ant-design-vue/components/back-top/index.jsx

import PropTypes from '../_util/vue-types'
import addEventListener from '../_util/Dom/addEventListener'
import getScroll from '../_util/getScroll'
import getRequestAnimationFrame from '../_util/getRequestAnimationFrame'
import BaseMixin from '../_util/BaseMixin'
import getTransitionProps from '../_util/getTransitionProps'

const reqAnimFrame = getRequestAnimationFrame()

const easeInOutCubic = (t, b, c, d) => {
  const cc = c - b
  t /= d / 2
  if (t < 1) {
    return cc / 2 * t * t * t + b
  } else {
    return cc / 2 * ((t -= 2) * t * t + 2) + b
  }
}

function getDefaultTarget () {
  return window
}

const BackTopProps = {
  visibilityHeight: PropTypes.number,
  // onClick?: React.MouseEventHandler<any>;
  target: PropTypes.func,
  prefixCls: PropTypes.string,
}

export default {
  name: 'BackTop',
  mixins: [BaseMixin],
  props: {
    ...BackTopProps,
    visibilityHeight: PropTypes.number.def(400),
  },
  data () {
    this.scrollEvent = null
    return {
      visible: false,
    }
  },

  mounted () {
    this.$nextTick(() => {
      const getTarget = this.target || getDefaultTarget
      this.scrollEvent = addEventListener(getTarget(), 'scroll', this.handleScroll)
      this.handleScroll()
    })
  },

  beforeDestroy () {
    if (this.scrollEvent) {
      this.scrollEvent.remove()
    }
  },
  methods: {
    getCurrentScrollTop () {
      const getTarget = this.target || getDefaultTarget
      const targetNode = getTarget()
      if (targetNode === window) {
        return window.pageYOffset || document.body.scrollTop || document.documentElement.scrollTop
      }
      return targetNode.scrollTop
    },

    scrollToTop (e) {
      const scrollTop = this.getCurrentScrollTop()
      const startTime = Date.now()
      const frameFunc = () => {
        const timestamp = Date.now()
        const time = timestamp - startTime
        this.setScrollTop(easeInOutCubic(time, scrollTop, 0, 450))
        if (time < 450) {
          reqAnimFrame(frameFunc)
        }
      }
      reqAnimFrame(frameFunc)
      this.$emit('click', e)
    },

    setScrollTop (value) {
      const getTarget = this.target || getDefaultTarget
      const targetNode = getTarget()
      if (targetNode === window) {
        document.body.scrollTop = value
        document.documentElement.scrollTop = value
      } else {
        targetNode.scrollTop = value
      }
    },

    handleScroll () {
      const { visibilityHeight, target = getDefaultTarget } = this
      const scrollTop = getScroll(target(), true)
      this.setState({
        visible: scrollTop > visibilityHeight,
      })
    },
  },

  render () {
    const { prefixCls = 'ant-back-top', $slots, $listeners } = this

    const defaultElement = (
      <div class={`${prefixCls}-content`}>
        <div class={`${prefixCls}-icon`} />
      </div>
    )
    const divProps = {
      on: {
        ...$listeners,
        click: this.scrollToTop,
      },
      class: prefixCls,
    }

    const backTopBtn = this.visible ? (
      <div {...divProps}>
        {$slots.default || defaultElement}
      </div>
    ) : null
    const transitionProps = getTransitionProps('fade')
    return (
      <transition {...transitionProps}>
        {backTopBtn}
      </transition>
    )
  },
}