React Native Performance

Problem Statement

React Native performance issues often stem from unnecessary re-renders, unoptimized lists, and expensive computations on the JS thread. This codebase has performance-critical areas (shot mastery, player lists) with established optimization patterns.

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Pattern: FlatList Optimization

keyExtractor - Stable Keys

// ✅ CORRECT: Stable function reference
const keyExtractor = useCallback((item: Session) => item.id, []);

<FlatList
  data={sessions}
  keyExtractor={keyExtractor}
  renderItem={renderItem}
/>

// ❌ WRONG: Creates new function every render
<FlatList
  data={sessions}
  keyExtractor={(item) => item.id}
  renderItem={renderItem}
/>

// ❌ WRONG: Using index (causes issues with reordering/deletion)
keyExtractor={(item, index) => `${index}`}

getItemLayout - Fixed Height Items

const ITEM_HEIGHT = 80;
const SEPARATOR_HEIGHT = 1;

const getItemLayout = useCallback(
  (data: Session[] | null | undefined, index: number) => ({
    length: ITEM_HEIGHT,
    offset: (ITEM_HEIGHT + SEPARATOR_HEIGHT) * index,
    index,
  }),
  []
);

<FlatList
  data={sessions}
  getItemLayout={getItemLayout}
  // ... other props
/>

Why it matters: Without getItemLayout, FlatList must measure each item, causing scroll jank.

renderItem - Memoized

// Extract to named component
const SessionItem = memo(function SessionItem({ 
  session, 
  onPress 
}: { 
  session: Session; 
  onPress: (id: string) => void;
}) {
  return (
    <Pressable onPress={() => onPress(session.id)}>
      <Text>{session.title}</Text>
    </Pressable>
  );
});

// Stable callback
const handlePress = useCallback((id: string) => {
  navigation.push(`/session/${id}`);
}, [navigation]);

// Stable renderItem
const renderItem = useCallback(
  ({ item }: { item: Session }) => (
    <SessionItem session={item} onPress={handlePress} />
  ),
  [handlePress]
);

<FlatList
  data={sessions}
  renderItem={renderItem}
  // ...
/>

Additional Optimizations

<FlatList
  data={sessions}
  renderItem={renderItem}
  keyExtractor={keyExtractor}
  getItemLayout={getItemLayout}
  
  // Performance props
  removeClippedSubviews={true}           // Unmount off-screen items
  maxToRenderPerBatch={10}               // Items per render batch
  windowSize={5}                         // Render window (screens)
  initialNumToRender={10}                // Initial render count
  updateCellsBatchingPeriod={50}         // Batch update delay (ms)
  
  // Prevent extra renders
  extraData={selectedId}                 // Only re-render when this changes
/>

---

Pattern: FlashList for Large Lists

When to use: 1000+ items, complex item components, or FlatList still janky.

import { FlashList } from '@shopify/flash-list';

<FlashList
  data={players}
  renderItem={renderItem}
  estimatedItemSize={80}  // Required - estimate item height
  keyExtractor={keyExtractor}
/>

Note: This codebase doesn't currently use FlashList. Consider for coach player lists.

---

Pattern: Memoization

useMemo - Expensive Computations

// ✅ CORRECT: Memoize expensive calculation
const sortedAndFilteredItems = useMemo(() => {
  return items
    .filter(item => item.active)
    .sort((a, b) => b.score - a.score)
    .slice(0, 100);
}, [items]);

// ❌ WRONG: Recalculates every render
const sortedAndFilteredItems = items
  .filter(item => item.active)
  .sort((a, b) => b.score - a.score);

// ❌ WRONG: Memoizing simple access (overhead > benefit)
const userName = useMemo(() => user.name, [user.name]);

When to use useMemo:

• Array transformations (filter, sort, map chains)
• Object creation passed to memoized children
• Computations with O(n) or higher complexity

useCallback - Stable Function References

// ✅ CORRECT: Stable callback for child props
const handlePress = useCallback((id: string) => {
  setSelectedId(id);
}, []);

// Pass to memoized child
<MemoizedItem onPress={handlePress} />

// ❌ WRONG: useCallback with unstable deps
const handlePress = useCallback((id: string) => {
  doSomething(unstableObject); // unstableObject changes every render
}, [unstableObject]); // Defeats the purpose

When to use useCallback:

• Callbacks passed to memoized children
• Callbacks in dependency arrays
• Event handlers that would cause child re-renders

---

Pattern: React.memo

// Wrap components that receive stable props
const PlayerCard = memo(function PlayerCard({ 
  player, 
  onSelect 
}: Props) {
  return (
    <Pressable onPress={() => onSelect(player.id)}>
      <Text>{player.name}</Text>
      <Text>{player.rating}</Text>
    </Pressable>
  );
});

// Custom comparison for complex props
const PlayerCard = memo(
  function PlayerCard({ player, onSelect }: Props) {
    // ...
  },
  (prevProps, nextProps) => {
    // Return true if props are equal (skip re-render)
    return (
      prevProps.player.id === nextProps.player.id &&
      prevProps.player.rating === nextProps.player.rating
    );
  }
);

When to use React.memo:

• List item components
• Components receiving stable primitive props
• Components that render frequently but rarely change

When NOT to use:

• Components that always receive new props
• Simple components (overhead > benefit)
• Root-level screens

---

Pattern: Zustand Selector Optimization

Problem: Selecting entire store causes re-render on any state change.

// ❌ WRONG: Re-renders on ANY store change
const store = useAssessmentStore();
// or
const { userAnswers, isLoading, retakeAreas, ... } = useAssessmentStore();

// ✅ CORRECT: Only re-renders when selected values change
const userAnswers = useAssessmentStore((s) => s.userAnswers);
const isLoading = useAssessmentStore((s) => s.isLoading);

// ✅ CORRECT: Multiple values with shallow comparison
import { useShallow } from 'zustand/react/shallow';

const { userAnswers, isLoading } = useAssessmentStore(
  useShallow((s) => ({ 
    userAnswers: s.userAnswers, 
    isLoading: s.isLoading 
  }))
);

See also: rn-zustand-patterns/SKILL.md for more Zustand patterns.

---

Pattern: Image Optimization

import { Image } from 'expo-image';

// expo-image provides caching and performance optimizations
<Image
  source={{ uri: player.avatarUrl }}
  style={{ width: 50, height: 50 }}
  contentFit="cover"
  placeholder={blurhash}           // Show while loading
  transition={200}                  // Fade in duration
  cachePolicy="memory-disk"         // Cache strategy
/>

// For lists, add priority
<Image
  source={{ uri: player.avatarUrl }}
  priority={isVisible ? 'high' : 'low'}
/>

---

Pattern: Avoiding Re-Renders

Object/Array Stability

// ❌ WRONG: New object every render
<ChildComponent style={{ padding: 10 }} />
<ChildComponent config={{ enabled: true }} />

// ✅ CORRECT: Stable reference
const style = useMemo(() => ({ padding: 10 }), []);
const config = useMemo(() => ({ enabled: true }), []);

<ChildComponent style={style} />
<ChildComponent config={config} />

// ✅ CORRECT: Or use StyleSheet
const styles = StyleSheet.create({
  container: { padding: 10 },
});

<ChildComponent style={styles.container} />

Children Stability

// ❌ WRONG: Inline function creates new element each render
<Parent>
  {() => <Child />}
</Parent>

// ✅ CORRECT: Stable element
const child = useMemo(() => <Child />, [deps]);
<Parent>{child}</Parent>

---

Pattern: Detecting Re-Renders

React DevTools Profiler

1. Open React DevTools
2. Go to Profiler tab
3. Click record, interact, stop
4. Review "Flamegraph" for render times
5. Look for components rendering unnecessarily

why-did-you-render

// Setup in development
import React from 'react';

if (__DEV__) {
  const whyDidYouRender = require('@welldone-software/why-did-you-render');
  whyDidYouRender(React, {
    trackAllPureComponents: true,
  });
}

// Mark specific component for tracking
PlayerCard.whyDidYouRender = true;

Console Logging

// Quick check for re-renders
function PlayerCard({ player }: Props) {
  console.log('PlayerCard render:', player.id);
  // ...
}

---

Pattern: Heavy Computation Off Main Thread

Problem: JS thread blocked causes UI jank.

// ❌ WRONG: Blocks JS thread
const result = heavyComputation(data); // Takes 500ms

// ✅ CORRECT: Use InteractionManager
import { InteractionManager } from 'react-native';

InteractionManager.runAfterInteractions(() => {
  const result = heavyComputation(data);
  setResult(result);
});

// ✅ CORRECT: requestAnimationFrame for visual updates
requestAnimationFrame(() => {
  // Update after current frame
});

---

Performance Checklist

Before shipping list-heavy screens:

• FlatList has keyExtractor (stable callback)
• FlatList has getItemLayout (if fixed height)
• List items are memoized with React.memo
• Callbacks passed to items use useCallback
• Zustand selectors are specific (not whole store)
• Images use expo-image with caching
• No inline object/function props to memoized children
• Profiler shows no unnecessary re-renders

---

Common Issues

Issue	Solution
List scroll jank	Add getItemLayout, memoize items
Component re-renders too often	Check selector specificity, memoize props
Slow initial render	Reduce initialNumToRender, defer computation
Memory growing	Check for state accumulation, image cache
UI freezes on interaction	Move computation off main thread

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Relationship to Other Skills

• rn-zustand-patterns: Selector optimization patterns
• rn-styling: StyleSheet.create for stable style references