前端每日实战：17# 视频演示如何用纯 CSS 创作炫酷的同心矩形旋转动画

效果预览

按下右侧的“点击预览”按钮可以在当前页面预览，点击链接可以全屏预览。

https://codepen.io/comehope/pen/bMvbRp

可交互视频教程

此视频是可以交互的，你可以随时暂停视频，编辑视频中的代码。

请用 chrome, safari, edge 打开观看。

https://scrimba.com/p/pEgDAM/cp2dZcQ

源代码下载

请从 github 下载。

https://github.com/comehope/front-end-daily-challenges/tree/master/017-swapping-colors-loader-animation

代码解读

定义 dom，一个容器中包含一个 span：

<div class="loader">
    <span></span>
</div>

居中显示：

html,
body,
.loader {
    height: 100%;
    display: flex;
    align-items: center;
    justify-content: center;
    background-color: black;
}

设置 span 的样式：

.loader {
    width: 10em;
    height: 10em;
    font-size: 28px;
    position: relative;
}
.loader span {
    position: absolute;
    width: 100%;
    height: 100%;
    background-color: rgba(100%, 0%, 0%, 0.3);
    box-sizing: border-box;
    border: 0.5em solid;
    border-color: white rgba(100%, 100%, 100%, 0.2);
}

在 dom 中把 span 增加到 5 个：

<div class="loader">
    <span></span>
    <span></span>
    <span></span>
    <span></span>
    <span></span>
</div>

分别设置 5 个 span 的尺寸：

.loader span:nth-child(1) {
    width: calc(20% + 20% * (5 - 1));
    height: calc(20% + 20% * (5 - 1));
}
.loader span:nth-child(2) {
    width: calc(20% + 20% * (5 - 2));
    height: calc(20% + 20% * (5 - 2));
}
.loader span:nth-child(3) {
    width: calc(20% + 20% * (5 - 3));
    height: calc(20% + 20% * (5 - 3));
}
.loader span:nth-child(4) {
    width: calc(20% + 20% * (5 - 4));
    height: calc(20% + 20% * (5 - 4));
}
.loader span:nth-child(5) {
    width: calc(20% + 20% * (5 - 5));
    height: calc(20% + 20% * (5 - 5));
}

增加颜色变幻的动画效果：

.loader span {
    animation: animate 5s ease-in-out infinite alternate;
}
@keyframes animate {
    0% {
        /* red */
        background-color: rgba(100%, 0%, 0%, 0.3);
    }
    25% {
        /* yellow */
        background-color: rgba(100%, 100%, 0%, 0.3);
    }
    50% {
        /* green */
        background-color: rgba(0%, 100%, 0%, 0.3);
    }
    75% {
        /* blue */
        background-color: rgba(0%, 0%, 100%, 0.3);
    }
    100% {
        /* purple */
        background-color: rgba(100%, 0%, 100%, 0.3);
    }
}

再增加旋转效果：

@keyframes animate {
    0% {
        transform: rotate(0deg);
    }
    100% {
        transform: rotate(720deg);
    }
}

最后，为每个 span 设置动画延时，增加动感：

.loader span:nth-child(1) {
    animation-delay: calc(0.2s * (5 - 1));
}
.loader span:nth-child(2) {
    animation-delay: calc(0.2s * (5 - 2));
}
.loader span:nth-child(3) {
    animation-delay: calc(0.2s * (5 - 3));
}
.loader span:nth-child(4) {
    animation-delay: calc(0.2s * (5 - 4));
}
.loader span:nth-child(5) {
    animation-delay: calc(0.2s * (5 - 5));
}

知识点

• border-color https://developer.mozilla.org/en-US/docs/Web/CSS/border-color
• calc() https://developer.mozilla.org/en-US/docs/Web/CSS/calc
• rotate() https://developer.mozilla.org/en-US/docs/Web/CSS/transform-function/rotate
• animation-delay https://developer.mozilla.org/en-US/docs/Web/CSS/animation-delay