前言

在了解 cubic-bezier 之前，你需要对 CSS3 中的动画效果有所认识，它是 animation-timing-function 和 transition-timing-function 中一个重要的内容。

本体

简介

cubic-bezier 又称三次贝塞尔，主要是为 animation 生成速度曲线的函数，规定是 cubic-bezier(, , , )。

我们可以从下图中简要理解一下 cubic-bezier：

从上图我们需要知道的是 cubic-bezier 的取值范围:

P0：默认值 (0, 0)
P1：动态取值 (x1, y1)
P2：动态取值 (x2, y2)
P3：默认值 (1, 1)

我们需要关注的是 P1 和 P2 两点的取值，而其中 X 轴的取值范围是 0 到 1，当取值超出范围时 cubic-bezier 将失效；Y 轴的取值没有规定，当然也毋须过大。

最直接的理解是，将以一条直线放在范围只有 1 的坐标轴中，并从中间拿出两个点来拉扯（X 轴的取值区间是 [0, 1]，Y 轴任意），最后形成的曲线就是动画的速度曲线。

使用

在测试例子中：

<!DOCTYPE html><html lang="zh-cn"><head>  <meta charset="UTF-8">  <title>Document</title>  <style>    .animation {      width: 50px;      height: 50px;      background-color: #ed3;      -webkit-transition:  all 2s;           -o-transition:  all 2s;              transition:  all 2s;    }    .animation:hover {      -webkit-transform:  translateX(100px);          -ms-transform:  translateX(100px);           -o-transform:  translateX(100px);              transform:  translateX(100px);    }  </style></head><body>  <div class="animation"></div></body></html>

我们可以在浏览器中看到，当鼠标移到元素上时，元素开始向右移动，开始比较慢，之后则比较快，移开时按原曲线回到原点。

在例子中，当我们不为 transition 添加 cubic-bezier 或是其他 timing-function 时，默认的速度曲线是 ease，此时的速度曲线是：

那么让我们在代码中加入 cubic-bezier(.17, .86, .73, .14)：

....animation {  ...  -webkit-transition:  all 2s cubic-bezier(.17, .86, .73, .14);       -o-transition:  all 2s cubic-bezier(.17, .86, .73, .14);          transition:  all 2s cubic-bezier(.17, .86, .73, .14);}...

再刷新页面观察效果，会看到动画在执行过程中有一段很缓慢的移动，前后的速度相似，此时的运动曲线是：

几个常用的固定值对应的 cubic-bezier 值以及速度曲线

ease：cubic-bezier(.25, .1, .25, 1)
liner：cubic-bezier(0, 0, 1, 1) / cubic-bezier(1, 1, 0, 0)
ease-in：cubic-bezier(.42, 0, 1, 1)
ease-out：cubic-bezier(0, 0, .58, 1)
ease-in-out：cubic-bezier(.42, 0, .58, 1)
In Out . Back（来回的缓冲效果）：cubic-bezier(0.68, -0.55, 0.27, 1.55)

效果参考

文章所提到的动画效果可以在下面站点中看到，当然你也可以大胆尝试：

英文版在线预览（Lea Verou）
中文版在线预览（更多效果）
在线生成系列
作者的《Loading》库

参考

MDNW3School

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