2018-10-10 22:03:03 +00:00
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---
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id: 587d78a9367417b2b2512ae8
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2020-12-16 07:37:30 +00:00
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title: 学习贝塞尔曲线的原理
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2018-10-10 22:03:03 +00:00
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challengeType: 0
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2020-02-11 07:46:34 +00:00
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videoUrl: 'https://scrimba.com/c/c9bDrs8'
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forumTopicId: 301058
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2021-01-13 02:31:00 +00:00
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dashedName: learn-how-bezier-curves-work
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2018-10-10 22:03:03 +00:00
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---
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2020-12-16 07:37:30 +00:00
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# --description--
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2018-10-10 22:03:03 +00:00
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2021-01-08 19:20:48 +00:00
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上一个挑战中,我们介绍了 `animation-timing-function` 以及它的一些预设值,这些值定义了不同时间内的动画速度。除了预定义值之外,CSS 还提供了贝塞尔曲线(Bezier curves)来更细致地控制动画的速度曲线。
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2018-10-10 22:03:03 +00:00
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2021-01-08 19:20:48 +00:00
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在 CSS 动画里,我们可以用 `cubic-bezier` 来定义贝塞尔曲线。曲线的形状代表了动画的速度。曲线在 `1 * 1` 的坐标系统内,其中 X 轴代表动画的时间间隔(类似于时间比例尺),Y 轴代表动画的改变。
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2018-10-10 22:03:03 +00:00
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2021-01-08 19:20:48 +00:00
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`cubic-bezier` 函数包含了 `1 * 1` 网格里的4个点:`p0`、`p1`、`p2`、`p3`。其中 `p0` 和 `p3` 是固定值,代表曲线的起始点和结束点,坐标值依次为 (0, 0) 和 (1, 1)。你只需设置另外两点的 x 值和 y 值,设置的这两点确定了曲线的形状从而确定了动画的速度曲线。在 CSS 里面通过 `(x1, y1, x2, y2)` 来确定 `p1` 和 `p2`。以下就是 CSS 贝塞尔曲线的例子:
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2018-10-10 22:03:03 +00:00
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2020-12-16 07:37:30 +00:00
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`animation-timing-function: cubic-bezier(0.25, 0.25, 0.75, 0.75);`
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2018-10-10 22:03:03 +00:00
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2021-01-08 19:20:48 +00:00
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在上面的例子里,两个点的 x 和 y 值相等(x1 = 0.25 = y1 和 x2 = 0.75 = y2),如果你还记得初中几何,结果是从原点到点 (1, 1) 的一条直线。动画速度呈线性,效果和 `linear` 一致。换言之,元素会匀速运动。
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2018-10-10 22:03:03 +00:00
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2020-12-16 07:37:30 +00:00
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# --instructions--
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2018-10-10 22:03:03 +00:00
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2021-01-08 19:20:48 +00:00
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对于 id 为 `ball1` 的元素,请把 `animation-timing-function` 属性值从 `linear` 变成等价的 `cubic-bezier` 函数值。也就是使用上面例子给的值。
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2018-10-10 22:03:03 +00:00
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2020-12-16 07:37:30 +00:00
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# --hints--
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2018-10-10 22:03:03 +00:00
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2020-12-16 07:37:30 +00:00
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id 为 `ball1` 的元素的 `animation-timing-function` 属性值应该为和 linear 预定值等价的贝塞尔函数值。
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2018-10-10 22:03:03 +00:00
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2020-12-16 07:37:30 +00:00
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```js
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assert(
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$('#ball1').css('animation-timing-function') ==
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'cubic-bezier(0.25, 0.25, 0.75, 0.75)'
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);
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2018-10-10 22:03:03 +00:00
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```
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2021-01-08 19:20:48 +00:00
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id 为 `ball2` 的元素的 `animation-timing-function` 属性值应该保持不变。
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2018-10-10 22:03:03 +00:00
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2020-12-16 07:37:30 +00:00
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```js
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const ball2Animation = $('#ball2')
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.css('animation-timing-function')
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.replace(/\s/g, '');
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assert(
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ball2Animation == 'ease-out' || ball2Animation == 'cubic-bezier(0,0,0.58,1)'
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);
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2018-10-10 22:03:03 +00:00
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```
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2020-02-11 07:46:34 +00:00
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2021-01-13 02:31:00 +00:00
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# --seed--
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## --seed-contents--
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```html
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<style>
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.balls{
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border-radius: 50%;
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background: linear-gradient(
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35deg,
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#ccffff,
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#ffcccc
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);
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position: fixed;
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width: 50px;
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height: 50px;
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margin-top: 50px;
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animation-name: bounce;
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animation-duration: 2s;
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animation-iteration-count: infinite;
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}
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#ball1 {
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left: 27%;
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animation-timing-function: linear;
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}
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#ball2 {
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left: 56%;
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animation-timing-function: ease-out;
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}
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@keyframes bounce {
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0% {
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top: 0px;
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}
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100% {
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top: 249px;
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}
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}
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</style>
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<div class="balls" id="ball1"></div>
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<div class="balls" id="ball2"></div>
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```
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2020-12-16 07:37:30 +00:00
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# --solutions--
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2021-01-13 02:31:00 +00:00
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```html
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<style>
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.balls{
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border-radius: 50%;
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background: linear-gradient(
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35deg,
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#ccffff,
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#ffcccc
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);
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position: fixed;
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width: 50px;
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height: 50px;
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margin-top: 50px;
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animation-name: bounce;
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animation-duration: 2s;
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animation-iteration-count: infinite;
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}
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#ball1 {
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left: 27%;
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animation-timing-function: cubic-bezier(0.25, 0.25, 0.75, 0.75);
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}
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#ball2 {
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left: 56%;
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animation-timing-function: ease-out;
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}
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@keyframes bounce {
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0% {
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top: 0px;
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}
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100% {
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top: 249px;
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}
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}
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</style>
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<div class="balls" id="ball1"></div>
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<div class="balls" id="ball2"></div>
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```
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