum.map(&(elem(&1, 1) |> to_string()))

    all_checked? = all_checked?(socket)
    new_selection = if all_checked?, do: [], else: all_ids

    changeset =
      %__MODULE__{}
      |> changeset(%{"job_types" => new_selection})
      |> Map.put(:action, :validate)

    {:noreply, assign(socket, changeset: changeset, all_checked?: !all_checked?)}
  end

  defp all_checked?(socket) do
    current_ids = get_field(socket.assigns.changeset, :job_types) || []
    all_checked?(socket, current_ids)
  end

  defp all_checked?(socket, current_ids) do
    all_ids =
      socket.assigns.job_type_options
      |> Enum.map(&(elem(&1, 0) |> to_string()))

    length(current_ids) == length(all_ids)
  end

@impl true
  def render(assigns) do
    ~H"""
       (...)

          <div class="flex justify-center">
          <button type="button" class="btn btn--link btn--compact" phx-click="toggle_all" phx-target= 
               {@myself}>
            <%= if @all_checked?, do: gettext("Deselect all"), else: gettext("Select all") %>
          </button>
        </div>
    """
  end
end

So handle_params being called the first time is “special”. I’d have an assign preselected, which you fill in mount, in handle_params you adjust the result of parse_filter with all preselected categories and at the end of handle_params you set preselected: [] to essentially disable preselection going forward.