perf-virtual-lists（性能虚拟列表）

Performance: Virtualized Lists on TV

Rendering large lists (movies, episodes) on Android TV requires virtualization to maintain 60FPS. Standard map() rendering will freeze the UI on low-end TV boxes (1GB RAM).

🚀 Core Technology

Use react-window + react-virtualized-auto-sizer. It is lighter and faster than react-virtualized.

📺 TV-Specific Optimizations

1. Overscan is Critical

On TV, users scroll FAST (holding the Down button). You MUST set overscanCount (or overscanRowCount) to at least 2-3 rows to prevent "blank" areas during rapid scrolling.

<FixedSizeGrid
  overscanRowCount={3} // Prevents blank cells during fast scroll
  ...
/>

2. Focus Management Strategy

Problem: Virtualization unmounts off-screen items. If the focused item is scrolled out, focus is lost, and the browser resets it to body. Solution: Do NOT rely on DOM focus (document.activeElement) for state.

1. Use useTVNavigation to track focusedIndex (data index).
2. Programmatically scroll to the focused item *before* it gets unmounted? No, just keep the index.
3. When the item re-mounts (scrolled back properly), the ref callback should re-apply focus if needed, or simply render it as .focused.

3. Fixed vs Variable Size

ALWAYS prefer FixedSizeGrid or FixedSizeList. Variable sizing triggers expensive layout recalculations on every scroll frame, killing performance on TV hardware.

4. Image Handling

• Use loading="lazy" on <img>.
• Even better: Use a custom <FadeInImage> that only sets src when inView (though react-window handles visibility well).
• Crucial: Set explicit width and height on images to prevent layout shifts.

🛠 Usage Pattern

import { FixedSizeGrid } from 'react-window';
import AutoSizer from 'react-virtualized-auto-sizer';

// ... inside your component
<div style={{ height: '100vh', width: '100%' }}>
  <AutoSizer>
    {({ height, width }) => (
      <FixedSizeGrid
        columnCount={4}
        columnWidth={width / 4}
        height={height}
        rowCount={Math.ceil(items.length / 4)}
        rowHeight={300} // Fixed height!
        width={width}
        overscanRowCount={3} // CRITICAL FOR TV
      >
        {({ columnIndex, rowIndex, style }) => {
          const index = rowIndex * 4 + columnIndex;
          if (index >= items.length) return null;

          return (
            <div style={style}>
              <MovieCard
                 item={items[index]}
                 isFocused={index === focusedIndex}
              />
            </div>
          );
        }}
      </FixedSizeGrid>
    )}
  </AutoSizer>
</div>

⚠️ Anti-Patterns

• ❌ react-virtualized (standard): Too heavy for some older TVs.
• ❌ Inline Functions: itemData={items} is fine, but avoid defining the render function inline if possible (hoist it).
• ❌ Complex Cards: Keep the item component simple. Heavy CSS effects (blur, shadows) on 20 items at once will drop frames.