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Re: Open Source Cross-Browser setImmediate in JavaScript

From: Domenic Denicola <domenic@domenicdenicola.com>
Date: Thu, 7 Jul 2011 00:08:33 -0400
Message-ID: <BLU150-ds12766509CEE0B638D429C0B2410@phx.gbl>
To: <public-web-perf@w3.org>
> Thanks both of you for the feedback.  Could you give some examples
(ideally with code) of when you would want to use setImmediate() and why
existing mechanisms (like setTimeout()) are not ideal?

> - James

One example in my own projects is a pub-sub system. pub('eventName') is
implemented, by design, to yield immediately to the calling code and execute
any subscribing callbacks whenever there is a break in the event loop that
allows for the (possibly computationally-intensive) callbacks to be
executed. And since they are each fired with separate setImmediate calls,
they yield independently.

Another common use case is to work around rendering timing issues. It seems
that in some cases, at least according to comments that I've left for
myself, adding a class to an element does not immediately change the
computed characteristics of the element---especially when e.g. scrolling
things into view is involved. I have previously used setTimeout(, 0) to work
around this and yield until such a time as the rendering has completed and
the event loop comes back around to process my code that depends on the
now-updated computed characteristics. setImmediate is more efficient and
much clearer as to why I am delaying, and I don't believe this is the
intended use of requestAnimationFrame either.

A final common case is to fire off an expensive operation, often interfacing
with a browser plugin or some other piece of code that will hang the browser
for a half-second. In this case, calling setImmediate allows the UI to
update in response to the triggering event, instead of stalling until the
plugin returns control to the browser UI thread and only then showing both
the result of the plugin's work and the updated UI. This of course applies
equally well to computationally-intensive JavaScript code.

 I can continue trawling my source code for uses of setTimeout(, 0), or
nextTick, but in general they are all instances where I need to yield to
calling code, or to the UI, which I believe was the intent of setImmediate.

-Domenic
Received on Friday, 8 July 2011 13:40:25 UTC

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