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Herminio Torres
Herminio Torres

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How to take leverage from on_mount to reduce code

#elixir #erlang #phoenix #liveview

Phoenix LiveView has implemented some fantastic features, and one of them is the on_mount/1 callback function.

This callback function will run before the mount/3 function in your LiveView.

There are two ways to set the on_mount callback function:

  1. In router using live_session/3.
  2. In your LiveView modules with on_mount macro.

If you need to do something before the mount/3 in all your LiveViews, live_session/3 is likely the best fit. However, if only for a few of them, the on_mount macro will be better for your needs.

on_mount helps reduce repetitive code in your LiveViews. Let's look at an example.

defmodule ExampleWeb.UserHook do import Phoenix.LiveView def on_mount(:default, _params, %{"current_user" => current_user} = _session, socket) do if authorized?(current_user) do {:cont, socket} else {:halt, socket} end end def on_mount(:admin, _params, %{"current_user" => current_user} = _session, socket) do if admin?(current_user) do {:cont, socket} else {:halt, socket} end end end
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The live_session/3 on Router:

live_session :default, on_mount: ExampleWeb.UserHook do scope "/", ExampleWeb do pipe_through [:browser, :auth] live "/", HomeLive, :page end end
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The on_mount macro:

defmodule ExampleWeb.HomeLive do use ExampleWeb, :live_view on_mount {ExampleWeb.UserHook, :admin} def mount(_params, _session, socket) do # ... end end
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Top comments (2)

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azyzz profile image
aziz abdullaev •

You can also pass on_mount directly in the routes/live_session:

 live_session :vendor_dashboard, on_mount: [{MyAppWeb.VendorAuth, :ensure_not_suspended}] do live "/new", Vendor.NewOrdersLive end
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where:

 def on_mount(:ensure_not_suspended, _params, session, socket) do socket = Phoenix.Component.assign_new(socket, :current_vendor, fn -> if vendor_token = session["vendor_token"] do Accounts.get_vendor_by_session_token(vendor_token) end end) case socket.assigns.current_vendor.status do "SUSPENDED" -> {:halt, Phoenix.LiveView.redirect(socket, to: "/error" )} _ -> {:cont, socket} end end
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herminiotorres profile image
Herminio Torres •

Thank you, @azyzz, for pointing it out; I put it this way since in one of my previous blog posts, I put more on that direction to use live_session, and I keep it this way to show some other alternatives to use the on_mount.