The end goal is to add a scrolling-up class to the body element when the user is scrolling up the page and remove said class when scrolling down.
I am achieving this the old school way, by attaching a throttled(via lodash) callback to the scroll event, like so:
var lastScrollTop = 0;
var checkScrollDirection = function() {
var currentOffset = window.pageYOffset;
document.body.classList.toggle('scrolling-up', currentOffset < lastScrollTop );
lastScrollTop = currentOffset;
};
window.addEventListener('scroll', _.throttle(
checkScrollDirection,
100,
{
'leading': true,
'trailing': true
}
)
);
This works (very) well, but I am wondering if it is possible to achieve this by using a modern Observer, and therefore taking all this off the main thread. Even if throttled, the above logic still takes alot more CPU time than makes sense from a purely logical standpoint.
Thank you!
If we place some target elements strategically in the body we can have them observed by an IntersectionObserver to sense whether the body is being scrolled up or down.
The minimum number of such targets would seem to be one viewport height one every other 100vh going down the body (with an adjustment for a remaining bit at the end). This way there is always one, but only one, target in the viewport at any one time. This target can be observed so it triggers code at various thresholds.
There is a balance to be struck between the number of 'stops' in the viewport and the time taken by observing. This snippet has observation every 5% of the viewport and on the devices tried (laptop, iPad) this seems to give no perceptible problem in practice.
This method has a slightly hacky feel to it as it entails adding elements to the document, and on a resize we have to resort to JS to recalculate their number, height and position.
However, the method seems to work, the absolute maximum GPU usage I saw with manic continuous scrolling and change of direction was around 7%. 'Ordinary' sort of scrolling hardly registers. The targets in the snippet have width 1px, I do not know whether it's better or worse processor-usage wise to have them wide or thin. They are placed in the center of the viewport just in case there are any problems with observing right at the edges.
This snippet just adds class scrolling-up to the body as appropriate and this shows/hides a fixed header.
//Set up action on intersection being observed
let previousTarget, previousTop, scrollingUp;
const header = document.querySelector('.header');
function scrolledFn(entries) {
entries.forEach(entry => {
if (entry.isIntersecting) { //note, only one can intersect at a time
if (entry.target == previousTarget) {
const newScrolling = (entry.boundingClientRect.top > previousTop);
if (newScrolling != scrollingUp) {
document.body.classList.toggle('scrolling-up');
scrollingUp = !scrollingUp;
}
}
else {
previousTarget = entry.target;
}
previousTop = entry.boundingClientRect.top;
}
});
}
const step = 0.05;
let thresholds =[];
for ( let t = 0; t <= 1; t += step) { thresholds.push(t); }
let observer = new IntersectionObserver(scrolledFn, {threshold: thresholds});
function setupTargets() {
// first remove any targets we may have already
const oldTargets = document.querySelectorAll('.observed');
oldTargets.forEach(oldTarget => { oldTarget.remove();
});
// Insert targets into the document
const numWholeViewports = Math.floor(document.body.offsetHeight/window.innerHeight);
const numFullHeightTargets = Math.floor((numWholeViewports + 1) / 2);
let i = 0;
function createTarget(h) {
const el = document.createElement('div');
document.body.appendChild(el);
observer.observe(el);
el.classList.add('observed');
el.style.top = i*200 + 'vh';
el.style.height = h + 'vh';
}
for (i; i < numFullHeightTargets ; i++) {
createTarget('100');
}
if (numWholeViewports%2 == 0) { createTarget((document.body.offsetHeight%window.innerHeight) * 100 / window.innerHeight); }
previousTop = 0;
scrollingUp = document.body.scrollTop == 0;
if (scrollingUp) document.body.classList.add('scrolling-up');
else document.body.classList.remove('scrolling-up');
}
window.onload = setupTargets;
window.onresize = setupTargets;body {
position: relative;
width: 100vw;
height: auto;
overflow-y: auto;
}
.header {
display: none;
position: fixed;
top: 0px;
left: 0px;
background-color: lime;
width: 100%;
height: 10%;
align-items: center;
justify-content: center;
font-size: 4rem;
z-index: 1;
}
.scrolling-up .header {
display: flex;
}
.content {
position: relative;
width: 80vw;
margin: 0 auto;
top: 10%;
padding: 10px 20px;
font-size: 3rem;
}
.observed {
width: 1px;
position: absolute;
left: 50%;
margin: 0;
padding: 0;
border-width: 0;
z-index: -99999;
}<body>
<header class="header">HEADER</header>
<div class="content">
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</p><p>
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</p>
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</p>
</div>Note, on IOS there may be a slight 'edge' case on scrolling back up to the very top - needs investigating - may be related to 100vh not being the viewport height when there are navbars at the top in the browser.