bsky-app/src/lib/custom-animations/CountWheel.tsx

import React from 'react'
import {View} from 'react-native'
import Animated, {
  Easing,
  LayoutAnimationConfig,
  useReducedMotion,
  withTiming,
} from 'react-native-reanimated'
import {i18n} from '@lingui/core'

import {decideShouldRoll} from 'lib/custom-animations/util'
import {s} from 'lib/styles'
import {formatCount} from 'view/com/util/numeric/format'
import {Text} from 'view/com/util/text/Text'
import {atoms as a, useTheme} from '#/alf'

const animationConfig = {
  duration: 400,
  easing: Easing.out(Easing.cubic),
}

function EnteringUp() {
  'worklet'
  const animations = {
    opacity: withTiming(1, animationConfig),
    transform: [{translateY: withTiming(0, animationConfig)}],
  }
  const initialValues = {
    opacity: 0,
    transform: [{translateY: 18}],
  }
  return {
    animations,
    initialValues,
  }
}

function EnteringDown() {
  'worklet'
  const animations = {
    opacity: withTiming(1, animationConfig),
    transform: [{translateY: withTiming(0, animationConfig)}],
  }
  const initialValues = {
    opacity: 0,
    transform: [{translateY: -18}],
  }
  return {
    animations,
    initialValues,
  }
}

function ExitingUp() {
  'worklet'
  const animations = {
    opacity: withTiming(0, animationConfig),
    transform: [
      {
        translateY: withTiming(-18, animationConfig),
      },
    ],
  }
  const initialValues = {
    opacity: 1,
    transform: [{translateY: 0}],
  }
  return {
    animations,
    initialValues,
  }
}

function ExitingDown() {
  'worklet'
  const animations = {
    opacity: withTiming(0, animationConfig),
    transform: [{translateY: withTiming(18, animationConfig)}],
  }
  const initialValues = {
    opacity: 1,
    transform: [{translateY: 0}],
  }
  return {
    animations,
    initialValues,
  }
}

export function CountWheel({
  likeCount,
  big,
  isLiked,
}: {
  likeCount: number
  big?: boolean
  isLiked: boolean
}) {
  const t = useTheme()
  const shouldAnimate = !useReducedMotion()
  const shouldRoll = decideShouldRoll(isLiked, likeCount)

  // Incrementing the key will cause the `Animated.View` to re-render, with the newly selected entering/exiting
  // animation
  // The initial entering/exiting animations will get skipped, since these will happen on screen mounts and would
  // be unnecessary
  const [key, setKey] = React.useState(0)
  const [prevCount, setPrevCount] = React.useState(likeCount)
  const prevIsLiked = React.useRef(isLiked)
  const formattedCount = formatCount(i18n, likeCount)
  const formattedPrevCount = formatCount(i18n, prevCount)

  React.useEffect(() => {
    if (isLiked === prevIsLiked.current) {
      return
    }

    const newPrevCount = isLiked ? likeCount - 1 : likeCount + 1
    setKey(prev => prev + 1)
    setPrevCount(newPrevCount)
    prevIsLiked.current = isLiked
  }, [isLiked, likeCount])

  const enteringAnimation =
    shouldAnimate && shouldRoll
      ? isLiked
        ? EnteringUp
        : EnteringDown
      : undefined
  const exitingAnimation =
    shouldAnimate && shouldRoll
      ? isLiked
        ? ExitingUp
        : ExitingDown
      : undefined

  return (
    <LayoutAnimationConfig skipEntering skipExiting>
      {likeCount > 0 ? (
        <View style={[a.justify_center]}>
          <Animated.View entering={enteringAnimation} key={key}>
            <Text
              testID="likeCount"
              style={[
                big ? a.text_md : {fontSize: 15},
                a.user_select_none,
                isLiked
                  ? [a.font_bold, s.likeColor]
                  : {color: t.palette.contrast_500},
              ]}>
              {formattedCount}
            </Text>
          </Animated.View>
          {shouldAnimate && (likeCount > 1 || !isLiked) ? (
            <Animated.View
              entering={exitingAnimation}
              // Add 2 to the key so there are never duplicates
              key={key + 2}
              style={[a.absolute, {width: 50, opacity: 0}]}
              aria-disabled={true}>
              <Text
                style={[
                  big ? a.text_md : {fontSize: 15},
                  a.user_select_none,
                  isLiked
                    ? [a.font_bold, s.likeColor]
                    : {color: t.palette.contrast_500},
                ]}>
                {formattedPrevCount}
              </Text>
            </Animated.View>
          ) : null}
        </View>
      ) : null}
    </LayoutAnimationConfig>
  )
}