How to Pass Arbitrary Values Into a LiveSession

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Sometimes it’s helpful to be able to pass simple values through to a group of LiveViews. This could be for any number of reasons. Here are a few examples:

• You want to support switching between pre-set themes with your styles.
• You want to only turn on a feature in certain section of your app.

Today I learned you can do this using live_session and its session option. For example, here we can set the theme for the admin portion of our app:

scope "/admin", MyAppWeb.Admin, as: :admin do
  live_session :admin, session: %{"theme" => "admin"}, on_mount: MyAppWeb.RequireAdminUser do
    live "/", DashboardLive, :show
    live "/assets", AssetsLive, :index
    live "/pages", PagesLive, :index
  end
end

Then in the LiveView:

@impl Phoenix.LiveView
def mount(_params, %{"theme" => theme} = _session, socket) do
  {:ok, assign(socket, :theme, theme)}
end

This is great for cases where you need to pass in some simple state to a group of LiveViews without a lot of ceremony.

Sources:

• https://hexdocs.pm/phoenix_live_view/Phoenix.LiveView.Router.html#live_session/3