Abstract
This document defines an API web page authors can use to write script-based
animations where the user agent is in control of limiting the update
rate of the animation. The user agent is in a better position to determine
the ideal animation rate based on whether the page is currently in a foreground
or background tab, what the current load on the CPU is, and so on. Using this
API should therefore result in more appropriate utilization of the CPU
by the browser.
Status of this Document
This section describes the status of this document at the time of its
publication. Other documents may supersede this document. A list of current W3C
publications and the latest revision of this technical report can be found in
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http://www.w3.org/TR/.
Beware. This specification is no longer in active maintenance and the Web
Performance Working Group does not intend to maintain it further. See HTML instead.
This document is the 22 September 2015 Working Group Note of the
Timing control for script-based animations specification.
Comments about this document may be sent to
public-web-perf@w3.org
(archived)
with [RequestAnimationFrame] at the start of the subject line but really, see the HTML specification instead.
This document is produced by
the Web Performance
Working Group.
Publication as a Working Group Note does not imply endorsement by the W3C Membership. This is a draft document and may be updated, replaced or obsoleted by other documents at any time. It is inappropriate to cite this document as other than work in progress.
This document is governed by the 1 September 2015 W3C Process Document.
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the 5
February 2004 W3C Patent Policy. W3C maintains
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Claim(s) must disclose the information in accordance
with section
6 of the W3C Patent Policy.
1. Introduction
This section is informative.
Animations in web browsers come in two forms: native, declarative ones, such
as the <animate> element in SVG, and those that are implemented
in script. These script-based animations are most often performed by scheduling
a callback using setTimeout or setInterval and making
changes to the DOM to effect the animation in that callback.
A disadvantage of this approach is that the author of the animation script has
no idea what the ideal frequency for updating their animation is. Instead, the
easiest way forward for the author is to simply call setTimeout with
a very small value, which in practice will be clamped to some minimum time like
10ms anyway. It likely won’t be the case that 100 updates per second are required
for the animation, especially if the page is in a background tab or the browser
window is minimized.
The API described in this document allows script authors to request the user
agent schedule an animation frame update. The user agent is in a better position
to determine how many frames per second to allocate to all of the animations running
in the entire browser. If there are many animations active, the user agent can
select a frame rate such that all of the animations will run as smoothly as possible.
If the page is not currently visible, animations on that page can be throttled
heavily so that they do not update often and thus consume little CPU power.
Here is an example of using the API to write a script-based animation.
HTML
<!DOCTYPE html>
<title>Script-based animation using requestAnimationFrame</title>
<style>
div { position: absolute; left: 10px; padding: 50px;
background: crimson; color: white }
</style>
<script>
var requestId = 0;
function animate(time) {
document.getElementById("animated").style.left =
(time - animationStartTime) % 2000 / 4 + "px";
requestId = window.requestAnimationFrame(animate);
}
function start() {
animationStartTime = window.performance.now();
requestId = window.requestAnimationFrame(animate);
}
function stop() {
if (requestId)
window.cancelAnimationFrame(requestId);
requestId = 0;
}
</script>
<button onclick="start()">Click me to start!</button>
<button onclick="stop()">Click me to stop!</button>
<div id="animated">Hello there.</div>
4. Window interface extensions
The partial interface in the IDL fragment below is used to expose the
requestAnimationFrame
operation on the Window
object. In the definition of requestAnimationFrame below, references to the
Document
object are to be taken to be references to the Window
object’s active document. [HTML5]
partial interface Window {
long requestAnimationFrame(FrameRequestCallback callback);
void cancelAnimationFrame(long handle);
};
callback FrameRequestCallback = void (DOMHighResTimeStamp time);
The requestAnimationFrame method is
used to signal to the user agent that a script-based animation
needs to be resampled. When requestAnimationFrame(callback) is called, the user agent MUST schedule a script-based animation
resampling by appending to the end of the animation frame request callback list
an entry whose handle is a user-agent-defined integer greater than zero that uniquely identifies the entry in the list and
whose callback is callback.
Each FrameRequestCallback object has a cancelled boolean flag. This
flag is initially false and is not exposed by any interface.
requestAnimationFrame only schedules
a single update to the script-based animation. If subsequent animation frames are needed, then
requestAnimationFrame will need to be called
again from within the callback.
Also note that multiple calls to requestAnimationFrame with the same callback
(before callbacks are invoked and
the list is cleared) will result in multiple entries being in the list with that same callback,
and thus will result in that callback being invoked more than once for the animation frame.
The cancelAnimationFrame method is
used to cancel a previously made request to schedule an animation frame update. When cancelAnimationFrame(handle) is called,
the user agent MUST set the cancelled flag to true for the
callback registered on this Document whose handle is handle. The cancelled
flag is set whether the callback is in a animation frame request callback list or not.
If there is no callback with the given handle, then this function does nothing.
6. Acknowledgements
This section is informative.
The editors would like to thank the following people for contributing to
this specification: Boris Zbarsky, Jonas Sicking, Robert O’Callahan.
This specification was written based on the mozRequestAnimationFrame
feature implemented in Firefox.