Assigns and LiveEEx templates

All of the data in a LiveView is stored in the socket as assigns. The Phoenix.LiveView.assign/2 and Phoenix.LiveView.assign/3 functions help store those values. Those values can be accessed in the LiveView as socket.assigns.name but they are accessed inside LiveView templates as @name.

Phoenix.LiveView's built-in templates are identified by the .leex extension (Live EEx) or ~L sigil. They are similar to regular .eex templates except they are designed to minimize the amount of data sent over the wire by splitting static and dynamic parts and tracking changes.

When you first render a .leex template, it will send all of the static and dynamic parts of the template to the client. After that, any change you do on the server will now send only the dynamic parts, and only if those parts have changed.

The tracking of changes is done via assigns. Imagine this template:

<h1><%= expand_title(@title) %></h1>

If the @title assign changes, then LiveView will execute expand_title(@title) and send the new content. If @title is the same, nothing is executed and nothing is sent.

Change tracking also works when accessing map/struct fields. Take this template:

<div id="user_<%= @user.id %>">
  <%= @user.name %>
</div>

If the @user.name changes but @user.id doesn't, then LiveView will re-render only @user.name and it will not execute or resend @user.id at all.

The change tracking also works when rendering other templates as long as they are also .leex templates:

<%= render "child_template.html", assigns %>

The assign tracking feature also implies that you MUST avoid performing direct operations in the template. For example, if you perform a database query in your template:

<%= for user <- Repo.all(User) do %>
  <%= user.name %>
<% end %>

Then Phoenix will never re-render the section above, even if the number of users in the database changes. Instead, you need to store the users as assigns in your LiveView before it renders the template:

assign(socket, :users, Repo.all(User))

Generally speaking, data loading should never happen inside the template, regardless if you are using LiveView or not. The difference is that LiveView enforces this best practice.

LiveEEx pitfalls

There are two common pitfalls to keep in mind when using the ~L sigil or .leex templates.

When it comes to do/end blocks, change tracking is supported only on blocks given to Elixir's basic constructs, such as if, case, for, and friends. If the do/end block is given to a library function or user function, such as content_tag, change tracking won't work. For example, imagine the following template that renders a div:

<%= content_tag :div, id: "user_#{@id}" do %>
  <%= @name %>
  <%= @description %>
<% end %>

LiveView knows nothing about content_tag, which means the whole div will be sent whenever any of the assigns change. This can be easily fixed by writing the HTML directly:

<div id="user_<%= @id %>">
  <%= @name %>
  <%= @description %>
</div>

Another pitfall of .leex templates is related to variables. Due to the scope of variables, LiveView has to disable change tracking whenever variables are used in the template, with the exception of variables introduced by Elixir basic case, for, and other block constructs. Therefore, you must avoid code like this in your LiveEEx:

<% some_var = @x + @y %>
<%= some_var %>

Instead, use a function:

<%= sum(@x, @y) %>

Similarly, do not define variables at the top of your render function:

def render(assigns) do
  sum = assigns.x + assigns.y

  ~L"""
  <%= sum %>
  """
end

Instead explicitly precompute the assign in your LiveView, outside of render:

assign(socket, sum: socket.assigns.x + socket.assigns.y)

Generally speaking, avoid accessing variables inside LiveViews, as code that access variables is always executed on every render. This also applies to the assigns variable. The exception are variables introduced by Elixir's block constructs. For example, accessing the post variable defined by the comprehension below works as expected:

<%= for post <- @posts do %>
  ...
<% end %>

As are the variables matched defined in a case or cond:

<%= cond do %>
  <% is_nil(@post) -> %>
    ...
  <% @post -> %>
    ...
<% end %>

To sum up:

1. Avoid passing block expressions to library and custom functions

2. Never do anything on def render(assigns) besides rendering a template or invoking the ~L sigil

3. Avoid defining local variables, except within for, case, and friends