Real-Time UX Patterns

Optimistic UI: Making It Feel Instant

The core tension: LiveView is server-rendered, but users expect client-side speed.

Level 0: CSS Loading Feedback (Built-in)

LiveView automatically adds phx-click-loading, phx-submit-loading, and phx-change-loading classes while waiting for server response. Use these for instant visual feedback — this is the minimum for every interactive element:

.phx-click-loading { opacity: 0.6; pointer-events: none; }
.phx-submit-loading .submit-label { display: none; }
.phx-submit-loading .submit-spinner { display: inline-flex; }

<button phx-click="save" phx-disable-with="Saving...">
  Save
</button>

<button
  phx-click="toggle_favorite"
  phx-value-id={@item.id}
  class="transition-opacity phx-click-loading:opacity-50"
>
  <.icon name={if @item.favorited, do: "hero-heart-solid", else: "hero-heart"} />
</button>

Available loading classes:

• phx-click-loading — on the clicked element
• phx-submit-loading — on the submitted form
• phx-change-loading — on the changed form
• phx-loading — on any element with a phx-loading target

Level 1: JS Commands for Instant State Changes

Use Phoenix.LiveView.JS to apply visual changes immediately on the client, before the server round trip:

<button
  phx-click={
    JS.push("toggle_favorite", value: %{id: @item.id})
    |> JS.toggle_class("text-yellow-500", to: "#star-#{@item.id}")
  }
>
  <.icon name="hero-star" id={"star-#{@item.id}"} class={if @item.favorited, do: "text-yellow-500"} />
</button>

The star turns yellow immediately. When the server responds, the DOM is patched to the correct state. If the server rejects it, the class is corrected on the next patch.

Level 2: Dual-State Optimistic Pattern

For actions where rollback matters (deleting items, toggling important state), track both "optimistic" and "confirmed" state:

def handle_event("delete_item", %{"id" => id}, socket) do
  # Immediately mark as "deleting" (optimistic)
  send(self(), {:confirm_delete, id})

  {:noreply,
    socket
    |> update(:items, fn items ->
      Enum.map(items, fn
        %{id: ^id} = item -> Map.put(item, :deleting, true)
        item -> item
      end)
    end)
  }
end

def handle_info({:confirm_delete, id}, socket) do
  case Items.delete(id) do
    :ok ->
      {:noreply, update(socket, :items, &Enum.reject(&1, fn i -> i.id == id end))}
    {:error, _} ->
      # Rollback: unmark the item
      {:noreply,
        socket
        |> update(:items, fn items ->
          Enum.map(items, &Map.delete(&1, :deleting))
        end)
        |> put_flash(:error, "Could not delete item")
      }
  end
end

In the template, items with :deleting true get a fade-out animation:

<div
  :for={item <- @items}
  class={["transition-opacity duration-300", item[:deleting] && "opacity-30 pointer-events-none"]}
>
  {item.name}
</div>

Level 3: Optimistic Assigns with Rollback

For complex optimistic updates, apply the change immediately and let the server confirm or correct:

def handle_event("toggle_favorite", %{"id" => id}, socket) do
  item = get_item(socket.assigns.items, id)

  # Optimistically update the UI immediately
  socket = update(socket, :items, fn items ->
    update_item(items, id, &Map.update!(&1, :favorited, fn v -> !v end))
  end)

  # Then do the actual work (may fail)
  case Items.toggle_favorite(item, socket.assigns.current_user) do
    {:ok, updated_item} ->
      # Server confirms — assign the real value (may be same as optimistic)
      {:noreply, update(socket, :items, &update_item(&1, id, fn _ -> updated_item end))}

    {:error, _reason} ->
      # Server rejects — revert to original
      {:noreply, update(socket, :items, &update_item(&1, id, fn _ -> item end))}
  end
end

Level 4: Optimistic Streams

For stream-based lists, insert optimistically and correct on confirmation:

def handle_event("add_comment", %{"body" => body}, socket) do
  # Create a temporary optimistic item
  temp_id = "temp-#{System.unique_integer([:positive])}"
  optimistic_comment = %{
    id: temp_id,
    body: body,
    user: socket.assigns.current_user,
    inserted_at: DateTime.utc_now(),
    pending: true
  }

  # Insert immediately
  socket = stream_insert(socket, :comments, optimistic_comment, at: 0)

  # Create for real
  case Comments.create(body, socket.assigns.current_user) do
    {:ok, real_comment} ->
      {:noreply,
       socket
       |> stream_delete(:comments, optimistic_comment)
       |> stream_insert(:comments, real_comment, at: 0)}

    {:error, _changeset} ->
      {:noreply,
       socket
       |> stream_delete(:comments, optimistic_comment)
       |> put_flash(:error, "Failed to post comment")}
  end
end

<div :for={{dom_id, comment} <- @streams.comments} id={dom_id}
     class={if comment.pending, do: "opacity-60", else: ""}>
  <p>{comment.body}</p>
  <span :if={comment.pending} class="text-xs text-text-secondary">Sending...</span>
</div>

Level 5: Undo Buffer (Deferred Execution)

Instead of immediately executing destructive actions, delay execution and offer an undo window:

def handle_event("delete_item", %{"id" => id}, socket) do
  # Hide the item immediately, but don't delete yet
  timer = Process.send_after(self(), {:confirm_delete, id}, 5_000)

  {:noreply,
    socket
    |> update(:items, fn items ->
      Enum.map(items, fn
        %{id: ^id} = item -> Map.put(item, :pending_delete, true)
        item -> item
      end)
    end)
    |> assign(undo_timer: timer, undo_item_id: id)
  }
end

def handle_event("undo_delete", _params, socket) do
  Process.cancel_timer(socket.assigns.undo_timer)

  {:noreply,
    socket
    |> update(:items, fn items ->
      Enum.map(items, &Map.delete(&1, :pending_delete))
    end)
    |> assign(undo_timer: nil, undo_item_id: nil)
  }
end

def handle_info({:confirm_delete, id}, socket) do
  Items.delete!(id)
  {:noreply,
    socket
    |> update(:items, &Enum.reject(&1, fn i -> i.id == id end))
    |> assign(undo_timer: nil, undo_item_id: nil)
  }
end

<div :if={@undo_item_id}
  class="fixed bottom-md left-1/2 -translate-x-1/2 z-50
         bg-foreground text-background rounded-lg px-lg py-sm
         shadow-xl flex items-center gap-md">
  <span>Item deleted</span>
  <button phx-click="undo_delete" class="font-medium underline">Undo</button>
</div>

When to Optimize (Latency Thresholds)

Users perceive delays differently by interaction type:

• Clicks/toggles: Tolerate ~100ms before feeling sluggish
• Typing/dragging: Tolerate ~50ms before feeling laggy
• Navigation: Tolerate ~200ms before feeling slow

Optimization by measured latency:

• < 100ms latency: No optimization needed. User won't notice.
• 100–300ms latency: CSS loading states (Level 0) are sufficient.
• > 300ms latency: JS commands (Level 1) or optimistic patterns (Level 2-5) improve perceived speed.
• Destructive actions: Use undo buffer (Level 5) or confirm on server first.

When NOT to Use Optimistic UI

• Destructive actions (delete, cancel subscription) — confirm first, then execute
• Actions with side effects (send email, charge payment) — show loading, wait for confirmation
• Validation-dependent actions (submit form with server-side validation) — wait for server
• Actions where rollback is confusing (reordering a list that other users see) — show loading instead

Rollback Strategies

Patterns by Use Case

Presence Indicators

Setting Up Phoenix Presence

# lib/my_app_web/presence.ex
defmodule MyAppWeb.Presence do
  use Phoenix.Presence,
    otp_app: :my_app,
    pubsub_server: MyApp.PubSub
end

Tracking in a LiveView

def mount(_params, _session, socket) do
  topic = "document:#{socket.assigns.document_id}"

  if connected?(socket) do
    Phoenix.PubSub.subscribe(MyApp.PubSub, topic)

    MyAppWeb.Presence.track(self(), topic, socket.assigns.current_user.id, %{
      name: socket.assigns.current_user.name,
      avatar_url: socket.assigns.current_user.avatar_url,
      color: user_color(socket.assigns.current_user.id),
      joined_at: System.system_time(:second),
      status: "active"
    })
  end

  presences = if connected?(socket) do
    MyAppWeb.Presence.list(topic) |> simplify_presences()
  else
    []
  end

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

def handle_info(%{event: "presence_diff", payload: _diff}, socket) do
  topic = "document:#{socket.assigns.document_id}"
  presences = MyAppWeb.Presence.list(topic) |> simplify_presences()
  {:noreply, assign(socket, presences: presences)}
end

defp simplify_presences(presences) do
  Enum.map(presences, fn {user_id, %{metas: [meta | _]}} ->
    Map.put(meta, :user_id, user_id)
  end)
end

# Assign a consistent color per user for avatars/cursors/field indicators
defp user_color(user_id) do
  colors = ~w(#3B82F6 #EF4444 #10B981 #F59E0B #8B5CF6 #EC4899 #06B6D4 #F97316)
  index = :erlang.phash2(user_id, length(colors))
  Enum.at(colors, index)
end

# Reusable helper for updating presence metadata
defp update_presence_meta(socket, updates) do
  topic = "document:#{socket.assigns.document_id}"
  user_id = socket.assigns.current_user.id
  MyAppWeb.Presence.update(self(), topic, user_id, fn meta ->
    Map.merge(meta, updates)
  end)
end

Avatar Stacks

Show who's present with a compact avatar stack:

<div class="flex items-center">
  <%!-- Avatar stack --%>
  <div class="flex -space-x-2">
    <div
      :for={user <- Enum.take(@presences, 5)}
      class="relative"
      title={user.name}
    >
      <img
        src={user.avatar_url}
        alt={user.name}
        class={[
          "size-8 rounded-full border-2 border-surface object-cover",
          user.status == "idle" && "opacity-50"
        ]}
        style={"border-color: #{user.color}"}
      />
      <div class={[
        "absolute -bottom-0.5 -right-0.5 size-2.5 rounded-full border-2 border-surface",
        status_color(user.status)
      ]} />
    </div>
  </div>

  <%!-- Overflow count --%>
  <div :if={length(@presences) > 5}
    class="ml-1 flex size-8 items-center justify-center rounded-full bg-muted text-xs font-medium">
    +{length(@presences) - 5}
  </div>

  <%!-- User count label --%>
  <span class="ml-sm text-sm text-muted-foreground">
    {length(@presences)} {if length(@presences) == 1, do: "person", else: "people"} viewing
  </span>
</div>

defp status_color("active"), do: "bg-green-500"
defp status_color("idle"), do: "bg-yellow-500"
defp status_color(_), do: "bg-gray-400"

Typing Indicators

Debounce the typing event to avoid flooding the server:

<textarea
  phx-keyup="typing"
  phx-debounce="500"
  phx-blur="stop_typing"
/>

def handle_event("typing", _params, socket) do
  update_presence_meta(socket, %{typing: true, typing_at: System.system_time(:second)})

  if timer = socket.assigns[:typing_timer] do
    Process.cancel_timer(timer)
  end
  timer = Process.send_after(self(), :clear_typing, 3_000)

  {:noreply, assign(socket, typing_timer: timer)}
end

def handle_event("stop_typing", _params, socket) do
  update_presence_meta(socket, %{typing: false})
  {:noreply, socket}
end

def handle_info(:clear_typing, socket) do
  update_presence_meta(socket, %{typing: false})
  {:noreply, socket}
end

Display with bouncing dots and grammatically correct text:

<div class="h-6 flex items-center">
  {#case typing_users(@presences, @current_user.id)}
    {%{count: 0}} ->

    {%{names: [name], count: 1}} ->
      <div class="flex items-center gap-xs text-sm text-muted-foreground">
        <span class="flex gap-0.5">
          <span class="size-1.5 rounded-full bg-muted-foreground animate-bounce [animation-delay:0ms]"></span>
          <span class="size-1.5 rounded-full bg-muted-foreground animate-bounce [animation-delay:150ms]"></span>
          <span class="size-1.5 rounded-full bg-muted-foreground animate-bounce [animation-delay:300ms]"></span>
        </span>
        {name} is typing
      </div>

    {%{names: [a, b], count: 2}} ->
      <span class="text-sm text-muted-foreground">{a} and {b} are typing</span>

    {%{names: [a | _], count: n}} ->
      <span class="text-sm text-muted-foreground">{a} and {n - 1} others are typing</span>
  {/case}
</div>

defp typing_users(presences, current_user_id) do
  typers = presences
    |> Enum.filter(&(&1.user_id != current_user_id and &1[:typing]))
    |> Enum.map(& &1.name)

  %{names: typers, count: length(typers)}
end

Active Field Indicators

Show which fields other users are currently editing:

def handle_event("focus_field", %{"field" => field_name}, socket) do
  update_presence_meta(socket, %{active_field: field_name})
  {:noreply, socket}
end

def handle_event("blur_field", _params, socket) do
  update_presence_meta(socket, %{active_field: nil})
  {:noreply, socket}
end

<div class="relative">
  <input
    name="title"
    phx-focus={JS.push("focus_field", value: %{field: "title"})}
    phx-blur={JS.push("blur_field")}
    class="input"
  />
  <%!-- Show who else is editing this field --%>
  <div :for={user <- field_editors(@presences, "title", @current_user.id)}
    class="absolute -top-2 -right-2 flex items-center gap-xs rounded-full px-xs py-0.5 text-xs text-white"
    style={"background-color: #{user.color}"}
  >
    <img src={user.avatar_url} class="size-4 rounded-full" />
    {user.name |> String.split() |> hd()}
  </div>
</div>

defp field_editors(presences, field_name, current_user_id) do
  Enum.filter(presences, &(&1.user_id != current_user_id and &1[:active_field] == field_name))
end

Idle Detection

Track user activity and update presence status:

Hooks.ActivityTracker = {
  mounted() {
    this._idleTimeout = null;
    this._isIdle = false;

    const resetIdle = () => {
      if (this._isIdle) {
        this._isIdle = false;
        this.pushEvent("user_active", {});
      }
      clearTimeout(this._idleTimeout);
      this._idleTimeout = setTimeout(() => {
        this._isIdle = true;
        this.pushEvent("user_idle", {});
      }, 60000); // 1 minute
    };

    this._events = ["mousemove", "keydown", "scroll", "touchstart"];
    this._events.forEach(e => window.addEventListener(e, resetIdle, { passive: true }));
    resetIdle();
  },

  destroyed() {
    clearTimeout(this._idleTimeout);
    this._events.forEach(e => window.removeEventListener(e, this._resetIdle));
  }
};

def handle_event("user_idle", _params, socket) do
  update_presence_meta(socket, %{status: "idle"})
  {:noreply, socket}
end

def handle_event("user_active", _params, socket) do
  update_presence_meta(socket, %{status: "active"})
  {:noreply, socket}
end

Cursor/Selection Presence

For collaborative editing — show where other users are:

def handle_event("cursor_moved", %{"line" => line, "col" => col}, socket) do
  update_presence_meta(socket, %{cursor_line: line, cursor_col: col, last_active: DateTime.utc_now()})
  {:noreply, socket}
end

UX Guidelines for Presence

1. Animate presence changes. Don't pop avatars in/out. Fade them.
2. Show, don't tell. An avatar with a green dot communicates "online" better than text.
3. Auto-expire indicators. Typing indicators must clear after 3-5s of inactivity.
4. Debounce aggressively. Don't send presence updates on every keystroke. Debounce to 300-500ms.
5. Limit visual noise. Don't show cursors unless the feature specifically needs them (e.g., collaborative whiteboard). For most apps, avatar stacks + typing indicators are sufficient.
6. Handle edge cases. A user with two tabs open appears as one presence (use the first meta). A user who closes their laptop stays "present" until the heartbeat timeout (~60s) — consider showing "idle" state after 30s of no activity.
7. Color coding. Assign consistent colors to users (hash the user ID). Use these colors for avatar borders, active field indicators, and cursors.
8. Be honest about staleness. If presence data is > 30s old, fade or hide it.
9. Respect privacy. Not every feature needs to show who's viewing.
10. Throttle cursor broadcasts. Max 10/sec — don't broadcast on every mousemove at 60fps.

Conflict Resolution

The Problem

Two users edit the same resource simultaneously. Without conflict handling, the last save wins and the first user's changes are silently lost.

Strategy 1: Optimistic Locking (Last Write Wins with Detection)

# Schema has a :lock_version field (integer, default 0)
def update_document(document, attrs, expected_version) do
  if document.lock_version != expected_version do
    {:error, :stale}
  else
    document
    |> Document.changeset(attrs)
    |> Ecto.Changeset.optimistic_lock(:lock_version)
    |> Repo.update()
  end
end

# In LiveView
def handle_event("save", %{"document" => params}, socket) do
  case Documents.update_document(
    socket.assigns.document,
    params,
    socket.assigns.document.lock_version
  ) do
    {:ok, document} ->
      {:noreply, assign(socket, :document, document)}

    {:error, :stale} ->
      {:noreply,
       socket
       |> put_flash(:error, "This document was updated by someone else. Your changes have been preserved below.")
       |> assign(:conflict, %{
         yours: params,
         theirs: Documents.get_document!(socket.assigns.document.id)
       })}

    {:error, changeset} ->
      {:noreply, assign(socket, :changeset, changeset)}
  end
end

Strategy 2: Field-Level Merging

For forms with independent fields, merge non-conflicting changes:

def merge_changes(server_doc, user_changes, original_doc) do
  Enum.reduce(user_changes, server_doc, fn {field, new_value}, acc ->
    original_value = Map.get(original_doc, field)
    server_value = Map.get(server_doc, field)

    cond do
      # User didn't change this field — keep server value
      new_value == original_value -> acc
      # Server didn't change this field — take user's change
      server_value == original_value -> Map.put(acc, field, new_value)
      # Both changed — conflict
      true -> Map.put(acc, field, {:conflict, server_value, new_value})
    end
  end)
end

Strategy 3: Real-Time Broadcasting (Prevent Conflicts)

Broadcast changes as they happen so users see each other's edits:

def handle_event("field_changed", %{"field" => field, "value" => value}, socket) do
  # Save the change
  {:ok, document} = Documents.update_field(socket.assigns.document, field, value)

  # Broadcast to other users viewing this document
  Phoenix.PubSub.broadcast(
    MyApp.PubSub,
    "document:#{document.id}",
    {:field_updated, field, value, socket.assigns.current_user.id}
  )

  {:noreply, assign(socket, :document, document)}
end

def handle_info({:field_updated, field, value, updater_id}, socket) do
  if updater_id != socket.assigns.current_user.id do
    document = Map.put(socket.assigns.document, String.to_existing_atom(field), value)
    {:noreply,
     socket
     |> assign(:document, document)
     |> push_event("field-updated-by-other", %{field: field, user: updater_id})}
  else
    {:noreply, socket}
  end
end

Strategy 4: Locking (Pessimistic)

For critical sections where concurrent editing must be prevented:

def handle_event("start_editing", %{"section" => section}, socket) do
  case Documents.acquire_lock(socket.assigns.document, section, socket.assigns.current_user) do
    {:ok, lock} ->
      Phoenix.PubSub.broadcast(
        MyApp.PubSub,
        "document:#{socket.assigns.document.id}",
        {:section_locked, section, socket.assigns.current_user}
      )
      {:noreply, assign(socket, :active_lock, lock)}

    {:error, :locked_by, other_user} ->
      {:noreply, put_flash(socket, :info, "#{other_user.name} is editing this section")}
  end
end

<div class={[
  "p-4 rounded border-2 transition-colors",
  @locked_by && @locked_by.id != @current_user.id && "border-yellow-400 bg-yellow-50",
  @locked_by && @locked_by.id == @current_user.id && "border-brand-500",
  !@locked_by && "border-transparent hover:border-gray-200"
]}>
  <div :if={@locked_by && @locked_by.id != @current_user.id}
       class="text-xs text-yellow-700 mb-2 flex items-center gap-1">
    <span class="h-2 w-2 rounded-full bg-yellow-500 animate-pulse" />
    {@locked_by.name} is editing
  </div>
  ...
</div>

Choosing a Strategy

Disconnect & Recovery

How LiveView Reconnection Works

When the WebSocket disconnects, LiveView's client-side library automatically:

1. Attempts reconnection immediately
2. Falls back to exponential backoff: 0s → 2s → 5s → 10s → ...
3. On reconnection, calls mount/3 and handle_params/3 again
4. Replays form state via phx-auto-recover
5. Re-renders the full page from the new server state

CSS classes applied during connection state changes:

• phx-connected — added to the LiveView container when connected
• phx-disconnected — added when connection is lost
• phx-error — added when an unrecoverable error occurs

Hook lifecycle callbacks:

• disconnected() — called on each hook when the LiveView disconnects
• reconnected() — called on each hook when the LiveView reconnects

Connection State UI

Delayed Disconnect Banner

Don't show a disconnect banner immediately — brief disconnects during deploys shouldn't alarm users. Show after 2-3 seconds:

// app.js
let disconnectTimeout;

window.addEventListener("phx:page-loading-start", (info) => {
  if (info.detail?.kind === "error" || info.detail?.kind === "initial") {
    disconnectTimeout = setTimeout(() => {
      document.getElementById("connection-banner")?.classList.remove("hidden");
    }, 2000);
  }
});

window.addEventListener("phx:page-loading-stop", () => {
  clearTimeout(disconnectTimeout);
  document.getElementById("connection-banner")?.classList.add("hidden");
});

<div id="connection-banner" class="hidden fixed top-0 inset-x-0 z-50">
  <div class="bg-yellow-500 text-yellow-950 text-sm text-center py-2 px-4 flex items-center justify-center gap-2">
    <svg class="animate-spin h-4 w-4" viewBox="0 0 24 24">
      <circle class="opacity-25" cx="12" cy="12" r="10" stroke="currentColor" stroke-width="4" fill="none"/>
      <path class="opacity-75" fill="currentColor" d="M4 12a8 8 0 018-8V0C5.373 0 0 5.373 0 12h4z"/>
    </svg>
    Reconnecting to server...
  </div>
</div>

Extended Disconnect with Retry

After 15-30 seconds, offer a manual refresh:

// app.js
let longDisconnectTimeout;

window.addEventListener("phx:page-loading-start", (info) => {
  longDisconnectTimeout = setTimeout(() => {
    document.getElementById("connection-banner-extended")?.classList.remove("hidden");
  }, 15000);
});

window.addEventListener("phx:page-loading-stop", () => {
  clearTimeout(longDisconnectTimeout);
  document.getElementById("connection-banner-extended")?.classList.add("hidden");
});

<div id="connection-banner-extended" class="hidden fixed inset-0 z-50 bg-black/50 flex items-center justify-center">
  <div class="bg-surface-raised rounded-xl shadow-xl p-6 max-w-sm text-center">
    <div class="text-lg font-semibold mb-2">Connection Lost</div>
    <p class="text-text-secondary mb-4">
      We're having trouble connecting to the server. Your unsaved changes are preserved.
    </p>
    <button onclick="window.location.reload()"
            class="bg-brand-600 text-white rounded-md px-4 py-2 font-medium hover:bg-brand-700">
      Refresh Page
    </button>
  </div>
</div>

Disabling Actions While Disconnected

Use CSS to disable destructive actions that require a server connection:

#app.phx-disconnected [data-requires-connection] {
  opacity: 0.4;
  pointer-events: none;
  cursor: not-allowed;
}

#app.phx-disconnected [data-requires-connection]::after {
  content: " (offline)";
  font-size: 0.75rem;
}

<button phx-click="delete" data-requires-connection>Delete</button>
<button phx-click="save" data-requires-connection>Save</button>

Form Recovery

LiveView automatically recovers form state on reconnection by re-submitting the last phx-change event. This works for simple forms out of the box.

Auto-Recovery (Default)

<form phx-change="validate" phx-submit="save">
  <input name="user[name]" value={@form[:name].value} />
  <input name="user[email]" value={@form[:email].value} />
</form>

On reconnect, LiveView collects all form field values and re-submits the phx-change event, restoring server state to match what the user sees.

Custom Recovery for Complex Forms

For multi-step wizards or forms with dynamic fields, use phx-auto-recover:

<form phx-change="validate_step" phx-submit="save" phx-auto-recover="recover_form">
  <!-- Dynamic fields based on current step -->
</form>

def handle_event("recover_form", params, socket) do
  # Reconstruct form state from params, possibly adjusting the step
  {:noreply, assign(socket, form: rebuild_form(params))}
end

Protecting Unsaved Work (Client-Side Backup)

For complex forms where auto-recovery isn't sufficient:

Hooks.UnsavedChanges = {
  mounted() {
    this._hasChanges = false;

    this.handleEvent("changes_saved", () => {
      this._hasChanges = false;
    });

    this.el.addEventListener("input", () => {
      this._hasChanges = true;
    });

    this._beforeUnload = (e) => {
      if (this._hasChanges) {
        e.preventDefault();
        e.returnValue = "";
      }
    };
    window.addEventListener("beforeunload", this._beforeUnload);
  },

  disconnected() {
    if (this._hasChanges) {
      const formData = new FormData(this.el);
      const data = Object.fromEntries(formData);
      localStorage.setItem(`unsaved-${this.el.id}`, JSON.stringify(data));
    }
  },

  reconnected() {
    const saved = localStorage.getItem(`unsaved-${this.el.id}`);
    if (saved) {
      this.pushEvent("restore_unsaved", JSON.parse(saved));
      localStorage.removeItem(`unsaved-${this.el.id}`);
    }
  },

  destroyed() {
    window.removeEventListener("beforeunload", this._beforeUnload);
  }
};

State Recovery Strategies

URL-Based State (Preferred)

Store important state in URL query params. This survives reconnection, page refresh, browser back, and link sharing:

# Instead of:
def handle_event("select_tab", %{"tab" => tab}, socket) do
  {:noreply, assign(socket, active_tab: tab)}
end

# Use:
def handle_event("select_tab", %{"tab" => tab}, socket) do
  {:noreply, push_patch(socket, to: ~p"/dashboard?tab=#{tab}")}
end

def handle_params(%{"tab" => tab}, _uri, socket) do
  {:noreply, assign(socket, active_tab: tab)}
end

Server-Side Draft Saving

For state that can't live in the URL (draft content, complex filters):

def mount(_params, _session, socket) do
  draft = Drafts.get_for_user(socket.assigns.current_user.id)
  {:ok, assign(socket, form: form_from_draft(draft))}
end

def handle_event("validate", params, socket) do
  Drafts.save(socket.assigns.current_user.id, params)
  {:noreply, assign(socket, form: changeset(params))}
end

Client-Side State with Hooks

For ephemeral state like scroll position:

const ScrollRestore = {
  mounted() {
    const saved = sessionStorage.getItem(`scroll:${this.el.id}`);
    if (saved) this.el.scrollTop = parseInt(saved);
  },
  updated() {
    sessionStorage.setItem(`scroll:${this.el.id}`, this.el.scrollTop);
  },
  reconnected() {
    const saved = sessionStorage.getItem(`scroll:${this.el.id}`);
    if (saved) this.el.scrollTop = parseInt(saved);
  }
};

Reconnection State Refresh

Ensure mount/3 can rebuild state from URL params + database, not just socket assigns:

def mount(_params, _session, socket) do
  if connected?(socket) do
    Phoenix.PubSub.subscribe(MyApp.PubSub, "document:#{socket.assigns.document_id}")
  end

  # Always load fresh data on mount (handles reconnection)
  document = Documents.get_document!(socket.assigns.document_id)
  {:ok, assign(socket, :document, document)}
end

Deployment Handling

Detecting New Deployments

def mount(_params, _session, socket) do
  {:ok, assign(socket, static_changed?: static_changed?(socket))}
end

<div :if={@static_changed?} class="fixed bottom-4 inset-x-4 z-50 mx-auto max-w-md">
  <div class="bg-brand-600 text-white rounded-lg px-4 py-3 shadow-xl flex items-center justify-between">
    <span class="text-sm">A new version is available</span>
    <a href="" class="text-sm font-semibold underline">Refresh</a>
  </div>
</div>

Zero-Downtime Deploy Checklist

1. Use phx-track-static on CSS and JS includes in your root layout
2. Check static_changed?/1 in mount to detect stale clients
3. Use rolling deploys so at least one node is always available
4. Test form recovery: liveSocket.disconnect(); setTimeout(() => liveSocket.connect(), 500);
5. Ensure mount/3 can rebuild state from URL params + database (not just socket assigns)

Offline-Capable Actions

For critical actions that should survive disconnects:

Hooks.ResilientAction = {
  mounted() {
    this.el.addEventListener("click", (e) => {
      const action = this.el.dataset.action;
      const payload = JSON.parse(this.el.dataset.payload || "{}");

      this.pushEvent(action, payload, (reply) => {
        this._removeFromQueue(action, payload);
      });

      this._addToQueue(action, payload);
    });
  },

  reconnected() {
    const queue = JSON.parse(localStorage.getItem("action-queue") || "[]");
    queue.forEach(({ action, payload }) => {
      this.pushEvent(action, payload, () => {
        this._removeFromQueue(action, payload);
      });
    });
  },

  _addToQueue(action, payload) {
    const queue = JSON.parse(localStorage.getItem("action-queue") || "[]");
    queue.push({ action, payload, timestamp: Date.now() });
    localStorage.setItem("action-queue", JSON.stringify(queue));
  },

  _removeFromQueue(action, payload) {
    const queue = JSON.parse(localStorage.getItem("action-queue") || "[]");
    const filtered = queue.filter(q => !(q.action === action && JSON.stringify(q.payload) === JSON.stringify(payload)));
    localStorage.setItem("action-queue", JSON.stringify(filtered));
  }
};

Loading and Skeleton States

Loading State Hierarchy

From worst to best user experience:

1. Nothing — User wonders if anything is happening (never do this)
2. Spinner — User knows something is loading but not what
3. Progress bar — User knows roughly how long to wait
4. Skeleton — User can see the shape of what's coming, feels faster
5. Optimistic render — User sees the data immediately (stale or predicted)

Reusable Skeleton Primitives

defmodule MyAppWeb.UI.Skeleton do
  use Phoenix.Component

  attr :class, :string, default: nil

  def skeleton(assigns) do
    ~H"""
    <div class={["animate-pulse rounded-md bg-muted", @class]} />
    """
  end

  def skeleton_text(assigns) do
    ~H"""
    <div class="space-y-2">
      <.skeleton class="h-4 w-full" />
      <.skeleton class="h-4 w-5/6" />
      <.skeleton class="h-4 w-4/6" />
    </div>
    """
  end

  def skeleton_card(assigns) do
    ~H"""
    <div class="rounded-lg border border-border p-lg space-y-md">
      <div class="flex items-center gap-md">
        <.skeleton class="size-10 rounded-full" />
        <div class="space-y-xs flex-1">
          <.skeleton class="h-4 w-1/3" />
          <.skeleton class="h-3 w-1/4" />
        </div>
      </div>
      <.skeleton_text />
    </div>
    """
  end

  def skeleton_table(assigns) do
    assigns = assign_new(assigns, :rows, fn -> 5 end)
    assigns = assign_new(assigns, :cols, fn -> 4 end)

    ~H"""
    <div class="space-y-sm">
      <div class="flex gap-lg border-b border-border pb-sm">
        <.skeleton :for={_ <- 1..@cols} class="h-4 flex-1" />
      </div>
      <div :for={_ <- 1..@rows} class="flex gap-lg py-sm">
        <.skeleton :for={_ <- 1..@cols} class="h-4 flex-1" />
      </div>
    </div>
    """
  end
end

Using Skeletons with assign_async

def mount(_params, _session, socket) do
  {:ok,
    socket
    |> assign_async(:dashboard_stats, fn ->
      {:ok, %{dashboard_stats: Stats.compute_dashboard()}}
    end)
    |> assign_async(:recent_activity, fn ->
      {:ok, %{recent_activity: Activity.recent(limit: 10)}}
    end)
  }
end

<div class="grid grid-cols-3 gap-lg">
  <.async_result :let={stats} assign={@dashboard_stats}>
    <:loading>
      <.skeleton class="h-24 rounded-lg" />
      <.skeleton class="h-24 rounded-lg" />
      <.skeleton class="h-24 rounded-lg" />
    </:loading>
    <:failed :let={_reason}>
      <div class="col-span-3 text-center text-destructive py-lg">
        Failed to load stats.
        <button phx-click="retry_stats" class="underline ml-xs">Retry</button>
      </div>
    </:failed>
    <.stat_card :for={stat <- stats} title={stat.label} value={stat.value} />
  </.async_result>
</div>

<div class="mt-xl">
  <.async_result :let={activities} assign={@recent_activity}>
    <:loading>
      <.skeleton_card />
      <.skeleton_card />
      <.skeleton_card />
    </:loading>
    <:failed :let={_reason}>
      <p class="text-destructive">Could not load activity.</p>
    </:failed>
    <.activity_item :for={activity <- activities} activity={activity} />
  </.async_result>
</div>

Inline Loading Indicators

For actions that take time but don't need a full skeleton:

<button phx-click="export_csv" class="inline-flex items-center gap-2">
  <svg class="hidden phx-click-loading:block size-4 animate-spin" viewBox="0 0 24 24">
    <circle class="opacity-25" cx="12" cy="12" r="10" stroke="currentColor" stroke-width="4" fill="none" />
    <path class="opacity-75" fill="currentColor" d="M4 12a8 8 0 018-8V0C5.373 0 0 5.373 0 12h4z" />
  </svg>
  <span class="phx-click-loading:hidden">Export CSV</span>
  <span class="hidden phx-click-loading:inline">Exporting...</span>
</button>

Progress Indicators for Long Operations

def handle_event("start_import", %{"file" => file}, socket) do
  task = Task.async(fn ->
    Importer.import_file(file, fn progress ->
      send(socket.root_pid, {:import_progress, progress})
    end)
  end)

  {:noreply, assign(socket, import_progress: 0, import_task: task)}
end

def handle_info({:import_progress, progress}, socket) do
  {:noreply, assign(socket, :import_progress, progress)}
end

<div :if={@import_progress > 0 && @import_progress < 100}
     class="w-full bg-muted rounded-full h-2 overflow-hidden">
  <div class="bg-primary h-full rounded-full transition-all duration-300"
       style={"width: #{@import_progress}%"} />
</div>

Progressive Loading with Streams

For lists and feeds, load incrementally rather than showing a skeleton for the whole list:

def mount(_params, _session, socket) do
  {:ok,
    socket
    |> stream(:messages, Messages.list(limit: 20))
    |> assign(page: 1, loading_more: false, has_more: true)
  }
end

def handle_event("load_more", _params, socket) do
  next_page = socket.assigns.page + 1
  messages = Messages.list(page: next_page, limit: 20)

  {:noreply,
    socket
    |> stream(:messages, messages)
    |> assign(page: next_page, loading_more: false, has_more: length(messages) == 20)
  }
end

<div id="messages" phx-update="stream">
  <div :for={{dom_id, message} <- @streams.messages} id={dom_id}
    phx-mounted={JS.transition({"ease-out duration-200", "opacity-0", "opacity-100"}, time: 200)}
  >
    <.message_bubble message={message} />
  </div>
</div>

<div :if={@loading_more} class="flex justify-center py-md">
  <svg class="animate-spin size-5 text-muted-foreground" viewBox="0 0 24 24">
    <circle class="opacity-25" cx="12" cy="12" r="10" stroke="currentColor" stroke-width="4" fill="none"/>
    <path class="opacity-75" fill="currentColor" d="M4 12a8 8 0 018-8V0C5.373 0 0 5.373 0 12h4z"/>
  </svg>
</div>

<button
  :if={@has_more && !@loading_more}
  phx-click="load_more"
  class="w-full py-md text-sm text-muted-foreground hover:text-foreground transition-colors"
>
  Load more
</button>

Infinite Scroll with Viewport Hook

const InfiniteScroll = {
  mounted() {
    this._observer = new IntersectionObserver(
      (entries) => {
        if (entries[0].isIntersecting) {
          this.pushEvent("load_more", {});
        }
      },
      { rootMargin: "200px" }
    );
    this._observer.observe(this.el);
  },
  destroyed() {
    this._observer?.disconnect();
  }
};

<div :if={@has_more} id="scroll-sentinel" phx-hook="InfiniteScroll"></div>

Stale-While-Revalidate

Show previously loaded data immediately, then refresh in the background:

def mount(_params, _session, socket) do
  cached = Cache.get("dashboard:#{socket.assigns.current_user.id}")

  socket = if cached do
    assign(socket, data: cached, stale: true)
  else
    assign(socket, data: nil, stale: false)
  end

  if connected?(socket), do: send(self(), :refresh_data)

  {:ok, socket}
end

def handle_info(:refresh_data, socket) do
  fresh_data = DataSource.fetch()
  Cache.put("dashboard:#{socket.assigns.current_user.id}", fresh_data)
  {:noreply, assign(socket, data: fresh_data, stale: false)}
end

<div class="relative">
  <div :if={@stale} class="absolute top-xs right-xs">
    <div class="flex items-center gap-xs text-xs text-muted-foreground">
      <svg class="animate-spin size-3" viewBox="0 0 24 24">
        <circle class="opacity-25" cx="12" cy="12" r="10" stroke="currentColor" stroke-width="4" fill="none"/>
        <path class="opacity-75" fill="currentColor" d="M4 12a8 8 0 018-8V0C5.373 0 0 5.373 0 12h4z"/>
      </svg>
      Updating...
    </div>
  </div>

  <div :if={@data}>
    <%!-- Render data (cached or fresh) --%>
  </div>
  <div :if={!@data}>
    <.skeleton_card />
  </div>
</div>

Loading Anti-Patterns

1. Full-page spinner. Never block the entire page with a spinner. Use skeletons for the loading area and keep the rest of the page interactive.

2. Skeleton flash. If data loads in < 200ms, the skeleton appears and disappears too quickly, creating a flash. Add a minimum display time or delay the skeleton.

3. Mismatched skeleton shape. A skeleton that looks nothing like the real content confuses users. Match the approximate shape, spacing, and count of the real content.

4. No error state. Always handle the failed case of assign_async. Show a helpful error message with a retry option, not a blank page.

5. Loading state that blocks navigation. Users should be able to navigate away from a loading page. Don't trap them.

Real-Time Feedback Patterns

Toast Notifications for Background Events

def handle_info({:comment_added, comment}, socket) do
  if comment.user_id != socket.assigns.current_user.id do
    {:noreply,
     socket
     |> stream_insert(:comments, comment)
     |> put_flash(:info, "#{comment.user.name} added a comment")}
  else
    {:noreply, socket}
  end
end

Live Counters and Badges

def handle_info({:new_notification, _notification}, socket) do
  {:noreply, update(socket, :unread_count, &(&1 + 1))}
end

<div class="relative">
  <.icon name="hero-bell" />
  <span :if={@unread_count > 0}
        class="absolute -top-1 -right-1 flex h-4 w-4 items-center justify-center rounded-full bg-red-500 text-[10px] font-bold text-white">
    {if @unread_count > 9, do: "9+", else: @unread_count}
  </span>
</div>

Typing Indicators

def handle_event("typing", _params, socket) do
  topic = "chat:#{socket.assigns.room_id}"
  Phoenix.PubSub.broadcast(MyApp.PubSub, topic, {:typing, socket.assigns.current_user})

  # Auto-clear after 3 seconds
  if socket.assigns[:typing_timer] do
    Process.cancel_timer(socket.assigns.typing_timer)
  end
  timer = Process.send_after(self(), {:clear_typing, socket.assigns.current_user.id}, 3000)

  {:noreply, assign(socket, :typing_timer, timer)}
end

def handle_info({:typing, user}, socket) do
  if user.id != socket.assigns.current_user.id do
    typers = Map.put(socket.assigns.typers, user.id, user.name)
    {:noreply, assign(socket, :typers, typers)}
  else
    {:noreply, socket}
  end
end

def handle_info({:clear_typing, user_id}, socket) do
  typers = Map.delete(socket.assigns.typers, user_id)
  {:noreply, assign(socket, :typers, typers)}
end

<div :if={map_size(@typers) > 0} class="text-xs text-text-secondary flex items-center gap-1 h-5">
  <span class="flex gap-0.5">
    <span class="h-1.5 w-1.5 rounded-full bg-text-secondary animate-bounce [animation-delay:0ms]" />
    <span class="h-1.5 w-1.5 rounded-full bg-text-secondary animate-bounce [animation-delay:150ms]" />
    <span class="h-1.5 w-1.5 rounded-full bg-text-secondary animate-bounce [animation-delay:300ms]" />
  </span>
  {typing_text(@typers)}
</div>

defp typing_text(typers) do
  names = Map.values(typers)
  case names do
    [name] -> "#{name} is typing"
    [a, b] -> "#{a} and #{b} are typing"
    [a | _rest] -> "#{a} and #{length(names) - 1} others are typing"
    [] -> ""
  end
end

Live Data Updates with Highlights

def handle_info({:price_updated, symbol, price}, socket) do
  {:noreply,
   socket
   |> stream_insert(:prices, %{id: symbol, symbol: symbol, price: price, updated: true})
   |> push_event("highlight", %{id: "price-#{symbol}"})}
end

<tr :for={{dom_id, price} <- @streams.prices} id={dom_id}
    data-highlight={JS.transition("bg-yellow-100", to: "##{dom_id}")}>
  <td>{price.symbol}</td>
  <td class="tabular-nums">{price.price}</td>
</tr>

Anti-Patterns

Silent failures on disconnect

❌ User submits form → disconnect → nothing happens → user confused
✅ Show connection status banner + queue action for replay on reconnect

Optimistic UI for irreversible actions

❌ Optimistically showing "Email sent!" before server confirms
✅ Show spinner → wait for server → then confirm

Ignoring concurrent edits

❌ Two users edit same field → last save silently wins → first user's work lost
✅ Use optimistic locking + conflict UI, or broadcast changes in real-time

Full-page loading spinners

❌ Blank page with centered spinner while data loads
✅ Skeleton screens that match the shape of real content
   Progressive loading: show what you have, load the rest async

Blocking UI during background operations

❌ Modal: "Please wait while we process your request..."
✅ Non-blocking toast: "Processing..." → "Done!" with the UI still interactive

Not debouncing presence updates

# ❌ Broadcasting cursor position on every mousemove (60fps = 60 broadcasts/sec)
def handle_event("cursor_moved", coords, socket) do
  Phoenix.PubSub.broadcast(...)
end

# ✅ Throttle to reasonable frequency
def handle_event("cursor_moved", coords, socket) do
  now = System.monotonic_time(:millisecond)
  if now - (socket.assigns[:last_cursor_broadcast] || 0) > 100 do  # Max 10/sec
    Phoenix.PubSub.broadcast(...)
    {:noreply, assign(socket, :last_cursor_broadcast, now)}
  else
    {:noreply, socket}
  end
end

Related Skills

• elixir-liveview: LiveView lifecycle, streams, PubSub, async assigns
• elixir-otp: GenServer, PubSub, supervision for presence and background tasks
• liveview-js-interop: Hooks for disconnect handling, JS commands for loading states
• animation-transitions: Transition patterns for loading states and presence indicators
• svelte-core: LiveSvelte for complex real-time interactive components