LiveView Patterns Reference

> Ash projects: Use ash-framework skill for AshPhoenix.Form. Lifecycle: AshPhoenix.Form.validate/3 on phx-change, AshPhoenix.Form.submit/2 on submit, to_form/1 for HEEx. Do not use Ecto.Changeset.cast/3.

Reference for building with Phoenix LiveView 1.0/1.1.

Iron Laws — Never Violate These

1. NO UNCONDITIONAL DB QUERIES IN MOUNT — Mount runs TWICE. Default: assign_async. SEO routes: connected? guard + cache-backed disconnected branch (crawlers read that HTML)
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
10. MATCH {:error, %Ecto.Changeset{}} EXPLICITLY — Bare {:error, _} merges changeset and non-changeset errors; the form silently never re-renders validation errors. Handle other errors separately

Memory Impact

| Pattern | 3K items | 10K users × 10K items | |---------|----------|----------------------| | Regular assigns | ~5.1 MB | ~10+ GB | | Streams | ~1.1 MB | Minimal (O(1)) |

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

Quick Patterns

Async Assigns (CRITICAL)

elixir
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

elixir
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)

SEO Dead-Render (cache-backed disconnected branch)

For public/SEO-visible routes (marketing, articles, product listings) the disconnected render IS the HTML crawlers see. Fetch from a cache there, real data on connect:

elixir
def mount(_params, _session, socket) do
  products =
    if connected?(socket),
      do: Catalog.list_products(),
      else: Cache.get_products() || []

  {:ok, assign(socket, products: products)}
end

Empty list → <noscript>-friendly skeleton. Cache → :persistent_term, ETS, or Cachex. This satisfies Iron Law #1 AND keeps Googlebot/GPTBot happy.

PubSub with connected? check

elixir
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

| Wrong | Right | |-------|-------| | DB queries without assign_async | Use assign_async for all queries | | assign(socket, items: list) for lists | stream(socket, :items, list) | | PubSub subscribe without connected? | if connected?(socket), do: subscribe() | | Passing socket to context functions | Extract socket.assigns first | | Business logic in handle_event | Delegate to context | | assign_new for locale/user in hooks | assign/3 (must run every mount) |

References

For detailed patterns, see:

• references/async-streams.md - assign_async, stream_async, streams
• references/forms-uploads.md - Forms, validation, file uploads
• references/components.md - Function components, LiveComponents
• references/pubsub-navigation.md - PubSub, navigation, JS commands
• references/js-interop.md - Third-party JS libraries, phx-update="ignore", hooks
• references/channels-presence.md - Phoenix Channels, Presence, token auth