phoenix-liveview-essentials_skill

Overview

This skill is a mandatory checklist and compact reference for building reliable Phoenix LiveViews. It enforces lifecycle best practices, assign initialization, and safe side-effect handling to prevent common runtime crashes. Use it before writing LiveView modules or .heex templates to avoid typical pitfalls and ensure predictable behavior.

How this skill works

The skill inspects your LiveView patterns against a set of hard rules: always add @impl true to callbacks, initialize all assigns in mount/3, and guard side effects with connected?(socket). It explains two-phase rendering, when mount and handle_params run twice, and shows safe patterns for assigns, PubSub subscriptions, streams, forms, and error handling. Follow its examples to return correct tuples, use with for error flows, and avoid unsafe operations during the static render phase.

When to use it

• Before creating any LiveView module or .heex template
• When initializing assigns and planning side effects (PubSub, timers, expensive queries)
• When implementing mount/3, handle_event/3, handle_info/2, or handle_params/3
• When wiring forms, uploads, streams, or live navigation
• When testing LiveViews with Phoenix.LiveViewTest

Best practices

• Always add @impl true before every LiveView callback to satisfy compiler checks
• Initialize every assign in mount/3 so render/1 never reads uninitialized data
• Check connected?(socket) before subscribing, starting timers, or running expensive queries
• Use Map.get(socket.assigns, :key, default) in helpers for optional assigns
• Return correct tuples: {:ok, socket} from mount, {:noreply, socket} from event/info handlers
• Use with or pattern matching to handle errors and assign failures to the socket instead of crashing

Example use cases

• Mount a posts LiveView: initialize posts to [] and subscribe to PubSub only when connected
• Build a form LiveView: assign form = to_form(changeset) in mount and validate with phx-change handlers
• Stream large lists: use stream(socket, :posts, list) and stream_insert/stream_delete in handle_event
• Handle broadcasts: subscribe in mount when connected and update assigns in handle_info
• Implement navigation: use push_navigate/2 for full LiveView changes and push_patch/2 for intra-view patches