# css动画

## animation
CSS animation属性是如下属性的一个简写属性形式: <br/>
animation-name, animation-duration, animation-timing-function, animation-delay, animation-iteration-count, animation-direction 和 animation-fill-mode.
```css
/* @keyframes duration | timing-function | delay |
   iteration-count | direction | fill-mode | play-state | name */
  animation: 3s ease-in 1s 2 reverse both paused slidein;

/* @keyframes duration | timing-function | delay | name */
  animation: 3s linear 1s slidein;

/* @keyframes duration | name */
  animation: 3s slidein;
```

## animation-delay
定义:
```
CSS属性定义动画于何时开始，即从动画应用在元素上到动画开始的这段时间的长度。
```
语法:
```css
animation-delay: 3s;
animation-delay: 2s, 4ms;
```

## animation-direction
定义:
```
CSS 属性指示动画是否反向播放，它通常在简写属性animation中设定
```
语法:
```css
animation-direction: normal
animation-direction: reverse
animation-direction: alternate
animation-direction: alternate-reverse
animation-direction: normal, reverse
animation-direction: alternate, reverse, normal
```
|属性|作用|
|:-|-|
|normal|每个循环内动画向前循环，换言之，每个动画循环结束，动画重置到起点重新开始，这是默认属性。|
|alternate|动画交替反向运行，反向运行时，动画按步后退，同时，带时间功能的函数也反向，比如，ease-in 在反向时成为ease-out。计数取决于开始时是奇数迭代还是偶数迭代|
|reverse        |反向运行动画，每周期结束动画由尾到头运行。|
|alternate-reverse|反向交替， 反向开始交替;动画第一次运行时是反向的，然后下一次是正向，后面依次循环。决定奇数次或偶数次的计数从1开始。|

## animation-duration
定义:
```
属性指定一个动画周期的时长。
```
语法:
```css
animation-duration: 6s
animation-duration: 120ms
animation-duration: 1s, 15s
animation-duration: 10s, 30s, 230ms
```
|属性|作用|
|:-|-|
|time|一个动画周期的时长，单位为秒(s)或者毫秒(ms)，无单位值无效。|

## animation-fill-mode
定义:
```
这个 CSS 属性用来指定在动画执行之前和之后如何给动画的目标应用样式。
```
语法
```css
/* <single-animation-fill-mode> = none | forwards | backwards | both */
animation-fill-mode: none
animation-fill-mode: forwards
animation-fill-mode: backwards
animation-fill-mode: both
/* 可以应用多个参数，这个时候使用逗号隔开  */
/* 各个参数应用于与次序相对应的动画名 */
animation-fill-mode: none, backwards
animation-fill-mode: both, forwards, none
```
>none<br/>
动画执行前后不改变任何样式
>forwards<br/>
目标保持动画最后一帧的样式，最后一帧是哪个取决于animation-direction和 animation-iteration-count:

|animation-direction|animation-iteration-count|last keyframe encountered|
|:---------------:|:---------:|:--------:|
|normal           |even or odd|100% or to|
|reverse          |even or odd|0% or from|
|alternate        |even       |0% or from|
|alternate        |odd        |100% or to|
|alternate-reverse|even       |100% or to|
|alternate-reverse|odd        |0% or from|
>backwards<br/>
动画采用相应第一帧的样式，保持 animation-delay，第一帧取法如下：

|animation-direction|first relevant keyframe|
|:-|:-:|
|normal or alternate	        | 0% or from |
|reverse or alternate-reverse |	100% or to |
>both<br/>
动画将会执行 forwards 和 backwards 执行的动作。

animation-iteration-count
定义:
```
CSS 属性   定义动画在结束前运行的次数 可以是1次 无限循环.
默认属性 animation 默认播放动画循环一次.
```
语法:
```css
/* 使用规范 <single-animation-iteration-count> = infinite | <number> */
animation-iteration-count: infinite;
animation-iteration-count: 3;
animation-iteration-count: 2.3;
animation-iteration-count: 2, 0, infinite;
```

## animation-name
定义
```
属性指定应用的一系列动画，每个名称代表一个由@keyframes定义的动画序列。
```
```css
/* Single animation */
animation-name: none;
animation-name: test_05;
animation-name: -specific;
animation-name: sliding-vertically;

/* Multiple animations */
animation-name: test1, animation4;
animation-name: none, -moz-specific, sliding;

/* Global values */
animation-name: initial
animation-name: inherit
animation-name: unset
```

## animation-play-state
定义:
```
CSS 属性定义一个动画是否运行或者暂停。
可以通过查询它来确定动画是否正在运行。
另外，它的值可以被设置为暂停和恢复的动画的重放。
恢复一个已暂停的动画，将从它开始暂停的时候，而不是从动画序列的起点开始在动画。
```
语法
```css
/* 正式语法 => <single-animation-play-state> = running | paused */
/* Single animation=>单一动画 */
animation-play-state: running;
animation-play-state: paused;

/* Multiple animations=>多个动画 */
animation-play-state: paused, running, running;

/* Global values=>全球价值 */
animation-play-state: inherited;
animation-play-state: initial;
animation-play-state: unset;
```
|属性|注释|
|:-|-|
|running|当前动画正在运行。|
|paused|当前动画以被停止。|

## animation-timing-function
属性
```
定义CSS动画在每一动画周期中执行的节奏。
可能值为一或多个 <timing-function>。

对于关键帧动画来说，timing function作用于一个关键帧周期而非整个动画周期，
即从关键帧开始开始，到关键帧结束结束。

定义于一个关键帧区块的缓动函数(animation timing function)应用到改关键帧；
另外，若该关键帧没有定义缓动函数，则使用定义于整个动画的缓动函数。
```
示例
```css
/* Keyword values */
animation-timing-function: ease;
animation-timing-function: ease-in;
animation-timing-function: ease-out;
animation-timing-function: ease-in-out;
animation-timing-function: linear;
animation-timing-function: step-start;
animation-timing-function: step-end;

/* Function values */
animation-timing-function: cubic-bezier(0.1, 0.7, 1.0, 0.1);
animation-timing-function: steps(4, end);
animation-timing-function: frames(10);

/* Multiple animations */
animation-timing-function: ease, step-start, cubic-bezier(0.1, 0.7, 1.0, 0.1);

/* Global values */
animation-timing-function: inherit;
animation-timing-function: initial;
animation-timing-function: unset;
```
|值|描述|
|-|-|
|linear|动画从头到尾的速度是相同的|
|ease|默认。动画以低速开始，然后加快，在结束前变慢。|
|ease-in|动画以低速开始。|
|ease-out|动画以低速结束。|
|ease-in-out|动画以低速开始和结束.|
|cubic-bezier(n,n,n,n)|在 cubic-bezier 函数中自己的值。可能的值是从 0 到 1 的数值。|


## FAQ
[贝塞尔曲线(cubic-bezier)](https://segmentfault.com/a/1190000004618375)<br/>
疑惑点:
```css
animation-timing-function: cubic-bezier(0.1, 0.7, 1.0, 0.1);
```
[CSS3 timing-function: steps() 详解](https://idiotwu.me/understanding-css3-timing-function-steps/)
```css
animation-timing-function: steps(4, end);
```

## Cylon 眼睛
考虑到所有浏览器特定的前缀，这里是一个Cylon 眼睛动画合并线性渐变和动画，适用于所有主要的浏览器：
```html
<div class="view_port">
  <div class="polling_message">
    Listening for dispatches
  </div>
  <div class="cylon_eye"></div>
</div>
```
```css
.polling_message {
  color:white;
  float:left;
  margin-right:2%;            
}

.view_port {
  background-color:black;
  height:25px;
  width:100%;
  overflow: hidden;
}

.cylon_eye {
  color:white;
  height: 100%;
  width: 20%;

  background-color: red;
  background-image: -webkit-linear-gradient(left, rgba( 0,0,0,0.9 ) 25%, rgba( 0,0,0,0.1 ) 50%, rgba( 0,0,0,0.9 ) 75%);
  background-image:    -moz-linear-gradient(left, rgba( 0,0,0,0.9 ) 25%, rgba( 0,0,0,0.1 ) 50%, rgba( 0,0,0,0.9 ) 75%);
  background-image:     -ms-linear-gradient(left, rgba( 0,0,0,0.9 ) 25%, rgba( 0,0,0,0.1 ) 50%, rgba( 0,0,0,0.9 ) 75%);
  background-image:      -o-linear-gradient(left, rgba( 0,0,0,0.9 ) 25%, rgba( 0,0,0,0.1 ) 50%, rgba( 0,0,0,0.9 ) 75%);
  background-image:         linear-gradient(to right, rgba( 0,0,0,0.9 ) 25%, rgba( 0,0,0,0.1 ) 50%, rgba( 0,0,0,0.9 ) 75%);

  -webkit-animation: move_eye 4s linear 0s infinite alternate;
     -moz-animation: move_eye 4s linear 0s infinite alternate;
       -o-animation: move_eye 4s linear 0s infinite alternate;
          animation: move_eye 4s linear 0s infinite alternate;
}

@-webkit-keyframes move_eye { from { margin-left:-20%; } to { margin-left:100%; }  }
   @-moz-keyframes move_eye { from { margin-left:-20%; } to { margin-left:100%; }  }
     @-o-keyframes move_eye { from { margin-left:-20%; } to { margin-left:100%; }  }
        @keyframes move_eye { from { margin-left:-20%; } to { margin-left:100%; }  }
```