refactor(hooks, container): avoid reading .value outside worklet

This is a performance optimization to avoid blocking the JavaScript
thread.

BREAKING CHANGE: headerHeight, tabBarHeight, containerHeight, and
contentInset are no longer SharedValues.

If you consume useHeaderMeasurements and/or useTabsContext expect this
to impact you.

Author: AndreiCalazans

documentation/README_TEMPLATE.md

@@ -161,6 +161,8 @@ This hook returns the top distance and the header height. For an example of how
 const { top, height } = useHeaderMeasurements()
 ```
 
+Note top and height are numbers stored in a React state.
+
 ### useCurrentTabScrollY
 
 This hook returns the vertical scroll position of the current tab as an Animated SharedValue. 

example/src/AnimatedHeader.tsx

@@ -30,8 +30,8 @@ export const Header = () => {
         {
           translateY: interpolate(
             top.value,
-            [0, -(height.value || 0 - MIN_HEADER_HEIGHT)],
-            [0, (height.value || 0 - MIN_HEADER_HEIGHT) / 2]
+            [0, -(height || 0 - MIN_HEADER_HEIGHT)],
+            [0, (height || 0 - MIN_HEADER_HEIGHT) / 2]
           ),
         },
       ],

example/src/FlashList.tsx

@@ -30,8 +30,8 @@ export const Header = () => {
         {
           translateY: interpolate(
             top.value,
-            [0, -(height.value || 0 - MIN_HEADER_HEIGHT)],
-            [0, (height.value || 0 - MIN_HEADER_HEIGHT) / 2]
+            [0, -(height || 0 - MIN_HEADER_HEIGHT)],
+            [0, (height || 0 - MIN_HEADER_HEIGHT) / 2]
           ),
         },
       ],

example/src/MasonryFlashList.tsx

@@ -30,8 +30,8 @@ export const Header = () => {
         {
           translateY: interpolate(
             top.value,
-            [0, -(height.value || 0 - MIN_HEADER_HEIGHT)],
-            [0, (height.value || 0 - MIN_HEADER_HEIGHT) / 2]
+            [0, -(height || 0 - MIN_HEADER_HEIGHT)],
+            [0, (height || 0 - MIN_HEADER_HEIGHT) / 2]
           ),
         },
       ],

example/src/Shared/Contacts.tsx

@@ -101,11 +101,7 @@ const renderItem = ({ item }: { item: Item }) => <ContactItem item={item} />
 const ListEmptyComponent = () => {
   const { top, height } = Tabs.useHeaderMeasurements()
   const translateY = useDerivedValue(() => {
-    return interpolate(
-      -top.value,
-      [0, height.value || 0],
-      [-(height.value || 0) / 2, 0]
-    )
+    return interpolate(-top.value, [0, height || 0], [-(height || 0) / 2, 0])
   }, [height])
 
   const stylez = useAnimatedStyle(() => {

example/src/Shared/ContactsFlashList.tsx

@@ -101,11 +101,7 @@ const renderItem = ({ item }: { item: Item }) => <ContactItem item={item} />
 const ListEmptyComponent = () => {
   const { top, height } = Tabs.useHeaderMeasurements()
   const translateY = useDerivedValue(() => {
-    return interpolate(
-      -top.value,
-      [0, height.value || 0],
-      [-(height.value || 0) / 2, 0]
-    )
+    return interpolate(-top.value, [0, height || 0], [-(height || 0) / 2, 0])
   }, [height])
 
   const stylez = useAnimatedStyle(() => {

example/src/Shared/ExampleMasonry.tsx

@@ -69,11 +69,7 @@ const ItemSeparator = () => <View style={styles.separator} />
 const ListEmptyComponent = () => {
   const { top, height } = Tabs.useHeaderMeasurements()
   const translateY = useDerivedValue(() => {
-    return interpolate(
-      -top.value,
-      [0, height.value || 0],
-      [-(height.value || 0) / 2, 0]
-    )
+    return interpolate(-top.value, [0, height || 0], [-(height || 0) / 2, 0])
   }, [height])
 
   const stylez = useAnimatedStyle(() => {

example/src/Shared/SectionContacts.tsx

@@ -117,11 +117,7 @@ const renderItem = ({ item }: { item: Item }) => <ContactItem item={item} />
 const ListEmptyComponent = () => {
   const { top, height } = Tabs.useHeaderMeasurements()
   const translateY = useDerivedValue(() => {
-    return interpolate(
-      -top.value,
-      [0, height.value || 0],
-      [-(height.value || 0) / 2, 0]
-    )
+    return interpolate(-top.value, [0, height || 0], [-(height || 0) / 2, 0])
   }, [height])
 
   const stylez = useAnimatedStyle(() => {

src/Container.tsx

@@ -1,10 +1,5 @@
 import React from 'react'
-import {
-  LayoutChangeEvent,
-  StyleSheet,
-  useWindowDimensions,
-  View,
-} from 'react-native'
+import { StyleSheet, useWindowDimensions, View } from 'react-native'
 import PagerView from 'react-native-pager-view'
 import Animated, {
   runOnJS,
@@ -15,6 +10,7 @@ import Animated, {
   useSharedValue,
   withDelay,
   withTiming,
+  useFrameCallback,
 } from 'react-native-reanimated'
 
 import { Context, TabNameContext } from './Context'
@@ -27,6 +23,7 @@ import {
   useContainerRef,
   usePageScrollHandler,
   useTabProps,
+  useLayoutHeight,
 } from './hooks'
 import {
   CollapsibleProps,
@@ -93,25 +90,29 @@ export const Container = React.memo(
       const windowWidth = useWindowDimensions().width
       const width = customWidth ?? windowWidth
 
-      const containerHeight = useSharedValue<number | undefined>(undefined)
+      const [containerHeight, getContainerLayoutHeight] = useLayoutHeight()
 
-      const tabBarHeight = useSharedValue<number | undefined>(
-        initialTabBarHeight
-      )
+      const [tabBarHeight, getTabBarHeight] =
+        useLayoutHeight(initialTabBarHeight)
 
-      const headerHeight = useSharedValue<number | undefined>(
+      const [headerHeight, getHeaderHeight] = useLayoutHeight(
         !renderHeader ? 0 : initialHeaderHeight
       )
+      const initialIndex = React.useMemo(
+        () =>
+          initialTabName
+            ? tabNamesArray.findIndex((n) => n === initialTabName)
+            : 0,
+        [initialTabName, tabNamesArray]
+      )
 
-      const contentInset = useDerivedValue(() => {
+      const contentInset = React.useMemo(() => {
         if (allowHeaderOverscroll) return 0
 
         // necessary for the refresh control on iOS to be positioned underneath the header
         // this also adjusts the scroll bars to clamp underneath the header area
-        return IS_IOS
-          ? (headerHeight.value || 0) + (tabBarHeight.value || 0)
-          : 0
-      })
+        return IS_IOS ? (headerHeight || 0) + (tabBarHeight || 0) : 0
+      }, [headerHeight, tabBarHeight, allowHeaderOverscroll])
 
       const snappingTo: ContextType['snappingTo'] = useSharedValue(0)
       const offset: ContextType['offset'] = useSharedValue(0)
@@ -131,22 +132,16 @@ export const Container = React.memo(
         () => tabNamesArray,
         [tabNamesArray]
       )
-      const index: ContextType['index'] = useSharedValue(
-        initialTabName
-          ? tabNames.value.findIndex((n) => n === initialTabName)
-          : 0
-      )
+      const index: ContextType['index'] = useSharedValue(initialIndex)
 
       const focusedTab: ContextType['focusedTab'] =
         useDerivedValue<TabName>(() => {
           return tabNames.value[index.value]
         }, [tabNames])
-      const calculateNextOffset = useSharedValue(index.value)
+      const calculateNextOffset = useSharedValue(initialIndex)
       const headerScrollDistance: ContextType['headerScrollDistance'] =
         useDerivedValue(() => {
-          return headerHeight.value !== undefined
-            ? headerHeight.value - minHeaderHeight
-            : 0
+          return headerHeight !== undefined ? headerHeight - minHeaderHeight : 0
         }, [headerHeight, minHeaderHeight])
 
       const indexDecimal: ContextType['indexDecimal'] = useSharedValue(
@@ -167,7 +162,7 @@ export const Container = React.memo(
           scrollToImpl(
             refMap[name],
             0,
-            scrollYCurrent.value - contentInset.value,
+            scrollYCurrent.value - contentInset,
             false
           )
         }
@@ -213,6 +208,33 @@ export const Container = React.memo(
         [onIndexChange, onTabChange]
       )
 
+      const syncCurrentTabScrollPosition = () => {
+        'worklet'
+
+        const name = tabNamesArray[index.value]
+        scrollToImpl(
+          refMap[name],
+          0,
+          scrollYCurrent.value - contentInset,
+          false
+        )
+      }
+
+      /*
+       * We run syncCurrentTabScrollPosition in every frame after the index
+       * changes for about 1500ms because the Lists can be late to accept the
+       * scrollTo event we send. This fixes the issue of the scroll position
+       * jumping when the user changes tab.
+       * */
+      const toggleSyncScrollFrame = (toggle: boolean) =>
+        syncScrollFrame.setActive(toggle)
+      const syncScrollFrame = useFrameCallback(({ timeSinceFirstFrame }) => {
+        syncCurrentTabScrollPosition()
+        if (timeSinceFirstFrame > 1500) {
+          runOnJS(toggleSyncScrollFrame)(false)
+        }
+      }, false)
+
       useAnimatedReaction(
         () => {
           return calculateNextOffset.value
@@ -236,13 +258,14 @@ export const Container = React.memo(
               scrollYCurrent.value =
                 scrollY.value[tabNames.value[index.value]] || 0
             }
+            runOnJS(toggleSyncScrollFrame)(true)
           }
         },
         []
       )
 
       useAnimatedReaction(
-        () => headerHeight.value,
+        () => headerHeight,
         (_current, prev) => {
           if (prev === undefined) {
             // sync scroll if we started with undefined header height
@@ -267,32 +290,6 @@ export const Container = React.memo(
         }
       }, [revealHeaderOnScroll])
 
-      const getHeaderHeight = React.useCallback(
-        (event: LayoutChangeEvent) => {
-          const height = event.nativeEvent.layout.height
-          if (headerHeight.value !== height) {
-            headerHeight.value = height
-          }
-        },
-        [headerHeight]
-      )
-
-      const getTabBarHeight = React.useCallback(
-        (event: LayoutChangeEvent) => {
-          const height = event.nativeEvent.layout.height
-          if (tabBarHeight.value !== height) tabBarHeight.value = height
-        },
-        [tabBarHeight]
-      )
-
-      const onLayout = React.useCallback(
-        (event: LayoutChangeEvent) => {
-          const height = event.nativeEvent.layout.height
-          if (containerHeight.value !== height) containerHeight.value = height
-        },
-        [containerHeight]
-      )
-
       const onTabPress = React.useCallback(
         (name: TabName) => {
           const i = tabNames.value.findIndex((n) => n === name)
@@ -302,7 +299,7 @@ export const Container = React.memo(
             runOnUI(scrollToImpl)(
               ref,
               0,
-              headerScrollDistance.value - contentInset.value,
+              headerScrollDistance.value - contentInset,
               true
             )
           } else {
@@ -313,11 +310,14 @@ export const Container = React.memo(
         [containerRef, refMap, contentInset]
       )
 
-      React.useEffect(() => {
-        if (index.value >= tabNamesArray.length) {
-          onTabPress(tabNamesArray[tabNamesArray.length - 1])
+      useAnimatedReaction(
+        () => tabNamesArray.length,
+        (tabLength) => {
+          if (index.value >= tabLength) {
+            runOnJS(onTabPress)(tabNamesArray[tabLength - 1])
+          }
         }
-      }, [index.value, onTabPress, tabNamesArray])
+      )
 
       const pageScrollHandler = usePageScrollHandler({
         onPageScroll: (e) => {
@@ -381,7 +381,7 @@ export const Container = React.memo(
         >
           <Animated.View
             style={[styles.container, { width }, containerStyle]}
-            onLayout={onLayout}
+            onLayout={getContainerLayoutHeight}
             pointerEvents="box-none"
           >
             <Animated.View
@@ -430,7 +430,7 @@ export const Container = React.memo(
             <AnimatedPagerView
               ref={containerRef}
               onPageScroll={pageScrollHandler}
-              initialPage={index.value}
+              initialPage={initialIndex}
               {...pagerProps}
               style={[pagerProps?.style, StyleSheet.absoluteFill]}
             >

src/FlashList.tsx

@@ -11,7 +11,6 @@ import Animated, {
 import {
   useChainCallback,
   useCollapsibleStyle,
-  useConvertAnimatedToValue,
   useScrollHandlerY,
   useSharedAnimatedRef,
   useTabNameContext,
@@ -118,16 +117,14 @@ function FlashListImpl<R>(
     [progressViewOffset, refreshControl]
   )
 
-  const contentInsetValue = useConvertAnimatedToValue<number>(contentInset)
-
   const memoContentInset = React.useMemo(
-    () => ({ top: contentInsetValue }),
-    [contentInsetValue]
+    () => ({ top: contentInset }),
+    [contentInset]
   )
 
   const memoContentOffset = React.useMemo(
-    () => ({ x: 0, y: -contentInsetValue }),
-    [contentInsetValue]
+    () => ({ x: 0, y: -contentInset }),
+    [contentInset]
   )
 
   const memoContentContainerStyle = React.useMemo(