LiveView Patterns Reference
Reference for building with Phoenix LiveView 1.0/1.1.
Iron Laws — Never Violate These
- NO DATABASE QUERIES IN DISCONNECTED MOUNT — Queries run TWICE (HTTP + WebSocket). Use
assign_async - ALWAYS USE STREAMS FOR LISTS — Regular assigns = O(n) memory per user. Streams = O(1)
- CHECK connected?/1 BEFORE SUBSCRIPTIONS — Prevents double subscriptions
- EXTRACT VARIABLES BEFORE assign_async CLOSURE — Closures copy entire referenced variables
- LOAD PRIMARY DATA IN mount/3, PAGINATION IN handle_params/3 — handle_params runs on EVERY URL change
- NEVER PASS SOCKET TO BUSINESS LOGIC — Extract data before calling contexts
- CHECK CHANGESET ERRORS BEFORE UI DEBUGGING — Silent form save = check
{:error, changeset}first, not viewport/JS - HIDDEN INPUTS FOR ALL REQUIRED EMBEDDED FIELDS — Every required field in an embedded schema MUST have a
hidden_inputif not directly editable - NEVER USE
assign_newFOR LIFECYCLE VALUES —assign_newskips the function if key exists. Useassign/3for locale, current user, or any value refreshed every mount
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)
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
| 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:
${CLAUDE_SKILL_DIR}/references/async-streams.md- assign_async, stream_async, streams${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
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