feat: split updating state to batches on long lists

src/TabBar.tsx

@@ -11,6 +11,7 @@ import {
   Platform,
   FlatList,
   ListRenderItemInfo,
+  ViewToken,
 } from 'react-native';
 import TabBarItem, { Props as TabBarItemProps } from './TabBarItem';
 import TabBarIndicator, { Props as IndicatorProps } from './TabBarIndicator';
@@ -23,6 +24,7 @@ import type {
   Event,
 } from './types';
 import useAnimatedValue from './useAnimatedValue';
+import useLatestCallback from 'use-latest-callback';
 
 export type Props<T extends Route> = SceneRendererProps & {
   navigationState: NavigationState<T>;
@@ -247,6 +249,10 @@ const renderIndicatorDefault = (props: IndicatorProps<Route>) => (
 
 const getTestIdDefault = ({ route }: Scene<Route>) => route.testID;
 
+// How many items measurements should we update per batch.
+// Defaults to 10, since that's whats FlatList is using in initialNumToRender.
+const MEASURE_PER_BATCH = 10;
+
 export default function TabBar<T extends Route>({
   getLabelText = getLabelTextDefault,
   getAccessible = getAccessibleDefault,
@@ -279,7 +285,7 @@ export default function TabBar<T extends Route>({
 }: Props<T>) {
   const [layout, setLayout] = React.useState<Layout>({ width: 0, height: 0 });
   const [tabWidths, setTabWidths] = React.useState<Record<string, number>>({});
-  const flatListRef = React.useRef<FlatList>(null);
+  const flatListRef = React.useRef<FlatList | null>(null);
   const isFirst = React.useRef(true);
   const scrollAmount = useAnimatedValue(0);
   const measuredTabWidths = React.useRef<Record<string, number>>({});
@@ -296,28 +302,19 @@ export default function TabBar<T extends Route>({
     flattenedTabWidth,
   });
 
-  const hasMeasuredTabWidths =
-    Boolean(layout.width) &&
-    routes.every((r) => typeof tabWidths[r.key] === 'number');
-
   React.useEffect(() => {
     if (isFirst.current) {
       isFirst.current = false;
       return;
     }
 
-    if (isWidthDynamic && !hasMeasuredTabWidths) {
-      // When tab width is dynamic, only adjust the scroll once we have all tab widths and layout
-      return;
-    }
-
     if (scrollEnabled) {
       flatListRef.current?.scrollToOffset({
         offset: scrollOffset,
         animated: true,
       });
     }
-  }, [hasMeasuredTabWidths, isWidthDynamic, scrollEnabled, scrollOffset]);
+  }, [scrollEnabled, scrollOffset]);
 
   const handleLayout = (e: LayoutChangeEvent) => {
     const { height, width } = e.nativeEvent.layout;
@@ -373,13 +370,24 @@ export default function TabBar<T extends Route>({
           ? (e: LayoutChangeEvent) => {
               measuredTabWidths.current[route.key] = e.nativeEvent.layout.width;
 
-              // When we have measured widths for all of the tabs, we should updates the state
-              // We avoid doing separate setState for each layout since it triggers multiple renders and slows down app
+              // If we have more than 10 routes divide updating tabWidths into multiple batches. Here we update only first batch of 10 items.
               if (
+                routes.length > MEASURE_PER_BATCH &&
+                index === MEASURE_PER_BATCH &&
+                routes
+                  .slice(0, MEASURE_PER_BATCH)
+                  .every(
+                    (r) => typeof measuredTabWidths.current[r.key] === 'number'
+                  )
+              ) {
+                setTabWidths({ ...measuredTabWidths.current });
+              } else if (
                 routes.every(
                   (r) => typeof measuredTabWidths.current[r.key] === 'number'
                 )
               ) {
+                // When we have measured widths for all of the tabs, we should updates the state
+                // We avoid doing separate setState for each layout since it triggers multiple renders and slows down app
                 setTabWidths({ ...measuredTabWidths.current });
               }
             }
@@ -494,6 +502,25 @@ export default function TabBar<T extends Route>({
     [scrollAmount]
   );
 
+  const handleViewableItemsChanged = useLatestCallback(
+    ({ changed }: { changed: ViewToken[] }) => {
+      if (routes.length <= MEASURE_PER_BATCH) {
+        return;
+      }
+      // Get next vievable item
+      const item = changed[changed.length - 1];
+      const index = item?.index || 0;
+      if (
+        item.isViewable &&
+        (index % 10 === 0 ||
+          index === navigationState.index ||
+          index === routes.length - 1)
+      ) {
+        setTabWidths({ ...measuredTabWidths.current });
+      }
+    }
+  );
+
   return (
     <Animated.View onLayout={handleLayout} style={[styles.tabBar, style]}>
       <Animated.View
@@ -535,6 +562,7 @@ export default function TabBar<T extends Route>({
           data={routes as Animated.WithAnimatedValue<T>[]}
           keyExtractor={keyExtractor}
           horizontal
+          initialNumToRender={MEASURE_PER_BATCH}
           accessibilityRole="tablist"
           keyboardShouldPersistTaps="handled"
           scrollEnabled={scrollEnabled}
@@ -549,6 +577,7 @@ export default function TabBar<T extends Route>({
           scrollEventThrottle={16}
           renderItem={renderItem}
           onScroll={handleScroll}
+          onViewableItemsChanged={handleViewableItemsChanged}
           ref={flatListRef}
           testID={testID}
         />

src/TabBarIndicator.tsx

@@ -59,17 +59,20 @@ export default function TabBarIndicator<T extends Route>({
 
   const opacity = useAnimatedValue(isWidthDynamic ? 0 : 1);
 
-  const hasMeasuredTabWidths =
-    Boolean(layout.width) &&
-    navigationState.routes.every((_, i) => getTabWidth(i));
+  const indicatorVisible = isWidthDynamic
+    ? layout.width &&
+      navigationState.routes
+        .slice(0, navigationState.index)
+        .every((_, r) => getTabWidth(r))
+    : true;
 
   React.useEffect(() => {
     const fadeInIndicator = () => {
       if (
         !isIndicatorShown.current &&
         isWidthDynamic &&
         // We should fade-in the indicator when we have widths for all the tab items
-        hasMeasuredTabWidths
+        indicatorVisible
       ) {
         isIndicatorShown.current = true;
 
@@ -85,7 +88,7 @@ export default function TabBarIndicator<T extends Route>({
     fadeInIndicator();
 
     return () => opacity.stopAnimation();
-  }, [hasMeasuredTabWidths, isWidthDynamic, opacity]);
+  }, [indicatorVisible, isWidthDynamic, opacity]);
 
   const { routes } = navigationState;