GlobalEventHandlers.onanimationstart
An event handler for the animationstart
event. This event is sent when a CSS Animation starts to play.
Syntax
var animStartHandler = target.onanimationstart;
target.onanimationstart = Function
Value
A Function
to be called when an animationstart
event occurs
indicating that a CSS animation has begun on the target
, where the
target object is an HTML element (HTMLElement
), document
(Document
), or window (Window
). The function receives as
input a single parameter: an AnimationEvent
object describing the event
which occurred.
Example
CSS content
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.
#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;
}
The animation sequence is described next. First, the "slideAnimation"
class, which establishes the animation
that will cause the box to move
over the course of five seconds, one time, using the "slideBox"
keyframe
set. The 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.
.slideAnimation {
animation: 5s ease-in-out 0s 1 slideBox;
}
@keyframes slideBox {
from {
left:0;
top:0;
}
to {
left:calc(100% - var(--boxwidth));
top:calc(100% - var(--boxwidth))
}
}
Since the CSS describes the animation but doesn't connect it to the box, we'll need some JavaScript code to do that. We'll get to that shortly.
JavaScript content
Before we get to the animation code, we define a function which logs information to a
box on the user's screen. We'll use this to show information about the events we
receive. Note the use of AnimationEvent.animationName
and
AnimationEvent.elapsedTime
to get information about the event which occurred.
function log(msg, event) {
let logBox = document.getElementById("log");
logBox.innerHTML += msg;
if (event) {
logBox.innerHTML += " <code>"+ event.animationName +
"</code> at time " + event.elapsedTime.toFixed(2) +
" seconds.";
}
logBox.innerHTML += "\n";
};
Then we set up the event handlers for the animationstart
and
animationend
events:
let box = document.getElementById("box");
box.onanimationstart = function(event) {
log("Animation started", event);
}
box.onanimationend = function(event) {
log("Animation stopped", event);
};
Finally, we set up a handler for a click on the button that runs the animation:
document.getElementById("play").addEventListener("click", function(event) {
document.getElementById("box").className = "slideAnimation";
event.target.style.display = "none";
}, false);
This sets the class of the box we want to animate to the class that contains the
animation
description, then hides the play button because this example
will only run the animation once. For information about why, and how to support running
an animation more than once, see
Run an animation again in CSS Animations tips and tricks.
Result
Assembled together, you get this:
Specifications
Specification |
---|
CSS Animations Level 2 # dom-document-onanimationstart |
Browser compatibility
BCD tables only load in the browser
See also
- The
animationstart
event this event handler is triggered by. AnimationEvent