Live layouts

Your LiveView applications can be made of two layouts:

the root layout - 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. It calls {@inner_content} to inject the content rendered by the layout
the app layout - this is the dynamic layout part of your application, it often includes the menu, sidebar, flash messages, and more. From Phoenix v1.8, this layout is explicitly rendered in your templates by calling the <Layouts.app /> component. In Phoenix v1.7 and earlier, the layout was typically configured as part of the lib/my_app_web.ex file, such as use Phoenix.LiveView, layout: ...

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

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.Layouts, :root}

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

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:

heex

<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:

heex

<Phoenix.Component.live_title default="Welcome" prefix="MyApp – ">
  {assigns[:page_title]}
</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.