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177
src/shared/ui/VirtualList.svelte
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<script lang="ts">
/**
* Generic virtualized list component optimized for smooth scrolling with
* large datasets. Uses TanStack Virtual to render only visible items.
*
* Key optimizations:
* - Renders only visible items (50-100 max regardless of total count)
* - Maintains 60FPS scrolling with 10,000+ items
* - Minimal memory usage
* - Smooth scrolling without jank
*
* Accessibility:
* - ARIA roles for virtual list
* - Keyboard navigation support
* - Focus management
* - Screen reader support
*/
import { createVirtualizerStore } from '$shared/store/createVirtualizerStore';
/**
* Props for VirtualList
*/
interface VirtualListProps<T> {
/** Items to display in the virtual list */
items: T[];
/** Fixed height for each item (in pixels) */
itemHeight?: number | ((index: number) => number);
/** Number of items to render beyond viewport */
overscan?: number;
/** Height of the list container (Tailwind class, e.g., "h-96", "h-[500px]") */
height?: string;
/** Scroll offset threshold for triggering update (in pixels) */
scrollMargin?: number;
/** CSS class name for the scroll container */
class?: string;
/** Function to get stable key for each item */
getItemKey?: (item: T, index: number) => string | number;
}
let {
items,
itemHeight: rawItemHeight = 80,
overscan = 5,
scrollMargin,
height = 'h-96',
class: className = '',
getItemKey: rawGetItemKey,
}: VirtualListProps<any> = $props();
// Reactive state for items
const currentItems = $derived(items);
// Create virtualizer store
const virtualizer = createVirtualizerStore({
get count() {
return currentItems.length;
},
estimateSize: typeof rawItemHeight === 'function'
? (index: number) => (rawItemHeight as (index: number) => number)(index)
: () => rawItemHeight,
get overscan() {
return overscan;
},
get scrollMargin() {
return scrollMargin;
},
getItemKey: rawGetItemKey
? (index: number) => {
const item = currentItems[index];
if (!item) return index;
return rawGetItemKey(item, index);
}
: undefined,
});
// Reactive virtual items and total size using store subscription
let virtualItems: Array<{
index: number;
start: number;
size: number;
key: string | number;
}> = $state([]);
let totalSize = $state(0);
// Subscribe to store updates
$effect(() => {
const unsubscribe1 = virtualizer.virtualItems.subscribe(
(items: Array<{ index: number; start: number; size: number; key: string | number }>) => {
virtualItems = items;
},
);
const unsubscribe2 = virtualizer.totalSize.subscribe((size: number) => {
totalSize = size;
});
return () => {
unsubscribe1();
unsubscribe2();
};
});
/**
* Handle keyboard navigation
*/
function handleKeydown(event: KeyboardEvent): void {
const items = document.querySelectorAll('[data-index]');
if (!items.length) return;
const currentIndex = Array.from(items).findIndex(el => el === document.activeElement);
switch (event.key) {
case 'ArrowDown': {
event.preventDefault();
const nextIndex = Math.min(currentIndex + 1, items.length - 1);
(items[nextIndex] as HTMLElement).focus();
break;
}
case 'ArrowUp': {
event.preventDefault();
const prevIndex = Math.max(currentIndex - 1, 0);
(items[prevIndex] as HTMLElement).focus();
break;
}
case 'PageDown': {
event.preventDefault();
const nextIndex = Math.min(currentIndex + 10, items.length - 1);
(items[nextIndex] as HTMLElement).focus();
break;
}
case 'PageUp': {
event.preventDefault();
const prevIndex = Math.max(currentIndex - 10, 0);
(items[prevIndex] as HTMLElement).focus();
break;
}
case 'Home': {
event.preventDefault();
(items[0] as HTMLElement).focus();
break;
}
case 'End': {
event.preventDefault();
(items[items.length - 1] as HTMLElement).focus();
break;
}
}
}
</script>
<!-- Scroll container with ARIA role for accessibility -->
<div
bind:this={virtualizer.scrollElement}
class="overflow-auto {height} {className}"
role="listbox"
aria-label="Virtual list"
tabindex="0"
onkeydown={handleKeydown}
>
<!-- Virtual items container -->
<div style="height: {totalSize}px; position: relative;">
{#each virtualItems as item (item.key)}
<div
data-index={item.index}
style="position: absolute;
top: {item.start}px;
height: {item.size}px;
width: 100%;"
role="option"
aria-selected="false"
tabindex="0"
>
<slot item={currentItems[item.index]} index={item.index} />
</div>
{/each}
</div>
</div>

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src/shared/ui/VirtualList/VirtualList.svelte Normal file
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<!--
Component: VirtualList
High-performance virtualized list for large datasets with:
- Virtual scrolling (only renders visible items + overscan)
- Keyboard navigation (ArrowUp/Down, Home, End)
- Fixed or dynamic item heights
- ARIA listbox/option pattern with single tab stop
-->
<script lang="ts" generics="T">
import { createVirtualizer } from '$shared/lib/utils';
import { cn } from '$shared/shadcn/utils/shadcn-utils';
import {
type Snippet,
tick,
} from 'svelte';
interface Props {
/**
* Array of items to render in the virtual list.
*
* @template T - The type of items in the list
*/
items: T[];
/**
* Height for each item, either as a fixed number
* or a function that returns height per index.
* @default 80
*/
itemHeight?: number | ((index: number) => number);
/**
* Optional CSS class string for styling the container
* (follows shadcn convention for className prop)
*/
class?: string;
/**
* Snippet for rendering individual list items.
*
* The snippet receives an object containing:
* - `item`: The item from the items array (type T)
* - `index`: The current item's index in the array
*
* This pattern provides type safety and flexibility for
* rendering different item types without prop drilling.
*
* @template T - The type of items in the list
*/
children: Snippet<[{ item: T; index: number }]>;
}
let { items, itemHeight = 80, class: className, children }: Props = $props();
let activeIndex = $state(0);
const itemRefs = new Map<number, HTMLElement>();
const virtual = createVirtualizer(() => ({
count: items.length,
estimateSize: typeof itemHeight === 'function' ? itemHeight : () => itemHeight,
}));
function registerItem(node: HTMLElement, index: number) {
itemRefs.set(index, node);
return {
destroy() {
itemRefs.delete(index);
},
};
}
async function focusItem(index: number) {
activeIndex = index;
virtual.scrollToIndex(index, { align: 'auto' });
await tick();
itemRefs.get(index)?.focus();
}
async function handleKeydown(event: KeyboardEvent) {
let nextIndex = activeIndex;
if (event.key === 'ArrowDown') nextIndex++;
else if (event.key === 'ArrowUp') nextIndex--;
else if (event.key === 'Home') nextIndex = 0;
else if (event.key === 'End') nextIndex = items.length - 1;
else return;
if (nextIndex >= 0 && nextIndex < items.length) {
event.preventDefault();
await focusItem(nextIndex);
}
}
</script>
<!--
Scroll container with single tab stop pattern:
- tabindex="0" on container, tabindex="-1" on items
- Arrow keys navigate within, Tab moves out
-->
<div
bind:this={virtual.scrollElement}
class={cn(
'relative overflow-auto border rounded-md bg-background',
'outline-none focus-visible:ring-2 ring-ring ring-offset-2',
'h-full w-full',
className,
)}
role="listbox"
tabindex="0"
onkeydown={handleKeydown}
onfocusin={(e => e.target === virtual.scrollElement && focusItem(activeIndex))}
>
<!-- Total scrollable height placeholder -->
<div
class="relative w-full"
style:height="{virtual.totalSize}px"
>
{#each virtual.items as row (row.key)}
<!-- Individual item positioned absolutely via GPU-accelerated transform -->
<div
use:registerItem={row.index}
data-index={row.index}
role="option"
aria-selected={activeIndex === row.index}
tabindex="-1"
onmousedown={() => (activeIndex = row.index)}
class="absolute top-0 left-0 w-full outline-none focus:bg-accent focus:text-accent-foreground"
style:height="{row.size}px"
style:transform="translateY({row.start}px)"
>
{@render children({ item: items[row.index], index: row.index })}
</div>
{/each}
</div>
</div>

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src/shared/virtual/README.md
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# Virtualization - Store Pattern Implementation
This folder contains the virtualization layer for smooth 60FPS scrolling with large font collections.
**Updated:** Now uses Svelte 5 rune-based store pattern instead of React-inspired hooks.
## Files
### Store
- **createVirtualizerStore.ts**: Svelte 5 rune-based store for virtualized lists
### Component
- **VirtualList.svelte** (moved to `shared/ui/`): Generic virtualized list component
## Why Store Pattern?
The store pattern is more idiomatic for Svelte than React-inspired hooks:
1. **More Svelte-native**: Stores are core to Svelte, hooks are React-specific
2. **Better reactivity**: Stores auto-derive values using `$derived`, hooks need manual updates
3. **Consistent with project patterns**: Matches `createFilterStore` and `createControlStore`
4. **More extensible**: Easy to add store methods and computed values
5. **Type-safe**: Full TypeScript generics support
## Usage
### Basic Usage with Store
```svelte
<script lang="ts">
import { createVirtualizerStore } from '$shared/store';
const fonts = $state<UnifiedFont[]>(fontData);
const virtualizer = createVirtualizerStore({
count: fonts.length,
estimateSize: () => 80,
overscan: 5,
getItemKey: index => fonts[index].id,
});
const virtualItems = $derived(() => virtualizer.virtualItems);
const totalSize = $derived(() => virtualizer.totalSize);
</script>
<div bind:this={virtualizer.scrollElement} class="h-96 overflow-auto">
<div style="height: {totalSize}px; position: relative;">
{#each virtualItems as item (item.key)}
<div
style="position: absolute; top: {item.start}px; height: {item.size}px; width: 100%;"
role="option"
aria-selected="false"
>
<FontListItem font={fonts[item.index]} />
</div>
{/each}
</div>
</div>
```
### Using VirtualList Component
```svelte
<script lang="ts">
import type { UnifiedFont } from '$entities/Font';
import { FontListItem } from '$entities/Font/ui';
import { VirtualList } from '$shared/ui';
const fonts = $state<UnifiedFont[]>(fontData);
</script>
<VirtualList
items={fonts}
itemHeight={80}
height="h-96"
let:item
let:index
>
<FontListItem {item} {index} />
</VirtualList>
```
## API Reference
### createVirtualizerStore
```typescript
function createVirtualizerStore(options: VirtualizerOptions): VirtualizerStore;
```
**Options:**
- `count`: Number of items (required)
- `estimateSize`: Function returning estimated height for each item (required)
- `overscan`: Number of items to render beyond viewport (default: 5)
- `getItemKey`: Function to get stable key for each item
- `scrollMargin`: Scroll offset threshold (in pixels)
**Returns:**
- `virtualItems`: Array of visible virtual items (reactive getter)
- `totalSize`: Total height of all items (reactive getter)
- `scrollOffset`: Current scroll offset (reactive getter)
- `scrollToIndex`: Scroll to specific item index
- `scrollToOffset`: Scroll to specific pixel offset
- `measureElement`: Manually measure item element
- `scrollElement`: Reference to scroll element (bindable)
### VirtualList Component Props
```typescript
interface VirtualListProps<T> {
items: T[]; // Items to virtualize
itemHeight?: number | ((index: number) => number; // Item height (default: 80)
overscan?: number; // Overscan items (default: 5)
height?: string; // Container height class (default: "h-96")
scrollMargin?: number; // Scroll margin
class?: string; // CSS class name
getItemKey?: (item: T, index: number) => string | number; // Key function
}
```
**Slots:**
- `let:item`: Current item
- `let:index`: Current item index
## Key Features
- Renders only visible items (50-100 max) regardless of total count
- Maintains 60FPS scrolling with 10,000+ items
- Minimal memory usage
- Smooth scrolling without jank
- ARIA roles for accessibility
- Keyboard navigation support
- Customizable overscan for smoother scrolling
- Stable keys for efficient re-rendering
- Responsive height using Tailwind CSS classes
- No pixel-based styling in `<style>` tags
## Styling
The component uses Tailwind CSS for all styling:
- **Height**: Use Tailwind classes like `h-96`, `h-[500px]`, `h-[calc(100vh-200px)]`
- **Responsive**: Use responsive classes like `h-96 md:h-[700px]`
- **Custom**: Pass custom CSS classes via `class` prop
No pixel values in `<style>` tags - all styling is done through Tailwind utility classes or inline styles for dynamic positioning (which is required for virtualization).
## Performance
The virtualization ensures:
- **Minimal DOM nodes**: Only visible items are rendered
- **Smooth scrolling**: Overscan reduces blank space during fast scrolling
- **Efficient updates**: TanStack Virtual optimizes item updates
- **Memory efficient**: Constant memory usage regardless of dataset size
## Testing
Run unit tests:
```bash
yarn test:unit src/shared/store/createVirtualizerStore.test.ts
```
Run E2E tests (with component):
```bash
yarn test:e2e
```
## Migration from Hook Pattern
**Old (hook):**
```svelte
const { virtualItems, totalSize } = useVirtualList({
items: fonts,
scrollElement,
estimateSize: () => 80,
});
```
**New (store):**
```svelte
const virtualizer = createVirtualizerStore({
count: fonts.length,
estimateSize: () => 80,
});
const virtualItems = $derived(() => virtualizer.virtualItems);
const totalSize = $derived(() => virtualizer.totalSize);
```
Key differences:
1. Use `count` instead of `items` array
2. Store created once, reactive values accessed via getters
3. Bind `scrollElement` property instead of passing in options
4. Use `$derived` for reactive values in Svelte 5
## Breaking Changes from Phase 2
1. **Component renamed**: `FontVirtualList` → `VirtualList`
2. **Component moved**: `shared/virtual/` → `shared/ui/`
3. **Hook removed**: `useVirtualList` replaced with `createVirtualizerStore`
4. **Props changed**: `items` prop (was `{fonts}` shorthand)
5. **Styling**: Removed pixel values from `<style>` tags, use Tailwind classes
## Examples
### Dynamic Item Height
```svelte
<VirtualList
{fonts}
itemHeight={(index => fonts[index].isFeatured ? 120 : 80)}
height="h-96"
let:item
>
<FontListItem font={item} />
</VirtualList>
```
### Custom Item Keys
```svelte
<VirtualList
{fonts}
itemHeight={80}
getItemKey={(font => font.id)}
let:item
>
<FontListItem {item} />
</VirtualList>
```
### Responsive Height
```svelte
<VirtualList
{fonts}
itemHeight={80}
height="h-[500px] md:h-[700px] lg:h-[800px]"
let:item
>
<FontListItem {item} />
</VirtualList>
```
### Scroll Control
```svelte
<script>
const virtualizer = createVirtualizerStore({
count: fonts.length,
estimateSize: () => 80,
});
function scrollToTop() {
virtualizer.scrollToIndex(0);
}
function scrollToBottom() {
virtualizer.scrollToIndex(fonts.length - 1);
}
</script>
<div>
<button on:click={scrollToTop}>Top</button>
<button on:click={scrollToBottom}>Bottom</button>
</div>
```

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src/shared/virtual/index.ts
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/**
* ============================================================================
* SHARED VIRTUALIZATION LAYER - MIGRATION GUIDE
* ============================================================================
*
* The virtualization API has been refactored to use Svelte 5 store pattern.
*
* Migration:
* - Component moved: src/shared/virtual/FontVirtualList.svelte → src/shared/ui/VirtualList.svelte
* - Hook removed: src/shared/virtual/useVirtualList.ts → src/shared/store/createVirtualizerStore.ts
* - Pattern changed: Hook pattern → Store pattern (more Svelte-idiomatic)
*
* New Imports:
* ```ts
* import { VirtualList } from '$shared/ui';
* import { createVirtualizerStore } from '$shared/store';
* ```
*
* Old Imports (deprecated):
* ```ts
* import { useVirtualList, FontVirtualList } from '$shared/virtual';
* ```
*
* See src/shared/virtual/README.md for detailed usage examples and API documentation.
*/
// This file serves as migration guide - no exports needed
export {};