How do I know the IntersectionObserver scroll direction?
Solution 1:
I don't know if handling boundingClientRect will end up on performance issues.
MDN states that the IntersectionObserver
does not run on the main thread:
This way, sites no longer need to do anything on the main thread to watch for this kind of element intersection, and the browser is free to optimize the management of intersections as it sees fit.
MDN, "Intersection Observer API"
We can compute the scrolling direction by saving the value of IntersectionObserverEntry.boundingClientRect.y
and compare that to the previous value.
Run the following snippet for an example:
const state = document.querySelector('.observer__state')
const target = document.querySelector('.observer__target')
const thresholdArray = steps => Array(steps + 1)
.fill(0)
.map((_, index) => index / steps || 0)
let previousY = 0
let previousRatio = 0
const handleIntersect = entries => {
entries.forEach(entry => {
const currentY = entry.boundingClientRect.y
const currentRatio = entry.intersectionRatio
const isIntersecting = entry.isIntersecting
// Scrolling down/up
if (currentY < previousY) {
if (currentRatio > previousRatio && isIntersecting) {
state.textContent ="Scrolling down enter"
} else {
state.textContent ="Scrolling down leave"
}
} else if (currentY > previousY && isIntersecting) {
if (currentRatio < previousRatio) {
state.textContent ="Scrolling up leave"
} else {
state.textContent ="Scrolling up enter"
}
}
previousY = currentY
previousRatio = currentRatio
})
}
const observer = new IntersectionObserver(handleIntersect, {
threshold: thresholdArray(20),
})
observer.observe(target)
html,
body {
margin: 0;
}
.observer__target {
position: relative;
width: 100%;
height: 350px;
margin: 1500px 0;
background: rebeccapurple;
}
.observer__state {
position: fixed;
top: 1em;
left: 1em;
color: #111;
font: 400 1.125em/1.5 sans-serif;
background: #fff;
}
<div class="observer__target"></div>
<span class="observer__state"></span>
If the thresholdArray
helper function might confuse you, it builds an array ranging from 0.0
to 1.0
by the given amount of steps. Passing 5
will return [0.0, 0.2, 0.4, 0.6, 0.8, 1.0]
.
Solution 2:
Comparing boundingClientRect
and rootBounds
from entry
, you can easily know if the target is above or below the viewport.
During callback()
, you check isAbove/isBelow then, at the end, you store it into wasAbove/wasBelow.
Next time, if the target comes in viewport (for example), you can check if it was above or below. So you know if it comes from top or bottom.
You can try something like this:
var wasAbove = false;
function callback(entries, observer) {
entries.forEach(entry => {
const isAbove = entry.boundingClientRect.y < entry.rootBounds.y;
if (entry.isIntersecting) {
if (wasAbove) {
// Comes from top
}
}
wasAbove = isAbove;
});
}
Hope this helps.
Solution 3:
This solution is without the usage of any external state, hence simpler than solutions which keep track of additional variables:
const observer = new IntersectionObserver(
([entry]) => {
if (entry.boundingClientRect.top < 0) {
if (entry.isIntersecting) {
// entered viewport at the top edge, hence scroll direction is up
} else {
// left viewport at the top edge, hence scroll direction is down
}
}
},
{
root: rootElement,
},
);