给你的H5页面加上惯性滚动吧！-轻识

在移动端，如果你使用过 overflow: scroll 生成一个滚动容器，会发现它的滚动是比较卡顿，呆滞的。为什么会出现这种情况呢？

因为我们早已习惯了目前的主流操作系统和浏览器视窗的滚动体验，比如滚动到边缘会有回弹，手指停止滑动以后还会按惯性继续滚动一会，手指快速滑动时页面也会快速滚动。而这种原生滚动容器却没有，就会让人感到卡顿。

首先，让我们来看一下它是怎样让滚动更流畅的吧。

<html>  <head>    <meta charset="UTF-8" />    <meta name="viewport" content="width=device-width, initial-scale=1.0" />    <title>Document</title>  </head>  <body>    <div id="app"></div>    <template id="tpl">      <div        class="wrapper"        ref="wrapper"        @touchstart.prevent="onStart"        @touchmove.prevent="onMove"        @touchend.prevent="onEnd"        @touchcancel.prevent="onEnd"        @mousedown.prevent="onStart"        @mousemove.prevent="onMove"        @mouseup.prevent="onEnd"        @mousecancel.prevent="onEnd"        @mouseleave.prevent="onEnd"        @transitionend="onTransitionEnd"      >        <ul class="list" ref="scroller" :style="scrollerStyle">          <li class="list-item" v-for="item in list">{{item}}</li>        </ul>      </div>    </template>    <style>      body,      ul {        margin: 0;        padding: 0;      }
      ul {        list-style: none;      }
      .wrapper {        width: 100vw;        height: 100vh;        overflow: hidden;      }
      .list {        background-color: #70f3b7;      }
      .list-item {        height: 40px;        line-height: 40px;        width: 100%;        text-align: center;        border-bottom: 1px solid #ccc;      }</style>    <script src="https://cdn.jsdelivr.net/npm/vue@2"></script>
    <script>
      new Vue({        el: "#app",        template: "#tpl",        computed: {          list() {            let list = [];            for (let i = 0; i < 100; i++) {              list.push(i);            }            return list;          },          scrollerStyle() {            return {              transform: `translate3d(0, ${this.offsetY}px, 0)`,              "transition-duration": `${this.duration}ms`,              "transition-timing-function": this.bezier,            };          },        },        data() {          return {            minY: 0,            maxY: 0,            wrapperHeight: 0,            duration: 0,            bezier: "linear",            pointY: 0, // touchStart 手势 y 坐标            startY: 0, // touchStart 元素 y 偏移值            offsetY: 0, // 元素实时 y 偏移值            startTime: 0, // 惯性滑动范围内的 startTime            momentumStartY: 0, // 惯性滑动范围内的 startY            momentumTimeThreshold: 300, // 惯性滑动的启动 时间阈值            momentumYThreshold: 15, // 惯性滑动的启动 距离阈值            isStarted: false, // start锁          };        },        mounted() {          this.$nextTick(() => {            this.wrapperHeight =              this.$refs.wrapper.getBoundingClientRect().height;            this.minY =              this.wrapperHeight -              this.$refs.scroller.getBoundingClientRect().height;          });        },        methods: {          onStart(e) {            const point = e.touches ? e.touches[0] : e;            this.isStarted = true;            this.duration = 0;            this.stop();            this.pointY = point.pageY;            this.momentumStartY = this.startY = this.offsetY;            this.startTime = new Date().getTime();          },          onMove(e) {            if (!this.isStarted) return;            const point = e.touches ? e.touches[0] : e;            const deltaY = point.pageY - this.pointY;            this.offsetY = Math.round(this.startY + deltaY);            const now = new Date().getTime();            // 记录在触发惯性滑动条件下的偏移值和时间            if (now - this.startTime > this.momentumTimeThreshold) {              this.momentumStartY = this.offsetY;              this.startTime = now;            }          },          onEnd(e) {            if (!this.isStarted) return;            this.isStarted = false;            if (this.isNeedReset()) return;            const absDeltaY = Math.abs(this.offsetY - this.momentumStartY);            const duration = new Date().getTime() - this.startTime;            // 启动惯性滑动            if (              duration < this.momentumTimeThreshold &&              absDeltaY > this.momentumYThreshold            ) {              const momentum = this.momentum(                this.offsetY,                this.momentumStartY,                duration              );              this.offsetY = Math.round(momentum.destination);              this.duration = momentum.duration;              this.bezier = momentum.bezier;            }          },          onTransitionEnd() {            this.isNeedReset();          },          momentum(current, start, duration) {            const durationMap = {              noBounce: 2500,              weekBounce: 800,              strongBounce: 400,            };            const bezierMap = {              noBounce: "cubic-bezier(.17, .89, .45, 1)",              weekBounce: "cubic-bezier(.25, .46, .45, .94)",              strongBounce: "cubic-bezier(.25, .46, .45, .94)",            };            let type = "noBounce";            // 惯性滑动加速度            const deceleration = 0.003;            // 回弹阻力            const bounceRate = 10;            // 强弱回弹的分割值            const bounceThreshold = 300;            // 回弹的最大限度            const maxOverflowY = this.wrapperHeight / 6;            let overflowY;
            const distance = current - start;            const speed = (2 * Math.abs(distance)) / duration;            let destination =              current + (speed / deceleration) * (distance < 0 ? -1 : 1);            if (destination < this.minY) {              overflowY = this.minY - destination;              type =                overflowY > bounceThreshold ? "strongBounce" : "weekBounce";              destination = Math.max(                this.minY - maxOverflowY,                this.minY - overflowY / bounceRate              );            } else if (destination > this.maxY) {              overflowY = destination - this.maxY;              type =                overflowY > bounceThreshold ? "strongBounce" : "weekBounce";              destination = Math.min(                this.maxY + maxOverflowY,                this.maxY + overflowY / bounceRate              );            }
            return {              destination,              duration: durationMap[type],              bezier: bezierMap[type],            };          },          // 超出边界时需要重置位置          isNeedReset() {            let offsetY;            if (this.offsetY < this.minY) {              offsetY = this.minY;            } else if (this.offsetY > this.maxY) {              offsetY = this.maxY;            }            if (typeof offsetY !== "undefined") {              this.offsetY = offsetY;              this.duration = 500;              this.bezier = "cubic-bezier(.165, .84, .44, 1)";              return true;            }            return false;          },          // 停止滚动          stop() {            const matrix = window              .getComputedStyle(this.$refs.scroller)              .getPropertyValue("transform");            this.offsetY = Math.round(+matrix.split(")")[0].split(", ")[5]);          },        },      });</script>  </body></html>

可以发现，在增加惯性滚动，边缘回弹等效果之后，明显流畅、舒服了很多。那么，这些效果是怎么实现的呢？在用户滑动操作结束时，还会继续惯性滚动一段。首先看一下源码中的函数，这是 touchend 事件的处理函数，也就是用户滚动操作结束时的逻辑。