滚动到指定位置

import React, { Component } from 'react'

import PropTypes from 'prop-types'

import './index.scss'

const trail = {



  linear : (t, b, c, d) => {

    return c * t/d + b

  },

  easeIn : ( t, b, c, d ) => c * ( t /= d ) * t + b,

  easeOut : ( t, b, c, d ) => -c *( t/=d )*( t - 2 ) + b,

  easeInOut : ( t, b, c, d ) => {

    if ( ( t/=d/2 ) < 1 ) return c/2 * t * t + b

          return -c/2 * ( (--t) * (t-2) - 1 ) + b

  }

}

class Back extends Component {

  state = {

    isShow: false

  }

  scrollTo = () => {

    // const { y, duration, rate } = this.props

    const y = 100, duration = 900, rate = 30

    const start = this.getScrollTop()

    let end = y

    if(end < 0) {

      end = 0

    }

    this.getStep(start, end, (step) => {

      this.setScrollTop(step)

    }, duration, rate)

  }

  getStep = (start, end, callback, duration, rate) => {

    const diff = end - start

    if(duration == 0 || diff == 0) {

      callback && callback(end)

      return

    }

    let count = duration / rate

    let i = 0

    let t = 0

    this.props.onScrollStart()

    const timer = window.setInterval(() => {

      if(i < count - 1) {

        t += rate

        const value = trail.easeOut(t, start, diff, duration)

        callback && callback(value)

        i++

      } else {

        this.props.onScrollEnd()

        callback && callback(end)

        window.clearInterval(timer)

      }

    }, rate)

  }

  bindScroll = () => {

    const scrollTop = this.getScrollTop()

    if(scrollTop > this.props.offset){

      this.setState({ isShow: true })

    }else{

      this.setState({ isShow: false })

    }

  }

  setScrollTop = (y) => {

    document.documentElement.scrollTop = y

    document.body.scrollTop = y

  }



  getScrollTop = () => {

    const scrollTop = document.documentElement.scrollTop || document.body.scrollTop

    return scrollTop

  }

  componentDidMount(){

    window.addEventListener('scroll', this.bindScroll)

  }

  componentWillUnmount() {

    window.removeEventListener('scroll', this.bindScroll)

  }



  render() {

    const { icon } = this.props

    const { isShow } = this.state

    return (

      <div className="back" >

        { isShow &&

          <div className="back-warp" onClick={this.scrollTo}>

            { icon && <div className="back-icon">{icon}</div>}

          </div>

        }

      </div>

    )

  }

}

Back.porpTypes = {

  icon: PropTypes.string,

  offset: PropTypes.number,

  onScrollStart: PropTypes.func,

  onScrollEnd: PropTypes.func

}

Back.defaultProps = {

  icon: '',

  offset: 1000,

  onScrollStart: () => {},

  onScrollEnd: () => {}

}

export default Back