I'm using the new position: sticky
(info) to create an iOS-like list of content.
It's working well and far superior than the previous JavaScript alternative (example) however as far as I know no event is fired when it's triggered, which means I can't do anything when the bar hits the top of the page, unlike with the previous solution.
I'd like to add a class (e.g. stuck
) when an element with position: sticky
hits the top of the page. Is there a way to listen for this with JavaScript? Usage of jQuery is fine.
// get the sticky element
const stickyElm = document.querySelector('header')
const observer = new IntersectionObserver(
([e]) => e.target.classList.toggle('isSticky', e.intersectionRatio < 1),
{threshold: [1]}
);
observer.observe(stickyElm)
body{ height: 200vh; font:20px Arial; }
section{
background: lightblue;
padding: 2em 1em;
}
header{
position: sticky;
top: -1px; /* ➜ the trick */
padding: 1em;
padding-top: calc(1em + 1px); /* ➜ compensate for the trick */
background: salmon;
transition: .1s;
}
/* styles for when the header is in sticky mode */
header.isSticky{
font-size: .8em;
opacity: .5;
}
<section>Space</section>
<header>Sticky Header</header>
The top
value needs to be -1px
or the element will never intersect with the top of the browser window (thus never triggering the intersection observer).
To counter this 1px
of hidden content, an additional 1px
of space should be added to either the border or the padding of the sticky element.
scroll
event listener:scrollCallback
(to be able to unbind if needed)// get the sticky element
const stickyElm = document.querySelector('header');
// get the first parent element which is scrollable
const stickyElmScrollableParent = getScrollParent(stickyElm);
// save the original offsetTop. when this changes, it means stickiness has begun.
stickyElm._originalOffsetTop = stickyElm.offsetTop;
// compare previous scrollTop to current one
const detectStickiness = (elm, cb) => () => cb & cb(elm.offsetTop != elm._originalOffsetTop)
// Act if sticky or not
const onSticky = isSticky => {
console.clear()
console.log(isSticky)
stickyElm.classList.toggle('isSticky', isSticky)
}
// bind a scroll event listener on the scrollable parent (whatever it is)
// in this exmaple I am throttling the "scroll" event for performance reasons.
// I also use functional composition to diffrentiate between the detection function and
// the function which acts uppon the detected information (stickiness)
const scrollCallback = throttle(detectStickiness(stickyElm, onSticky), 100)
stickyElmScrollableParent.addEventListener('scroll', scrollCallback)
// OPTIONAL CODE BELOW ///////////////////
// find-first-scrollable-parent
// Credit: https://stackoverflow.com/a/42543908/104380
function getScrollParent(element, includeHidden) {
var style = getComputedStyle(element),
excludeStaticParent = style.position === "absolute",
overflowRegex = includeHidden ? /(auto|scroll|hidden)/ : /(auto|scroll)/;
if (style.position !== "fixed")
for (var parent = element; (parent = parent.parentElement); ){
style = getComputedStyle(parent);
if (excludeStaticParent && style.position === "static")
continue;
if (overflowRegex.test(style.overflow + style.overflowY + style.overflowX))
return parent;
}
return window
}
// Throttle
// Credit: https://jsfiddle.net/jonathansampson/m7G64
function throttle (callback, limit) {
var wait = false; // Initially, we're not waiting
return function () { // We return a throttled function
if (!wait) { // If we're not waiting
callback.call(); // Execute users function
wait = true; // Prevent future invocations
setTimeout(function () { // After a period of time
wait = false; // And allow future invocations
}, limit);
}
}
}
header{
position: sticky;
top: 0;
/* not important styles */
background: salmon;
padding: 1em;
transition: .1s;
}
header.isSticky{
/* styles for when the header is in sticky mode */
font-size: .8em;
opacity: .5;
}
/* not important styles*/
body{ height: 200vh; font:20px Arial; }
section{
background: lightblue;
padding: 2em 1em;
}
<section>Space</section>
<header>Sticky Header</header>