Sam Saffron Sam Saffron - 6 months ago 96
Javascript Question

What is the cleanest way to disable CSS transition effects temporarily?

I have a DOM element with some/all of the following effects applied:

#elem {
-webkit-transition: height 0.4s ease;
-moz-transition: height 0.4s ease;
-o-transition: height 0.4s ease;
-ms-transition: height 0.4s ease;
transition: height 0.4s ease;
}


I am writing a jQuery plugin that is resizing this element, I need to disable these effects temporarily so I can resize it smoothly.

What is the most elegant way of disabling these effects temporarily (and then re-enabling them), given they may be applied from parents or may not be applied at all.

Answer

Short Answer

Use this CSS:

.notransition {
  -webkit-transition: none !important;
  -moz-transition: none !important;
  -o-transition: none !important;
  transition: none !important;
}

Plus either this JS (with jQuery)...

$someElement.addClass('notransition'); // Disable transitions
doWhateverCssChangesYouWant($someElement);
$someElement[0].offsetHeight; // Trigger a reflow, flushing the CSS changes
$someElement.removeClass('notransition'); // Re-enable transitions

... or equivalent JavaScript using raw DOM functions or whatever other framework you're working with.

Explanation

This is actually a fairly subtle problem.

First up, you probably want to create a 'notransition' class that you can apply to elements to set their *-transition CSS attributes to none. For instance:

.notransition {
  -webkit-transition: none !important;
  -moz-transition: none !important;
  -o-transition: none !important;
  transition: none !important;
}

But that's just style, and is the easy bit. When you come to try and use this class, you'll run into a trap. The trap is that code like this won't work the way you might naively expect:

// Don't do things this way! It doesn't work!
$someElement.addClass('notransition')
$someElement.css('height', '50px') // just an example; could be any CSS change
$someElement.removeClass('notransition')

Naively, you might think that the change in height won't be animated, because it happens while the 'notransition' class is applied. In reality, though, it will be animated, at least in all modern browsers I've tried. The problem is that the browser is caching the styling changes that it needs to make until the JavaScript has finished executing, and then making all the changes in a single reflow. As a result, it does a reflow where there is no net change to whether or not transitions are enabled, but there is a net change to the height. Consequently, it animates the height change.

You might think a reasonable and clean way to get around this would be to wrap the removal of the 'notransition' class in a 1ms timeout, like this:

// Don't do things this way! It STILL doesn't work!
$someElement.addClass('notransition')
$someElement.css('height', '50px') // just an example; could be any CSS change
setTimeout(function () {$someElement.removeClass('notransition')}, 1);

but this doesn't reliably work either. I wasn't able to make the above code break in WebKit browsers, but on Firefox (on both slow and fast machines) you'll sometimes (seemingly at random) get the same behaviour as using the naive approach. I guess the reason for this is that it's possible for the JavaScript execution to be slow enough that the timeout function is waiting to execute by the time the browser is idle and would otherwise be thinking about doing an opportunistic reflow, and if that scenario happens, Firefox executes the queued function before the reflow.

The only solution I've found to the problem is to force a reflow of the element, flushing the CSS changes made to it, before removing the 'notransition' class. There are various ways to do this - see here for some. The closest thing there is to a 'standard' way of doing this is to read the offsetHeight property of the element.

One solution that actually works, then, is

$someElement.addClass('notransition'); // Disable transitions
doWhateverCssChangesYouWant($someElement);
$someElement[0].offsetHeight; // Trigger a reflow, flushing the CSS changes
$someElement.removeClass('notransition'); // Re-enable transitions

Here's a JS fiddle that illustrates the three possible approaches I've described here (both the one successful approach and the two unsuccessful ones): http://jsfiddle.net/2uVAA/35/