最近公司有一個專案的需要做出旋鈕的效果，旋鈕有十個檔，根據手指旋轉切換，旋轉時接近最近的檔會有吸附效果，具體效果如下：

html部分程式碼：

<div class="_touch">
  <div class="round_box" ref="box" @touchstart="touchStart" @touchmove="touchMove">
    <div v-if="able" style="position:absolute;width:100%;height:100%;top:0;left:0;">
      <div class
 
="round_right" ref="right" :style="{ transform: 'rotate(' + angle +'deg)' }">
        <div class="round_info"></div>
      </div>
      <div class="round_num">{{level}}</div>
    </div>
  </div>
</div>

less樣式部分程式碼：

._touch {
  padding-top: 48px
 
;
  .round_box {
    position: relative;
    width: 54%;
    padding-top: 54%;
    margin: 0 auto;
    background-image: url(../../assets/img/round_box.png);
    background-size: auto 100%;
    background-position: center top;
    background-repeat: no-repeat;
    .round_right {
      width: 8%;
      height
 
: 27%;
      position: absolute;
      left: 46%;
      top: 23%;
      transform-origin: 50% 100%;
      -webkit-transform-origin: 50% 100%;
      z-index: 2;
      .round_info {
        background: linear-gradient(#858585, #b3b3b3);
        background: -webkit-gradient(#858585, #b3b3b3);
        background: -moz-linear-gradient(#858585, #b3b3b3);
        width: 100%;
        padding-top: 100%;
        border-radius: 100%;
      }
    }
    .round_num {
      display: inline-block;
      position: absolute;
      z-index: 1;
      text-align: center;
      width: 30%;
      top: 38%;
      left: 35%;
      font-size: 2.4em;
      font-weight: 900;
      background: linear-gradient(#b0b0b0, #c8c8c8);
      background: -webkit-gradient(#b0b0b0, #c8c8c8);
      -webkit-background-clip: text;
      background-clip: text;
      -webkit-text-fill-color: transparent;
      text-fill-color: transparent;
    }
  }
}

js部分程式碼：

methods: {
  ......,
  touchStart(e) {
    e.preventDefault();
    e.stopPropagation();
    let round_box = this.$refs.box;
    var w = round_box.offsetWidth / 2;
    var h = round_box.offsetHeight / 2;
    this.px = round_box.getBoundingClientRect().left + w;
    this.py = round_box.getBoundingClientRect().top + h;
  },
  touchMove(e) {
    e.preventDefault();
    e.stopPropagation();
    this.getAngle(e.changedTouches[0].clientX, e.changedTouches[0].clientY);
  },
  touchEnd(e) {
    e.preventDefault();
    e.stopPropagation();
  },
  resetAngle(angle) {
    let list = [
      { angle: 0, level: 5 },
      { angle: 36, level: 6 },
      { angle: 72, level: 7 },
      { angle: 108, level: 8 },
      { angle: 144, level: 9 },
      { angle: 180, level: 10 },
      { angle: 216, level: 1 },
      { angle: 252, level: 2 },
      { angle: 288, level: 3 },
      { angle: 324, level: 4 },
      { angle: 360, level: 5 }
    ];
    let result = list.filter(function(currentVal, index, arr) {
      return Math.abs(angle - currentVal.angle) <= 18;
    });
    this.angle = result[0].angle;
    this.level = result[0].level;
  },
  getAngle(mx, my) {
    var px = this.px;
    var py = this.py;
    var x = Math.abs(px - mx);
    var y = Math.abs(py - my);
    var z = Math.sqrt(Math.pow(x, 2) + Math.pow(y, 2));
    var cos = y / z;
    var radina = Math.acos(cos); //用反三角函式求弧度
    var angle = Math.floor(180 / (Math.PI / radina)); //將弧度轉換成角度
    if (mx > px && my > py) {
      //滑鼠在第四象限
      angle = 180 - angle;
    }
    if (mx == px && my > py) {
      //滑鼠在y軸負方向上
      angle = 180;
    }
    if (mx > px && my == py) {
      //滑鼠在x軸正方向上
      angle = 90;
    }
    if (mx < px && my > py) {
      //滑鼠在第三象限
      angle = 180 + angle;
    }
    if (mx < px && my == py) {
      //滑鼠在x軸負方向
      angle = 270;
    }
    if (mx < px && my < py) {
      //滑鼠在第二象限
      angle = 360 - angle;
    }
    this.angle = angle;
    this.$nextTick(function() {
      this.resetAngle(this.angle);
    });
  },
......

主要的思路是根據監聽 .round_box 元素的 touchmove 事件獲取手指相對於圓心這條直線的旋轉角度（transform : rotate），

並把旋轉角度同步到水平居中於 .round_box 容器，底邊框中心與 .round_box重合的元素 ： .round_right 上，使它相對於

.round_box的圓心旋轉即可。

注：.round_box圓心如下：

注：.round_right 元素如下：

其中 吸附效果的程式碼在getAngle中。