当元素进入视口(viewport)时，JavaScript/jQuery 添加类？

Question

当我想应用类的元素进入视口(viewport)时，有没有办法添加类？或者当屏幕底部超过元素顶部一定距离时？

现在我有了我想要的效果:

if ($(document).scrollTop() > 100) {
                    $(".graph-line.two").addClass("graph-75");

这个问题是它相对于文档高度，所以当我缩小页面(或在移动设备上查看)并且元素堆叠在彼此之上时，页面变得更高并且类(动画)不当元素进入 View 时开始。

我见过其他人问过类似的问题，我尝试实现他们得到的答案，但我无法使任何工作正常进行，因此非常感谢您的帮助。

这是我的:

$(document).ready(function() {
  $(".graph-line.one").addClass("graph-75");
  $(".graph-line-2.one").addClass("opacity");
  $(window).scroll(function() {

    if ($(document).scrollTop() > 100) {
      $(".graph-line.two").addClass("graph-75");
      $(".graph-line-2.two").addClass("opacity");
    }

    if ($(document).scrollTop() > 450) {
      $(".graph-line.three").addClass("graph-75");
      $(".graph-line-2.three").addClass("opacity");
    }

    if ($(document).scrollTop() > 800) {
      $(".graph-line.four").addClass("graph-75");
      $(".graph-line-2.four").addClass("opacity");
    }

    if ($(document).scrollTop() > 1150) {
      $(".graph-line.five").addClass("graph-75");
      $(".graph-line-2.five").addClass("opacity");
    }

    if ($(document).scrollTop() > 1500) {
      $(".graph-line.six").addClass("graph-75");
      $(".graph-line-2.six").addClass("opacity");
    }
  });
});

.graph {
  display: block;
  margin: 100px auto;
  transform: rotate(-90deg);
  will-change: transform;
}
.graph-line {
  stroke-dasharray: 628px;
  stroke-dashoffset: -628px;
  transform-origin: center;
}
.graph-75 {
  animation: graph-75 1200ms ease forwards;
}
@keyframes graph-75 {
  0% {
    stroke-dashoffset: 0;
    transform: rotate(360deg);
  }
  100% {
    stroke-dashoffset: 471;
    transform: rotate(0deg);
  }
}
.graph-line-2 {
  transition: opacity 700ms;
  opacity: 0.1;
}
.opacity {
  opacity: 1;
}

<script src="https://ajax.googleapis.com/ajax/libs/jquery/3.1.0/jquery.min.js"></script>

<h1>Scroll Down</h2>

<svg width="250" height="250" class="graph photoshop">
						<circle class="graph-line-2 one" cx="50%" cy="50%" r="100" stroke-width="20" fill="none" stroke="#2ECBE4" />
						<circle class="graph-line one" cx="50%" cy="50%" r="100" stroke-width="22" fill="none" stroke="#fff" opacity="0.92" />
						<text class="graph-text" text-anchor="middle" x="50%" y="-46%" fill="#fff">Photoshop</text>
					</svg>



<svg width="250" height="250" class="graph photoshop">
						<circle class="graph-line-2 two" cx="50%" cy="50%" r="100" stroke-width="20" fill="none" stroke="#2ECBE4" />
						<circle class="graph-line two" cx="50%" cy="50%" r="100" stroke-width="22" fill="none" stroke="#fff" opacity="0.92" />
						<text class="graph-text" text-anchor="middle" x="50%" y="-46%" fill="#fff">Photoshop</text>
					</svg>



<svg width="250" height="250" class="graph photoshop">
						<circle class="graph-line-2 three" cx="50%" cy="50%" r="100" stroke-width="20" fill="none" stroke="#2ECBE4" />
						<circle class="graph-line three" cx="50%" cy="50%" r="100" stroke-width="22" fill="none" stroke="#fff" opacity="0.92" />
						<text class="graph-text" text-anchor="middle" x="50%" y="-46%" fill="#fff">Photoshop</text>
					</svg>



<svg width="250" height="250" class="graph photoshop">
						<circle class="graph-line-2 four" cx="50%" cy="50%" r="100" stroke-width="20" fill="none" stroke="#2ECBE4" />
						<circle class="graph-line four" cx="50%" cy="50%" r="100" stroke-width="22" fill="none" stroke="#fff" opacity="0.92" />
						<text class="graph-text" text-anchor="middle" x="50%" y="-46%" fill="#fff">Photoshop</text>
					</svg>



<svg width="250" height="250" class="graph photoshop">
						<circle class="graph-line-2 five" cx="50%" cy="50%" r="100" stroke-width="20" fill="none" stroke="#2ECBE4" />
						<circle class="graph-line five" cx="50%" cy="50%" r="100" stroke-width="22" fill="none" stroke="#fff" opacity="0.92" />
						<text class="graph-text" text-anchor="middle" x="50%" y="-46%" fill="#fff">Photoshop</text>
					</svg>



<svg width="250" height="250" class="graph photoshop">
						<circle class="graph-line-2 six" cx="50%" cy="50%" r="100" stroke-width="20" fill="none" stroke="#2ECBE4" />
						<circle class="graph-line six" cx="50%" cy="50%" r="100" stroke-width="22" fill="none" stroke="#fff" opacity="0.92" />
						<text class="graph-text" text-anchor="middle" x="50%" y="-46%" fill="#fff">Photoshop</text>
					</svg>

Here's the codepen if you prefer

Answer 1

你可以这样做:(参见 CodePen 实现)

函数取自此处:Check if element is visible after scrolling

CodePen

$(window).on('scroll', function() {
  $(".graph").each(function() {
    if (isScrolledIntoView($(this))) {
      $(this).find(".graph-line").addClass("graph-75");
      $(this).find(".graph-line-2").addClass("opacity");
    }
  });
});

function isScrolledIntoView(elem) {
  var docViewTop = $(window).scrollTop();
  var docViewBottom = docViewTop + $(window).height();

  var elemTop = $(elem).offset().top;
  var elemBottom = elemTop + $(elem).height();

  return ((elemBottom <= docViewBottom) && (elemTop >= docViewTop));
}

虽然这并不完美，但它应该能为您指明正确的方向。