Phoenix LiveView Essentials

RULES — Follow these with no exceptions

1. Always add @impl true before every callback (mount, handle_event, handle_info, render)
2. Initialize assigns before they're accessed in render/1 — use mount/3 for static defaults, handle_params/3 for URL-dependent assigns (pagination, filters, sorting)
3. Check connected?(socket) before PubSub subscriptions, timers, or side effects
4. Use Map.get(assigns, :key, default) for optional assigns in helper functions
5. Return proper tuples — {:ok, socket} from mount, {:noreply, socket} from handle_event
6. Use with for error handling in event handlers — assign errors to socket, don't crash
7. auto_upload: true works with submit, but entries must be fully uploaded before you consume them — check entry.done? / rely on consume_uploaded_entries only after progress completes; default to manual uploads unless you need incremental upload UX
8. Check core_components.ex for existing components before creating custom ones
9. Never query the database directly from LiveViews — call context functions instead

Critical Concept: Two-Phase Rendering

LiveView renders happen in TWO phases: