CSS 滚动捕捉与动画效果

Question

前提

我正在尝试制作一个用户可以在其上滚动的数字列表选择器。选定的数字应该与center容器的形状对齐。一旦选择了一个数字，就应该放大它并改变颜色。

---

问题

• 如何将第一个或最新的项目捕捉到容器的中心？
• 是否有 CSS 或 JavaScript 方法来检测数字是否对齐中心，然后selected为其分配类？
• 不要依赖任何插件。

* {
  box-sizing: border-box;
  font-family: Roboto, sans-serif;
}

.container {
  display: flex;
  flex-direction: row;
  width: 10rem;
  height: 22rem;
  border-radius: 3rem;
  border: solid 0.2rem #b2b2c2;
  background-color: #000000;
  display: flex;
  align-items: center;
  justify-content: center;
}

.scrollport {
  display: flex;
  flex-direction: column;
  flex-wrap: nowrap;
  width: 9.4rem;
  height: 22rem;
  overflow: auto;
  scroll-snap-type: y mandatory;
}

.cell {
  display: block;
  scroll-snap-align: center;
  flex-grow: 1;
  flex-shrink: 0;
  flex-basis: 33.3%;
  display: flex;
  justify-content: center;
  align-items: center;
  color: #e9e9f2;
  font-size: 2.4rem;
}

.selected {
  font-size: 3rem;
  font-weight: bold;
  color: #0073e6;
}

<div class="container">
  <div class="scrollport">
    <div class="cell">09</div>
    <div class="cell selected">10</div>
    <div class="cell">11</div>
    <div class="cell">12</div>
    <div class="cell">13</div>
    <div class="cell">14</div>
    <div class="cell">15</div>
    <div class="cell">16</div>
  </div>
</div>

---

期待结果

Answer 1

添加伪元素.scrollport并设置它们的样式.cell

.scrollport:before,
.scrollport:after {
  content: '';
}
.scrollport:before,
.scrollport:after,
.cell {
  /* ... */
}

我还创建了一个 JS 函数，它利用David Walsh的debounce函数来模拟滚动结束事件。然后，我检查每个cell中心是否位于 的scrollport中心周围，并相应地设置类

.scrollport:before,
.scrollport:after {
  content: '';
}
.scrollport:before,
.scrollport:after,
.cell {
  /* ... */
}

"use strict";
console.clear()

{
  const selector = 'scrollport'
  const selected = 'selected'
  const scrollports = document.getElementsByClassName(selector)

  for (const scrollport of scrollports) {
    scrollport.addEventListener('scroll', debounce(check, 250) /* simulate scrollend event */ )
  }

  function check(e) {
    // uses native elementFromPoint for better performance
    const rect = e.target.getBoundingClientRect();
    const centerCell = document.elementFromPoint(rect.left + e.target.offsetWidth / 2, rect.top + e.target.offsetHeight / 2)
    for (const cell of e.target.getElementsByClassName(selected)) {
      cell.classList.remove(selected)
    }
    centerCell.classList.add(selected)
    
    // Old version for backward compatibility
    // const rect = e.target.getBoundingClientRect();
    // for (const cell of e.target.children) {
    //   const cellRect = cell.getBoundingClientRect();
    //   const bounds = [rect.height/2 - cellRect.height/2, rect.height/2 + cellRect.height/2];
    //   const centerScrollOffset = cellRect.top - rect.top + cell.offsetHeight / 2
    //   if (bounds[0] < centerScrollOffset && centerScrollOffset < bounds[1]) {
    //     cell.classList.add(selected)
    //   } else {
    //     cell.classList.remove(selected)
    //   }
    // }
  }
}
// From: https://davidwalsh.name/javascript-debounce-function
// Returns a function, that, as long as it continues to be invoked, will not
// be triggered. The function will be called after it stops being called for
// N milliseconds. If `immediate` is passed, trigger the function on the
// leading edge, instead of the trailing.
function debounce(func, wait, immediate) {
  var timeout;
  return function() {
    var context = this,
      args = arguments;
    var later = function() {
      timeout = null;
      if (!immediate) func.apply(context, args);
    };
    var callNow = immediate && !timeout;
    clearTimeout(timeout);
    timeout = setTimeout(later, wait);
    if (callNow) func.apply(context, args);
  };
};

* {
  box-sizing: border-box;
  font-family: Roboto, sans-serif;
}

.container {
  display: flex;
  flex-direction: row;
  width: 10rem;
  height: 22rem;
  border-radius: 3rem;
  border: solid 0.2rem #b2b2c2;
  background-color: #000000;
  display: flex;
  align-items: center;
  justify-content: center;
}

.scrollport:before,
.scrollport:after {
  content: '';
}

.scrollport {
  display: flex;
  flex-direction: column;
  flex-wrap: nowrap;
  width: 9.4rem;
  height: 22rem;
  overflow: auto;
  scroll-snap-type: y mandatory;
}

.scrollport:before,
.scrollport:after,
.cell {
  display: block;
  scroll-snap-align: center;
  flex-grow: 1;
  flex-shrink: 0;
  flex-basis: 33.3%;
  display: flex;
  justify-content: center;
  align-items: center;
  color: #e9e9f2;
  font-size: 2.4rem;
}

.selected {
  font-size: 3rem;
  font-weight: bold;
  color: #0073e6;
}