Here’s how I did it in my project.

First, define a function in layout_view.ex,

  @doc """
  Set css class for active link.
  """
  @spec css_class(Plug.Conn.t(), String.t(), String.t(), String.t()) :: String.t()
  def css_class(conn, path, defaultClass, activeClass \\ "active") do
    if path == Phoenix.Controller.current_path(conn) do
      defaultClass <> " " <> activeClass
    else
      defaultClass
    end
  end

Then in my app.html.eex call the function:

<%= link "Dashboard", to: Routes.user_path(@conn, :show, @current_user), class: css_class(@conn, Routes.user_path(@conn, :show, @current_user), "nav-link") %>

My navbar is a list of data entries (with some dropdown that are just further lists) and they define what is active or note, built up from the Views themselves so they know that they are being called or not and of which template.

I have improved this to suit my case which having /action_name/slug/etc

  @doc """
  Sets additional css class when for the current page.
  """
  def css_class(conn, path, default_class \\ nil, active_class \\ "active") do
    if is_nav_item_actvie?(conn, path) do
      "#{default_class} #{active_class}"
    else
      default_class
    end
  end

  @doc """
  Determine if the nav item is active or not.
  """
  def is_nav_item_actvie?(conn, path) do
    input_action_name = Enum.at(String.split(path, "/"), 1)
    current_action_name = List.first(conn.path_info)
    IO.inspect(%{c: current_action_name, p: input_action_name})
    current_action_name === input_action_name
  end

Just sharing~