如何使用CSS和GSAP实现树枝发芽的loader动画（附源码）

效果预览

源代码下载

https://github.com/comehope/front-end-daily-challenges

代码解读

定义 dom，容器包含 2 个元素，branch 代表枝，leaves 代表叶，叶有 6 个子元素，代表 6 个叶片：

<figure class="sapling">
 <div class="branch"></div>
 <div class="leaves">
 <span></span>
 <span></span>
 <span></span>
 <span></span>
 <span></span>
 <span></span>
 </div>
</figure>

居中显示：

body {
 margin: 0;
 height: 100vh;
 display: flex;
 align-items: center;
 justify-content: center;
 background-color: black;
}

定义容器尺寸，并设置子元素水平居中：

.sapling {
 position: relative;
 width: 5em;
 height: 17.5em;
 font-size: 10px;
 display: flex;
 justify-content: center;
}

画出树枝：

.branch {
 position: absolute;
 width: 0.2em;
 height: inherit;
 border-radius: 25%;
 background: burlywood;
}

定义树叶容器，设置为叶片在垂直方向均匀分布，并且从下到上排列：

.leaves {
 position: absolute;
 width: inherit;
 height: 15em;
 top: 1em;
 display: flex;
 flex-direction: column-reverse;
}

设置叶片的尺寸和和背景颜色：

.leaves span {
 width: 2.5em;
 height: 2.5em;
 background-color: limegreen;
}

设置左右叶片的各自样式：

.leaves span:nth-child(odd) {
 border-bottom-left-radius: 3em;
 border-top-right-radius: 3em;
 transform-origin: right bottom;
 align-self: flex-start;
}

.leaves span:nth-child(even) {
 border-bottom-right-radius: 3em;
 border-top-left-radius: 3em;
 transform-origin: left bottom;
 align-self: flex-end;
}

至此，静态效果绘制完成，接下来开始写动画脚本。
引入 GSAP 库：

<script src="https://cdnjs.cloudflare.com/ajax/libs/gsap/2.0.2/TweenMax.min.js"></script>

声明一个时间线对象：

let animation = new TimelineMax();

增加树枝的入场动画效果，并为这个动画设置一个标签 branch：

animation.from('.branch', 4, {scaleY: 0, ease: Power1.easeOut}, 'branch');

增加树叶的入场动画效果，它的参数中有 3 个 0.5，从左到右的含义分别是动画时长、多个叶片动画的间隔时长、相对 branch 标签动画的延迟时间：

animation.from('.branch', 4, {scaleY: 0, ease: Power1.easeOut}, 'branch')
 .staggerFrom('.leaves span', 0.5, {scale: 0, ease: Power1.easeOut}, 0.5, 0.5, 'branch');

增加叶片变黄的动画效果：

animation.from('.branch', 4, {scaleY: 0, ease: Power1.easeOut}, 'branch')
 .staggerFrom('.leaves span', 0.5, {scale: 0, ease: Power1.easeOut}, 0.5, 0.5, 'branch')
 .to(['.branch', '.leaves span'], 3, {backgroundColor: 'yellow'});

增加淡出效果：

animation.from('.branch', 4, {scaleY: 0, ease: Power1.easeOut}, 'branch')
 .staggerFrom('.leaves span', 0.5, {scale: 0, ease: Power1.easeOut}, 0.5, 0.5, 'branch')
 .to(['.branch', '.leaves span'], 3, {backgroundColor: 'yellow'})
 .to(['.branch', '.leaves span'], 1, {autoAlpha: 0});

修改声明时间线的代码，使动画重复播放：

let animation = new TimelineMax({repeat: -1, repeatDelay: 0.5});

大功告成！