上一篇文章中，我们介绍了如何实现轮播图的无缝滚动

这一篇文章将会介绍如何实现自动播放，以及如何将自动播放和手指滑动这两个事件进行隔离

自动播放

假设自动播放的顺序为：图片无限向左滚动

那么当我们发现 this.current 指向第二组最后一张图片时，也应该让它瞬间移动到第一组的最后一张图片

直接看代码：

autoPlaySlider = () => {
    
    clearInterval(this.intervalID)
    this.intervalID = setInterval(() => {
        if(this.current === this.dotLength*2 -1){
            this.current = this.dotLength - 1
            this.translatex = this.current * -this.imgWidth
            this.list.current.style.transition = 'none'
            this.list.current.style.transform = `translateX(${this.translatex}px)`

            clearTimeout(this.timerID)
            this.timerID = setTimeout(() => {
                this.current++
                this.translatex = this.current * -this.imgWidth
                this.list.current.style.transition = '0.4s'
                this.list.current.style.transform = `translateX(${this.translatex}px)`
                this.formatDots()
            }, 0);

        }else{
            this.current++
            this.translatex = this.current * -this.imgWidth
            this.list.current.style.transition = '0.4s'
            this.list.current.style.transform = `translateX(${this.translatex}px)`
            this.formatDots()
        }

    }, 1500);
    
  }

当 this.current 指向第二组最后一张图片时，必须要做2次transform的动作

第一次：从“第二组最后一张” -> “第一组最后一张”

第二次：从“第一组最后一张” -> “第二组第一张”

为了将这两个动作完全隔离，我们用了一个计时器timerID，将 第二次的滚动动作 延迟执行

如果不这样做，第二次的动作将会覆盖第一次的动作，导致动画效果出错，变成“向右滚动”

兼容自动播放和手指触摸

我们的第一个目标是：当手指开始滑动图片时，停止自动播放

当 touchStart 被触发时，清空页面上所有与自动播放相关的计时器

然后给这个“进程”上一个锁，将this.touching设置为true

并在touchend 结束时，将 this.touching 改为 false

我们的第二个目标是：当手指停止滑动图片后，过一段时间，重新开始自动播放

当touchend结束时，开启一个名为 waitForAutoPlay 的计时器，3s之后，执行autoPlaySlider函数

并在autoPlaySlider函数开头，根据 this.touching的值 多加一层判断

autoPlaySlider = () => {
    if(this.touching){
      return
    }else{
      ...
    }
}

最后，别忘了在组件卸载之前，清空页面上的所有计时器

componentWillUnmount() {
    clearInterval(this.intervalID)
    clearTimeout(this.timerID)
    clearTimeout(this.waitForAutoPlay)
}

完整代码

Slider.js的完整代码如下

import React, {Component, Fragment} from 'react'
import {
  SliderWrapper,
  SliderList,
  SliderDot
} from './style'

class Slider extends Component {
  constructor(props) {
    super(props)
    this.wrap = React.createRef()
    this.list = React.createRef()
    this.dot = React.createRef()
    this.dotLength = 0
    this.startPoint = {}
    this.distance = {}
    this.current = 0
    this.translatex = 0
    this.startOffset = 0
    this.imgWidth = this.props.imgWidth
    this.threshold = 0.2 //滑动切换阈值
    this.touching = false
    this.isMove = false
    this.intervalID = null
    this.timerID = null
  }


  render() {
    return (
      <Fragment>
      <SliderWrapper
        ref={this.wrap}
        width={this.props.imgWidth}
        onTouchStart={this.handleTouchStart}
        onTouchMove={this.handleTouchMove}
        onTouchEnd={this.handleTouchEnd}
      >
        <SliderList ref={this.list} >
          {
            this.props.imgList.map((item,index) => {
              return (
                <li key={index}><img src={item} width={this.props.imgWidth}/></li>
              )
            })
          }
        </SliderList>
        <SliderDot ref={this.dot}>
          {
            this.props.imgList.map((item,index) => {
              return (
                <li key={index}
                ></li>
              )
            })
          }
        </SliderDot>
        
      </SliderWrapper>
      </Fragment>
    )
  }

  componentDidMount() {
    this.initSlider()
  }

  initSlider = () => {
    this.dotLength = this.dot.current.childNodes.length
    this.list.current.innerHTML+=this.list.current.innerHTML
    this.dot.current.childNodes[0].classList.add('active')

    this.autoPlaySlider()
  }


  autoPlaySlider = () => {
    if(this.touching){
      return
    }else{
      clearInterval(this.intervalID)
      this.intervalID = setInterval(() => {
    
        if(this.current === this.dotLength*2 -1){
          this.current = this.dotLength - 1
          this.translatex = this.current * -this.imgWidth
          this.list.current.style.transition = 'none'
          this.list.current.style.transform = `translateX(${this.translatex}px)`
          
  
          clearTimeout(this.timerID)
          this.timerID = setTimeout(() => {
            this.current++
            this.translatex = this.current * -this.imgWidth
            this.list.current.style.transition = '0.4s'
            this.list.current.style.transform = `translateX(${this.translatex}px)`
            this.formatDots()
          }, 0);
  
        }else{
          this.current++
          this.translatex = this.current * -this.imgWidth
          this.list.current.style.transition = '0.4s'
          this.list.current.style.transform = `translateX(${this.translatex}px)`
          this.formatDots()
        }
  
      }, 1500);
    }
  }

  handleTouchStart = (ev) => {
    clearTimeout(this.timerID)
    clearInterval(this.intervalID)
    clearTimeout(this.waitForAutoPlay)
    this.touching = true
    let touch = ev.changedTouches[0]
    this.startPoint = {
      x: touch.pageX,
      y: touch.pageY
    }

    if(this.current === 0){
      this.current = this.dotLength
    }else if(this.current === this.dotLength*2 -1){
      this.current = this.dotLength -1
    }

    this.translatex = this.current * -this.imgWidth
    this.startOffset = this.translatex
    this.list.current.style.transition = 'none'
    this.list.current.style.transform = `translateX(${this.translatex}px)`
  }

  handleTouchMove = (ev) => {
    let touch = ev.changedTouches[0]
    this.distance = {
      x: touch.pageX - this.startPoint.x,
      y: touch.pageY - this.startPoint.y
    }
    
    this.translatex = this.startOffset + this.distance.x
    this.list.current.style.transform = `translateX(${this.translatex}px)`
  }

  handleTouchEnd = (ev) => {
    if(Math.abs(this.distance.x) > this.imgWidth * this.threshold){

      if(this.distance.x>0){
        this.current--
      }else{
          this.current++
      }
    }

    this.translatex = this.current * -this.imgWidth
    this.list.current.style.transition = '0.3s'
    this.list.current.style.transform = `translateX(${this.translatex}px)`
    this.formatDots()
    this.touching = false
    
    this.waitForAutoPlay = setTimeout(() => {
      if(!this.touching){
        this.autoPlaySlider()
      }
    }, 3000);
  }

  formatDots() {
    Array.from(this.dot.current.childNodes).forEach((item,index) => {
      item.classList.remove('active')
      if(index === (this.current%this.dotLength)){
        item.classList.add('active')
      }
    })
  }

  componentWillUnmount() {
    clearInterval(this.intervalID)
    clearTimeout(this.timerID)
    clearTimeout(this.waitForAutoPlay)
  }
}

export default Slider