Introduction to complex animations

For animations that can't be driven by adding and removing classes, AMP offers several animation specific components. These components apply AMP's principles to animations: they're fast, efficient, and user first. AMP restricts what CSS properties inside keyframes are allowed, but grants benefits such as fine-grain control, seamless animations, and cross browser compatibility with no extra work.

Use amp-animation if you need to tightly control playback, as well as have precise timing with multiple elements animating at the same time.

Creating a basic AMP animation

The amp-animation component enables use of the Web Animation API in AMP.

A basic amp-animation is a JSON object made of the following key parts:

The element the component is animating, or selector.
Timing Properties
Keyframes
Trigger

<amp-animation layout="nodisplay" id="exampleAnimation">
<script type="application/json">
{
 "selector": "#elementID", //select the element to animate
 "duration": "1s", //timing property
 "iterations": 2, //timing property
 "fill": "both", //timing property
 "keyframes": {"opacity": 0, "transform": "scale(2)"} //keyframes
}
</script>
</amp-animation>
<!-- trigger -->
<button on="tap:exampleAnimation.start">

Selector

Much like CSS, the amp-animation component links the animation properties to the element by declaring the element's tag name, class, or id in the "selector" field. The component animates each element with the tag type or class name declared. Use an id to ensure you animate a single element.

Timing properties

The timing properties control how long an animation takes, the amount of times it plays, and which direction keyframes execute.

No timing properties are required, but an animation might not run if properties related to time and display are missing, such as duration and fill.

Keyframes

While CSS allows you to morph from one state to another via transitions, you must declare animation properties as keyframes to implement amp-animation (similar to CSS animations). To ensure smooth playback and cross browser compatibility, amp-animation restricts what keyframe properties are usable to GPU accelerated properties that do not cause a re-layout and can animate on the compositor thread. This prevents animations from interfering with AMP and the browser's render process.

Keyframes are either defined directly in an amp-animation or referenced from <amp style-keyframe> as long as they follow the property restrictions. Read more here about keyframes in amp-animation.

Trigger

The trigger starts the animation sequence. The amp-animation extension starts either when the <body> becomes visible on the page or by connecting it to an AMP action or event

Triggering on visibility of <body> is useful when the animation should run as soon as the page loads because it appears "above the fold", or within the first viewport of the page. Animations trigger through visibility by adding trigger="visibility" as an attribute to the component.

<amp-animation layout="nodisplay"
    trigger="visibility">
  ...
</amp-animation>

Animations connect to an action or event by assigning the amp-animation component an id and linking that id to the desired event trigger, such as tapping a button.

<amp-animation layout="nodisplay" id="exampleAnimation">
  ...
</amp-animation>

<button on="tap:exampleAnimation.start">

Building complex animations

Building an animation in amp-animation allows for fine grained control that goes beyond starting and stopping an animation: it can also pause, reverse, and direct to a specific point. You can even chain multiple animations together and animate elements in a sequence.

Subtargets

Elements of the same tag or class can have specified timing properties and override the values of variables defined in the top level animation.

<body>
  <h1>Hello World!</h1>
  <h1>Hello World!</h1>
  <h1 id="helloMe">Hello World!</h1>
  <h1>Hello World!</h1>
  <amp-animation layout="nodisplay" id="animateThis">
    <script type="application/json">
      {
        "selector": "h1",
        "duration": "3s",
        "fill": "both",
        "keyframes": [{"transform": "translateX(0px)"}, {"transform": "translateX(50%)"}],
        "subtargets": [
          {
            "index": 1,
            "duration": "1s"
          },
          {
            "selector": "#helloMe",
            "direction": "reverse",
            "duration": "5s"
          }
        ]
      }
    </script>
  </amp-animation>
  <button on="tap:animateThis.start">
   start
  </button>
</body>

在 playground 中打开此代码片段

Chained animations

Multiple animations can connect together to form a large sequence. You can create timed effects, such as overlays on a video, by writing animations in the animations array within the amp-animation component.

<amp-animation id="overlaysAnim" layout="nodisplay">
  <script type="application/json">
    {
      "duration": "3s",
      "fill": "both",
      "animations": [{
          "selector": ".one",
          "keyframes": [{
              "opacity": "1",
              "offset": 0
            },
            {
              "opacity": "1",
              "offset": 0.04
            },
            {
              "opacity": "0",
              "offset": 0.0401
            },
            {
              "opacity": "0",
              "offset": 1
            }
          ]
        },
      ]
    }
  </script>
</amp-animation>

This setup plays each animation for 3 seconds in a sequence.

For larger animations, animations inside the animations array are able to reference other amp-animation components.

<amp-animation id="addEnergy" layout="nodisplay">
  <script type="application/json">
  {
    "duration": "0.3s",
    "fill": "both",
    "direction": "alternate",
    "animations": [
      {
        "selector": "#energy",
        "keyframes": [
          {"transform": "scaleX(calc(num(width('#energy'))/10))"},
          {"transform": "scaleX(calc(num(width('#energy'))/10 + 3))"}
        ]
      },
      {
        "animation": "atomExcite"
      }
    ]
  }
  </script>
</amp-animation>


<amp-animation id="atomExcite" layout="nodisplay" trigger="visibility">
<script type="application/json">
  {
    "duration": "0.3s",
    "iterations": "2",
    "fill": "both",
    "direction": "alternate",
    "animations": [
      {
        "selector": ".atom",
        "keyframes": {
          "transform": "translate(20vw)"
        }
      }
    ]
  }
  </script>
</amp-animation>

Animating an unknown amount of elements

By using var() and calc() expressions along with CSS extensions, you can write complex and timed animations that work with any number of elements. This allows for dynamic and user generated data to be animated with ease and fluidity.

<head>
  <script async custom-element="amp-animation" src="https://cdn.ampproject.org/v0/amp-animation-0.1.js"></script>
  <style amp-custom>
    .parent {
      perspective: 1000px;
      transform-style: preserve-3d;
      position: relative;
      margin: 10px;
      width: 239px;
      height: 335px;
    }
    .card {
      transform-origin: left;
      height: 50%;
      width: 50%;
    }
  </style>  
</head>
<body>
  <amp-animation layout="nodisplay" id="cardAdmin">
    <script type="application/json">
      {
        "selector": ".card",
        "--duration": "2s",
        "duration": "var(--duration)",
        "delay": "calc((length() - index() - 1) * var(--duration))",
        "easing": "ease-in",
        "iterations": "1",
        "fill": "both",
        "keyframes": [
            {"transform": "translate3d(0px, 0px, 0px)"},
            {"transform": "translate3d(50%, 0px, 100px)"},
            {"transform": "translate3d(110%, 0px, 0px) rotateY(-20deg)"},
            {"transform": "translate3d(50%, 0px, -100px)"},
            {"transform": "translate3d(0px, 0px, -1px)"}
        ]
      }
    </script>
  </amp-animation>
  <div class="parent" on="tap:cardAdmin.start" tabindex=none role="animation">
    <amp-img class="card" src="https://upload.wikimedia.org/wikipedia/commons/7/70/3C.svg" layout="fill"></amp-img>
    <amp-img class="card" src="https://upload.wikimedia.org/wikipedia/commons/3/3a/3H.svg" layout="fill"></amp-img>
    <amp-img class="card" src="https://upload.wikimedia.org/wikipedia/commons/e/e1/KC.svg" layout="fill"></amp-img>
  </div>    
</body>

在 playground 中打开此代码片段

This example works by:

Declaring a variable, --duration, and gives it the value of two seconds.
Sets the duration to the var --duration's value.
Calculates the delay applied to each element with that meets the selector .card.
1. The length() extension calculates how many .card elements were selected
2. The length then subtracts each .card's index()
3. The resulting value is multiplied by the var --duration
4. The final total is applied in seconds to that element's delay
The animation is applied to each element individually so that the cards are shuffled one after another instead of all at the same time.

Open the animation in the AMP playground and add more amp-img elements to test this behavior.

Look great, everywhere

Animations can include conditions that allow customized effects. Tailor animations to any screen size through the media condition and supports backwards browser compatibility by enabling supports conditions in a switch statement.

<head>
 <style amp-custom>
    .drop {
      width: 20px;
      height: 20px;
      background: blue;
      margin-top: 1em;
      border-radius: 50%;
    }
    .right {
      position: absolute;
      right: 0;
      background: red;
    }
  </style>
  <script async custom-element="amp-animation" src="https://cdn.ampproject.org/v0/amp-animation-0.1.js"></script>
</head>
<body>
<amp-animation id="mediaAnimation" layout="nodisplay">
  <script type="application/json">
    {
      "duration": "1s",
      "iterations": "4",
      "fill": "both",
      "direction": "alternate",
      "animations": [
        {
          "media": "(min-width: 300px)",
          "selector": ".drop",
          "keyframes": {
            "transform": "translate(100vw)"
          }
        },
        {
          "media": "(max-width: 300px)",
          "selector": ".drop",
          "keyframes": {
            "transform": "translate(50vw)"
          }
        },
        {
          "media": "(min-width: 300px)",
          "selector": ".right",
          "keyframes": {
            "transform": "translate(-100vw)"
          }
        },
        {
          "media": "(max-width: 300px)",
          "selector": ".right",
          "keyframes": {
            "transform": "translate(-50vw)"
          }
        }
      ]
    }
  </script>
</amp-animation>  

  <div class="rain">
    <div class="drop"></div>
    <div class="drop right"></div>
    <div class="drop"></div>
    <div class="drop right"></div>
    <div class="drop"></div>
    <div class="drop right"></div>
    <div class="drop"></div>
    <div class="drop right"></div>
  </div>
  <button on="tap:mediaAnimation.start">Start</button>
</body>

在 playground 中打开此代码片段

Written by @CrystalOnScript

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Introduction to complex animations

Creating a basic AMP animation

Selector

Timing properties

Keyframes

Trigger

Building complex animations

Subtargets

Chained animations

Animating an unknown amount of elements

Look great, everywhere