# Patterns and Guidelines

## Searchable Grid with `useDeferredValue`

`useDeferredValue` makes filter updates a transition, activating `<ViewTransition>`:

```tsx
'use client';

import { useDeferredValue, useState, ViewTransition, Suspense } from 'react';

export default function SearchableGrid({ itemsPromise }) {
  const [search, setSearch] = useState('');
  const deferredSearch = useDeferredValue(search);

  return (
    <>
      <input value={search} onChange={(e) => setSearch(e.currentTarget.value)} />
      <ViewTransition>
        <Suspense fallback={<GridSkeleton />}>
          <ItemGrid itemsPromise={itemsPromise} search={deferredSearch} />
        </Suspense>
      </ViewTransition>
    </>
  );
}
```

Per-item `<ViewTransition name={...}>` inside a deferred list triggers cross-fades on every keystroke. Fix with `default="none"`:

```tsx
{filteredItems.map(item => (
  <ViewTransition key={item.id} name={`item-${item.id}`} share="morph" default="none">
    <ItemCard item={item} />
  </ViewTransition>
))}
```

## Card Expand/Collapse with `startTransition`

Toggle between grid and detail view with shared element morph:

```tsx
'use client';

import { useState, useRef, startTransition, ViewTransition } from 'react';

export default function ItemGrid({ items }) {
  const [expandedId, setExpandedId] = useState(null);
  const scrollRef = useRef(0);

  return expandedId ? (
    <ViewTransition enter="slide-in" name={`item-${expandedId}`}>
      <ItemDetail
        item={items.find(i => i.id === expandedId)}
        onClose={() => {
          startTransition(() => {
            setExpandedId(null);
            setTimeout(() => window.scrollTo({ behavior: 'smooth', top: scrollRef.current }), 100);
          });
        }}
      />
    </ViewTransition>
  ) : (
    <div className="grid grid-cols-3 gap-4">
      {items.map(item => (
        <ViewTransition key={item.id} name={`item-${item.id}`}>
          <ItemCard
            item={item}
            onSelect={() => {
              scrollRef.current = window.scrollY;
              startTransition(() => setExpandedId(item.id));
            }}
          />
        </ViewTransition>
      ))}
    </div>
  );
}
```

## Type-Safe Transition Helpers

Use `as const` arrays and derived types to prevent ID clashes:

```tsx
const transitionTypes = ['default', 'transition-to-detail', 'transition-to-list'] as const;
const animationTypes = ['auto', 'none', 'animate-slide-from-left', 'animate-slide-from-right'] as const;

type TransitionType = (typeof transitionTypes)[number];
type AnimationType = (typeof animationTypes)[number];
type TransitionMap = { default: AnimationType } & Partial<Record<Exclude<TransitionType, 'default'>, AnimationType>>;

export function HorizontalTransition({ children, enter, exit }: {
  children: React.ReactNode;
  enter: TransitionMap;
  exit: TransitionMap;
}) {
  return <ViewTransition enter={enter} exit={exit}>{children}</ViewTransition>;
}
```

## Cross-Fade Without Remount

Omit `key` to trigger an update (cross-fade) instead of exit + enter. Avoids Suspense remount/refetch:

```jsx
<ViewTransition>
  <TabPanel tab={activeTab} />
</ViewTransition>
```

Use `key` when content identity changes (state resets). Omit for cross-fades (tabs, panels, carousel).

## Isolate Elements from Parent Animations

### Persistent Layout Elements

Persistent elements (headers, navbars, sidebars) get captured in the page's transition snapshot. Fix with `viewTransitionName`:

```jsx
<nav style={{ viewTransitionName: "persistent-nav" }}>{/* ... */}</nav>
```

Then add the persistent element isolation CSS from `css-recipes.md`. For `backdrop-blur`/`backdrop-filter`, use the backdrop-blur workaround from `css-recipes.md`.

### Floating Elements

Give popovers/tooltips their own `viewTransitionName`:

```jsx
<SelectPopover style={{ viewTransitionName: 'popover' }}>{options}</SelectPopover>
```

Global fix: see persistent element isolation in `css-recipes.md`.

## Shared Controls Between Skeleton and Content

Give matching controls in fallback and content the same `viewTransitionName`:

```jsx
// Fallback
<input disabled placeholder="Search..." style={{ viewTransitionName: 'search-input' }} />
// Content
<input placeholder="Search..." style={{ viewTransitionName: 'search-input' }} />
```

Don't put manual `viewTransitionName` on the root DOM node inside `<ViewTransition>` — React's auto-generated name overrides it.

## Reusable Animated Collapse

```jsx
function AnimatedCollapse({ open, children }) {
  if (!open) return null;
  return (
    <ViewTransition enter="expand-in" exit="collapse-out">
      {children}
    </ViewTransition>
  );
}

// Usage: toggle with startTransition
<button onClick={() => startTransition(() => setOpen(o => !o))}>Toggle</button>
<AnimatedCollapse open={open}><SectionContent /></AnimatedCollapse>
```

## Preserve State with Activity

```jsx
<Activity mode={isVisible ? 'visible' : 'hidden'}>
  <ViewTransition enter="slide-in" exit="slide-out">
    <Sidebar />
  </ViewTransition>
</Activity>
```

## Exclude Elements with `useOptimistic`

`useOptimistic` values update before the transition snapshot, excluding them from animation. Use for controls (labels); use committed state for animated content:

```tsx
const [sort, setSort] = useState('newest');
const [optimisticSort, setOptimisticSort] = useOptimistic(sort);

function cycleSort() {
  const nextSort = getNextSort(optimisticSort);
  startTransition(() => {
    setOptimisticSort(nextSort);  // before snapshot — no animation
    setSort(nextSort);            // between snapshots — animates
  });
}

<button>Sort: {LABELS[optimisticSort]}</button>
{items.sort(comparators[sort]).map(item => (
  <ViewTransition key={item.id}><ItemCard item={item} /></ViewTransition>
))}
```

---

## View Transition Events

Imperative control via `onEnter`, `onExit`, `onUpdate`, `onShare`. Always return a cleanup function. `onShare` takes precedence over `onEnter`/`onExit`.

```jsx
<ViewTransition
  onEnter={(instance, types) => {
    const anim = instance.new.animate(
      [{ transform: 'scale(0.8)', opacity: 0 }, { transform: 'scale(1)', opacity: 1 }],
      { duration: 300, easing: 'ease-out' }
    );
    return () => anim.cancel();
  }}
>
  <Component />
</ViewTransition>
```

The `instance` object: `instance.old`, `instance.new`, `instance.group`, `instance.imagePair`, `instance.name`.

The `types` array (second argument) lets you vary animation based on transition type.

---

## Animation Timing

| Interaction | Duration |
|------------|----------|
| Direct toggle (expand/collapse) | 100–200ms |
| Route transition (slide) | 150–250ms |
| Suspense reveal (skeleton → content) | 200–400ms |
| Shared element morph | 300–500ms |

---

## Troubleshooting

**VT not activating:** Ensure `<ViewTransition>` comes before any DOM node. Ensure state update is inside `startTransition`.

**"Two ViewTransition components with the same name":** Names must be globally unique. Use IDs: `name={`hero-${item.id}`}`.

**`router.back()` and browser back/forward skip animation:** Use `router.push()` with an explicit URL instead. See SKILL.md "router.back() and Browser Back Button."

**`flushSync` skips animations:** Use `startTransition` instead.

**Only updates animate (no enter/exit):** Without `<Suspense>`, React treats swaps as updates. Conditionally render the VT itself, or wrap in `<Suspense>`.

**Layout VT prevents page VTs from animating:** Nested VTs never fire enter/exit inside a parent VT. If your layout has a VT wrapping `{children}`, page-level enter/exit will silently not work. Remove the layout VT.

**List reorder not animating with `useOptimistic`:** Optimistic values resolve before snapshot. Use committed state for list order.

**TS error "Property 'default' is missing":** Type-keyed objects require a `default` key.

**Hash fragments cause scroll jumps:** Navigate without hash; scroll programmatically after navigation.

**Backdrop-blur flickers:** Use the backdrop-blur workaround from `css-recipes.md`.

**`border-radius` lost during transitions:** Apply `border-radius` directly to the captured element.

**Skeleton controls slide away:** Give matching controls the same `viewTransitionName`.

**Batching:** Multiple updates during animation are batched. A→B→C→D becomes B→D.