Phoenix Liveview Essentials

Critical Concept: Two-Phase Rendering

LiveView renders happen in TWO phases:

1. Static/Disconnected Render - Initial HTTP request

   • No WebSocket connection
   • connected?(socket) returns false
   • Side effects (PubSub, timers) won't work

2. Connected Render - WebSocket established

   • Full live functionality active
   • connected?(socket) returns true
   • Events and live updates work

Common Bug: Accessing uninitialized assigns during static render crashes with KeyError.

Solution: Initialize assigns before render — use mount/3 for static defaults, handle_params/3 for URL-dependent state.

LiveView Lifecycle

Mount Callback

@impl true
def mount(_params, _session, socket) do
  # Initialize static defaults here; URL-dependent assigns go in handle_params
  socket =
    socket
    |> assign(:user, nil)
    |> assign(:loading, false)
    |> assign(:data, [])

  # Only subscribe when connected
  if connected?(socket) do
    Phoenix.PubSub.subscribe(MyApp.PubSub, "topic")
  end

  {:ok, socket}
end

Why check connected? PubSub subscriptions and timers only work with WebSocket connection.

Handle Event

Use pattern matching for different actions.

@impl true
def handle_event("save", %{"post" => post_params}, socket) do
  case Posts.create_post(post_params) do
    {:ok, post} ->
      socket =
        socket
        |> put_flash(:info, "Created!")
        |> assign(:post, post)

      {:noreply, socket}

    {:error, changeset} ->
      {:noreply, assign(socket, :changeset, changeset)}
  end
end

@impl true
def handle_event("delete", %{"id" => id}, socket) do
  Posts.delete_post(id)
  {:noreply, assign(socket, :posts, Posts.list_posts())}
end

Handle Info

Handle async messages and PubSub broadcasts.

@impl true
def handle_info({:post_created, post}, socket) do
  {:noreply, update(socket, :posts, fn posts -> [post | posts] end)}
end

@impl true
def handle_info(%{event: "presence_diff"}, socket) do
  {:noreply, assign(socket, :online_users, get_presence_count())}
end

Handle Params

Respond to URL changes (called in BOTH render phases).

@impl true
def handle_params(%{"id" => id}, _uri, socket) do
  # This runs during static AND connected render
  post = Posts.get_post!(id)

  if connected?(socket) do
    # Only subscribe when connected
    Phoenix.PubSub.subscribe(MyApp.PubSub, "post:#{id}")
  end

  {:noreply, assign(socket, :post, post)}
end

@impl true
def handle_params(_params, _uri, socket) do
  {:noreply, socket}
end

Socket Assigns

Use assign/2 or assign/3 to update socket state.

# Single assign
socket = assign(socket, :count, 0)

# Multiple assigns
socket = assign(socket, count: 0, name: "User", active: true)

# Update existing assign
socket = update(socket, :count, &(&1 + 1))

Safe Assign Access

In render/1: Direct access is safe if initialized in mount.

@impl true
def mount(_params, _session, socket) do
  {:ok, assign(socket, :count, 0)}
end

@impl true
def render(assigns) do
  ~H"""
  <p>Count: <%= @count %></p>  <!-- Safe -->
  """
end

In helper functions: Use Map.get for optional assigns.

# ❌ BAD - Crashes if not a map with :name
defp format_user(%{name: name}), do: name

# ✅ GOOD - Handles nil case
defp format_user(socket) do
  case Map.get(socket.assigns, :current_user) do
    nil -> "Guest"
    user -> user.name
  end
end

Temporary Assigns

Use temporary assigns for large collections that don't need to persist.

@impl true
def mount(_params, _session, socket) do
  socket = assign(socket, :posts, [])
  {:ok, socket, temporary_assigns: [posts: []]}
end

Flash Messages

Use put_flash/3 and clear_flash/2 for user feedback.

@impl true
def handle_event("save", params, socket) do
  case save_data(params) do
    {:ok, _} ->
      socket = put_flash(socket, :info, "Saved successfully!")
      {:noreply, socket}

    {:error, _} ->
      socket = put_flash(socket, :error, "Failed to save")
      {:noreply, socket}
  end
end

Live Navigation

Use push_navigate/2 or push_patch/2 for navigation.

# Full page reload (new LiveView)
{:noreply, push_navigate(socket, to: ~p"/users")}

# Patch (same LiveView, different params)
{:noreply, push_patch(socket, to: ~p"/posts/#{post}")}

Streams

Use streams for efficient rendering of large lists.

@impl true
def mount(_params, _session, socket) do
  {:ok, stream(socket, :posts, Posts.list_posts())}
end

@impl true
def handle_event("add", %{"post" => attrs}, socket) do
  {:ok, post} = Posts.create_post(attrs)
  {:noreply, stream_insert(socket, :posts, post, at: 0)}
end

@impl true
def handle_event("delete", %{"id" => id}, socket) do
  Posts.delete_post(id)
  {:noreply, stream_delete_by_dom_id(socket, :posts, "posts-#{id}")}
end

Components

Extract reusable UI into function components.

def card(assigns) do
  ~H"""
  <div class="card">
    <h3><%= @title %></h3>
    <p><%= @content %></p>
  </div>
  """
end

# Usage in template
<.card title="Hello" content="World" />

Form Binding

Bind forms to changesets for validation.

<.simple_form for={@form} phx-change="validate" phx-submit="save">
  <.input field={@form[:title]} label="Title" />
  <.input field={@form[:body]} type="textarea" label="Body" />
  <:actions>
    <.button>Save</.button>
  </:actions>
</.simple_form>

@impl true
def mount(_params, _session, socket) do
  changeset = Post.changeset(%Post{}, %{})
  {:ok, assign(socket, form: to_form(changeset))}
end

@impl true
def handle_event("validate", %{"post" => params}, socket) do
  changeset =
    %Post{}
    |> Post.changeset(params)
    |> Map.put(:action, :validate)

  {:noreply, assign(socket, form: to_form(changeset))}
end

Error Handling

Always handle errors gracefully in LiveViews.

@impl true
def handle_event("risky_operation", _params, socket) do
  case perform_operation() do
    {:ok, result} ->
      {:noreply, assign(socket, :result, result)}

    {:error, reason} ->
      {:noreply, put_flash(socket, :error, "Operation failed: #{reason}")}
  end
end

Error Boundaries

Handle errors in handle_event to prevent LiveView crashes.

@impl true
def handle_event("save", params, socket) do
  case save_record(params) do
    {:ok, record} ->
      socket =
        socket
        |> put_flash(:info, "Saved successfully")
        |> assign(:record, record)

      {:noreply, socket}

    {:error, %Ecto.Changeset{} = changeset} ->
      socket =
        socket
        |> put_flash(:error, "Please correct the errors")
        |> assign(:changeset, changeset)

      {:noreply, socket}

    {:error, reason} ->
      socket = put_flash(socket, :error, "An error occurred: #{reason}")
      {:noreply, socket}
  end
end

PubSub Broadcasting

Use PubSub for real-time updates across LiveViews.

# Subscribe in mount
@impl true
def mount(_params, _session, socket) do
  if connected?(socket) do
    Phoenix.PubSub.subscribe(MyApp.PubSub, "posts")
  end

  {:ok, assign(socket, :posts, list_posts())}
end

# Broadcast when data changes
def create_post(attrs) do
  with {:ok, post} <- Repo.insert(changeset) do
    Phoenix.PubSub.broadcast(MyApp.PubSub, "posts", {:post_created, post})
    {:ok, post}
  end
end

# Handle broadcast
@impl true
def handle_info({:post_created, post}, socket) do
  {:noreply, update(socket, :posts, fn posts -> [post | posts] end)}
end

Testing

When writing LiveView tests, invoke elixir-phoenix-guide:testing-essentials before writing any _test.exs file.

Common Lifecycle Mistakes

❌ Mistake 1: Assuming Assigns Exist

def render(assigns) do
  ~H"""
  <p>Count: <%= @count %></p>  <!-- Crash if @count not initialized -->
  """
end

✅ Fix: Initialize before render (mount or handle_params)

@impl true
def mount(_params, _session, socket) do
  {:ok, assign(socket, :count, 0)}
end

❌ Mistake 2: Subscribing in Both Phases

@impl true
def mount(_params, _session, socket) do
  # BAD - Subscribes during static render (doesn't work)
  Phoenix.PubSub.subscribe(MyApp.PubSub, "topic")
  {:ok, socket}
end

✅ Fix: Check connected?

@impl true
def mount(_params, _session, socket) do
  if connected?(socket) do
    Phoenix.PubSub.subscribe(MyApp.PubSub, "topic")
  end

  {:ok, socket}
end

❌ Mistake 3: Expensive Operations in Both Phases

@impl true
def mount(_params, _session, socket) do
  # BAD - Runs expensive query twice (static + connected)
  data = run_expensive_query()
  {:ok, assign(socket, :data, data)}
end

✅ Fix: Defer to connected phase

@impl true
def mount(_params, _session, socket) do
  socket =
    if connected?(socket) do
      # Only run when connected
      assign(socket, :data, run_expensive_query())
    else
      # Placeholder for static render
      assign(socket, :data, [])
    end

  {:ok, socket}
end

Lifecycle Flow

1. HTTP Request arrives
   ↓
2. mount/3 called (connected? = false)
   ↓
3. handle_params/3 called (connected? = false)
   ↓
4. render/1 called (STATIC HTML generated)
   ↓
5. HTML sent to browser
   ↓
6. Browser connects WebSocket
   ↓
7. mount/3 called AGAIN (connected? = true)
   ↓
8. handle_params/3 called AGAIN (connected? = true)
   ↓
9. render/1 called (sent over WebSocket)
   ↓
10. LiveView now active and reactive

Quick Reference

Safe Patterns

# ✅ Initialize in mount
assign(socket, :key, default_value)

# ✅ Use Map.get for optional
Map.get(socket.assigns, :key, default)

# ✅ Check connected for side effects
if connected?(socket), do: subscribe()

# ✅ Pattern match with fallback
def helper(%{name: name}), do: name
def helper(_), do: "default"

# ✅ Add @impl true
@impl true
def mount(...), do: ...

Unsafe Patterns

# ❌ Direct access without initialization
socket.assigns.key

# ❌ Subscribe without checking
Phoenix.PubSub.subscribe(...)

# ❌ Expensive ops in both phases
mount(...) do
  data = expensive_query()
end

# ❌ Missing @impl true
def mount(...), do: ...