View Source Live layouts

From Phoenix v1.7, your application is made of two layouts:

  • the root layout - this is a layout used by both LiveView and regular views. This layout typically contains the <html> definition alongside the head and body tags. Any content defined in the root layout will remain the same, even as you live navigate across LiveViews. The root layout is typically declared on the router with put_root_layout and defined as "root.html.heex" in your layouts folder

  • the app layout - this is the default application layout which is rendered on both regular HTTP requests and LiveViews. It defaults to "app.html.heex"

Overall, those layouts are found in components/layouts and are embedded within MyAppWeb.Layouts.

All layouts must call <%= @inner_content %> to inject the content rendered by the layout.

Root layout

The "root" layout is rendered only on the initial request and therefore it has access to the @conn assign. The root layout is typically defined in your router:

plug :put_root_layout, html: {MyAppWeb.LayoutView, :root}

The root layout can also be set via the :root_layout option in your router via Phoenix.LiveView.Router.live_session/2.

Application layout

The "app.html.heex" layout is rendered with either @conn or @socket. Both Controllers and LiveViews explicitly define the default layouts they will use. See the def controller and def live_view definitions in your MyAppWeb to learn how it is included.

For LiveViews, the default layout can be overridden in two different ways for flexibility:

  1. The :layout option in Phoenix.LiveView.Router.live_session/2, when set, will override the :layout option given via use Phoenix.LiveView

  2. The :layout option returned on mount, via {:ok, socket, layout: ...} will override any previously set layout option

The LiveView itself will be rendered inside the layout wrapped by the :container tag.

Updating document title

Because the root layout from the Plug pipeline is rendered outside of LiveView, the contents cannot be dynamically changed. The one exception is the <title> of the HTML document. Phoenix LiveView special cases the @page_title assign to allow dynamically updating the title of the page, which is useful when using live navigation, or annotating the browser tab with a notification. For example, to update the user's notification count in the browser's title bar, first set the page_title assign on mount:

def mount(_params, _session, socket) do
  socket = assign(socket, page_title: "Latest Posts")
  {:ok, socket}
end

Then access @page_title in the root layout:

<title><%= @page_title %></title>

You can also use the Phoenix.Component.live_title/1 component to support adding automatic prefix and suffix to the page title when rendered and on subsequent updates:

<Phoenix.Component.live_title prefix="MyApp – ">
  <%= assigns[:page_title] || "Welcome" %>
</Phoenix.Component.live_title>

Although the root layout is not updated by LiveView, by simply assigning to page_title, LiveView knows you want the title to be updated:

def handle_info({:new_messages, count}, socket) do
  {:noreply, assign(socket, page_title: "Latest Posts (#{count} new)")}
end

Note: If you find yourself needing to dynamically patch other parts of the base layout, such as injecting new scripts or styles into the <head> during live navigation, then a regular, non-live, page navigation should be used instead. Assigning the @page_title updates the document.title directly, and therefore cannot be used to update any other part of the base layout.