Using Keyframes – WebKit CSS Animation Examples

最新推荐文章于 2022-08-01 19:06:20 发布

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本文介绍如何使用CSS动画实现雪花飘落效果。通过@-webkit-keyframes定义动画关键帧，并结合-webkit-animation-name等属性控制动画播放。文章还展示了如何利用-webkit-transform进行旋转和平移。

Now WebKit supports explicit CSS animations! After seeing the new animation examples posted onWebKit.org, I needed to test keyframes by myself.
So I have created a dumb-downed version of the fallen leaves seen on webkit.org blog, called “Let it Snow”.

Unlike the fallen leaves example, I stick strictly with CSS only (means zero JavaScript). Also I tested on Webkit nightly and an iPhone (OS 2.0) Safari. On my iPhone (Mozilla/5.0 (iPhone; U; CPU iPhone OS 2_2 like Mac OS X; en-us) AppleWebKit/525.18.1 (KHTML, like Gecko) Version/3.1.1 Mobile/5G77 Safari/525.20), the animation is slow and some feature is ingored.

Well, let’s see the “Let It Snow” animation in action!

How to use Keyframes?

Keyframes are specified with the CSS “At-Rule” by using the keyword,@-webkit-keyframes, followed by an identifier (= animation-name)

	
@-webkit-keyframes animation-name {
 from {
   style definition ["Before"-state]
 }
 to {
   style definition ["After"-state]
 }
}

A keyframe defines the styles applied within the animation. To specify multiple frames, use “%” instead of “from” and “to” keywords.
Here’s an actual example I used for “Let it Snow”.

	
@-webkit-keyframes fade {
  0%   { opacity: 0; }
  10%  { opacity: 0.8; }
  100% { opacity: 0; }
}

This style is applie to create each snow flake appearance. A snowflake blurry appears (increase opacity) when 10% of the time elapsed (The total time is defined later. I’ll explain it next).
And at the end, the snowflake disappears (opacity back to zero).

Once the animation timeframe is defined, apply it using -webkit-animation-name and related properties.
I set total animation duration as 5 seconds, and the animatin goes forever (= infinite times. The default is 1).
See the simplified example below.

	
#snow div {
  -webkit-animation-name: fade;
  -webkit-animation-duration: 5s;
  -webkit-animation-iteration-count: infinite;
}

	
<div id="snow" class="snow">	
  <div>&#10053;</div> /* an entity for ❅ */
</div>

Using Transform

Let’s rotate and move around snowflakes by using -webkit-transform.
rotate, of course, rotate the element, and translate specifies a 2D translation by the vector [tx, ty]. (For more explanations, please see CSS transform spec page).
I used percent, 0 and 100% here, but of course you can use “from” and “to”.
Also note that transform doesn’t seem to work on current iPhone Safari yet.

	
@-webkit-keyframes spin{
  0%   { -webkit-transform: rotate(-180deg) translate(0px, 0px);}
  100% { -webkit-transform: rotate(180deg) translate(10px, 75px);}
}

You can just add the amination-name to the #snow div selector, separating with comma.

	
#snow div {
  -webkit-animation-name: fade, spin;
  ...
}

For the “Let it snow” example, I also include the cheesy “accumulate” keyframe to make snow accumulate on ground. Kinda ugly though.
Moreover, I gave the -webkit-animation-duration to individual snowflake so all flakes don’t fall all together!

	
.snowflake {
  color: #fff;
  font-size: 2em;
  position: absolute; (Note: The parent container is set relative positioned!)
}
.snowflake.f1 {
  left: 40px;
  -webkit-animation-duration: 5s;
}
.snowflake.f2 {
  font-size: 1.8em;
  left: 120px;
  -webkit-animation-duration: 7s;	
}
...

	
<div id="snow" class="snow">	
  <div class="snowflake f1">&#10053;</div> /* an entity for ❅ */
  <div class="snowflake f2">&#10052;</div> /* an entity for ❄ */
  ... (add two more snowflake-div in the actual sample)
</div>

To view the entire markup and CSS, just view source of the sample file!