Element：animationiteration 事件

Baseline Widely available

This feature is well established and works across many devices and browser versions. It’s been available across browsers since March 2020.

animationiteration 事件会在一次 CSS 动画迭代的结束和另一次迭代的开始时触发。此事件不会与 animationend 事件同时发生，因此其不会发生在 animation-iteration-count 为 1 的动画上。

语法

在类似 addEventListener() 这样的方法中使用事件名称，或设置事件处理器属性。

js
addEventListener("animationiteration", (event) => {});

onanimationiteration = (event) => {};

示例

Let's create an animation which automatically pauses at the end of each iteration, allowing the user to choose whether or not to start the next iteration. Much of this code is the same as in other examples of animation events, so it may look familiar.

CSS

Leaving out some bits of the CSS that don't matter for the discussion here, let's take a look at the styles for the box that we're animating. First is the box itself. We set its size, position, color, and layout. Note that there's nothing there about animation. That's because we don't want the box to start animating right away. We'll add the animation style later to start animating the box.

css
#box {
  width: var(--boxwidth);
  height: var(--boxwidth);
  left: 0;
  top: 0;
  border: 1px solid #7788ff;
  margin: 0;
  position: relative;
  background-color: #2233ff;
  display: flex;
  justify-content: center;
  animation: 2s ease-in-out 0s infinite alternate both paused slideBox;
}

The animation's keyframes are defined next; they describe an animation which causes the box to migrate from the top-left corner of the container to the bottom-right corner.

css
@keyframes slideBox {
  from {
    left: 0;
    top: 0;
  }
  to {
    left: calc(100% - var(--boxwidth));
    top: calc(100% - var(--boxwidth));
  }
}

JavaScript

Some JavaScript code will need to be written to handle the click on the button to start the next iteration. Let's have a look.

js
var box = document.getElementById("box");
var iterationCounter = 0;

box.onanimationiteration = function (event) {
  box.style.animationPlayState = "paused";
  document.getElementById("play").innerHTML =
    "Start Iteration #" + (iterationCounter + 1);
};

This sets up two global variables: box, which references the "box" element that we're animating, and iterationCounter, which is initially zero, which indicates how many iterations of the animation have occurred.

The onanimationiteration event handler is then set up. It simply sets the box's animation-play-state to "paused", then updates the text displayed in the button to indicate that clicking the button will start playing the next iteration of theanimation.

Finally, we set up a handler for a click on the button that runs the animation:

js
document.getElementById("play").addEventListener(
  "click",
  function (event) {
    box.style.animationPlayState = "running";
    iterationCounter++;
  },
  false,
);

This sets the box element's animation-play-state to "running" and increments the iteration counter. That's all there is to it!

结果

Assembled together, you get this:

Each time the box reaches the opposing corner, it stops, with the button reflecting which iteration number is up next, until you click the button to run the next iteration.

规范

Specification
CSS Animations Level 1
# eventdef-globaleventhandlers-animationiteration

浏览器兼容性

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参见