LiveView Patterns Reference

Reference for building with Phoenix LiveView 1.0/1.1.

Iron Laws — Never Violate These

1. NO DATABASE QUERIES IN DISCONNECTED MOUNT — Queries run TWICE (HTTP + WebSocket). Use

   assign_async

2. ALWAYS USE STREAMS FOR LISTS — Regular assigns = O(n) memory per user. Streams = O(1)
3. CHECK connected?/1 BEFORE SUBSCRIPTIONS — Prevents double subscriptions
4. EXTRACT VARIABLES BEFORE assign_async CLOSURE — Closures copy entire referenced variables
5. LOAD PRIMARY DATA IN mount/3, PAGINATION IN handle_params/3 — handle_params runs on EVERY URL change
6. NEVER PASS SOCKET TO BUSINESS LOGIC — Extract data before calling contexts
7. CHECK CHANGESET ERRORS BEFORE UI DEBUGGING — Silent form save = check

   {:error, changeset}

   first, not viewport/JS
8. HIDDEN INPUTS FOR ALL REQUIRED EMBEDDED FIELDS — Every required field in an embedded schema MUST have a

   hidden_input

   if not directly editable
9. NEVER USE

   assign_new

   FOR LIFECYCLE VALUES —

   assign_new

   skips the function if key exists. Use

   assign/3

   for locale, current user, or any value refreshed every mount

Memory Impact

Decision: Lists with >100 items → Use streams, not assigns

Quick Patterns

Async Assigns (CRITICAL)

def mount(%{"slug" => slug}, _session, socket) do
  # Extract needed values BEFORE the closure
  scope = socket.assigns.current_scope

  {:ok,
   socket
   |> assign_async(:org, fn -> {:ok, %{org: fetch_org(scope, slug)}} end)}
end

Streams for Lists

def mount(_params, _session, socket) do
  {:ok, stream(socket, :items, Items.list_items())}
end

# Insert/update/delete
stream_insert(socket, :items, item, at: 0)
stream_delete(socket, :items, item)

PubSub with connected? check

def mount(_params, _session, socket) do
  if connected?(socket), do: Chat.subscribe(room_id)
  {:ok, socket}
end

Navigation Decision Tree

Same LiveView, different params? → patch / push_patch
Different LiveView, same live_session? → navigate / push_navigate
Different live_session or non-LiveView? → href / redirect

Component Decision Tree

Does component need BOTH internal state AND event handling?
│
├── YES → Does it encapsulate APPLICATION logic (not just DOM)?
│   ├── YES → Use LiveComponent ✅
│   └── NO → Refactor to function component with parent handling
│
└── NO → Use Function Component ✅

Official guidance: "Prefer function components over live components"

Common Anti-patterns

assign_async

assign_async

assign(socket, items: list)

stream(socket, :items, list)

connected?

if connected?(socket), do: subscribe()

socket.assigns

handle_event

assign_new

assign/3

References

For detailed patterns, see:

• ${CLAUDE_SKILL_DIR}/references/async-streams.md

  - assign_async, stream_async, streams

• ${CLAUDE_SKILL_DIR}/references/forms-uploads.md

  - Forms, validation, file uploads

• ${CLAUDE_SKILL_DIR}/references/components.md

  - Function components, LiveComponents

• ${CLAUDE_SKILL_DIR}/references/pubsub-navigation.md

  - PubSub, navigation, JS commands

• ${CLAUDE_SKILL_DIR}/references/js-interop.md

  - Third-party JS libraries, phx-update="ignore", hooks

• ${CLAUDE_SKILL_DIR}/references/channels-presence.md

  - Phoenix Channels, Presence, token auth