Awesome
lazy-image
<lazy-image>
is a custom element that contains an image that has the
option to be loaded only on-demand, for performance reasons. If a <lazy-image>
is inactive, then its source is not loaded (the XHR won't be made until the
active
attribute is set on the element).
<lazy-image>
has 3 attributes:
src
the image source (like you would use with a<img>
alt
, the alternate text for the image the image source (like you would use with a<img>
)active
, whether the image should be loaded or not
How to use
You can install the element with bower
(npm
support coming soon, hold tight)
bower install notwaldorf/lazy-image
Then you can drop a <script src="bower_components/lazy-image.js"></script>
in the page where you want to use it. Custom elements/Shadow DOM are not yet supported in all browsers, so we recommend the webcomponentsjs polyfills. In my demo I use the webcomponents-sd-ce.js
bundle (because I am not using any HTML Imports and I didn't bother with IE11), but for a fancier way to load the polyfills, check the webcomponents-loader
and their docs.
On-demand loading
For a <lazy-image>
to load, it must have the active
property set to true
.
In the example below, the images will only load when clicked (you can check the network tab in
your favourite developer tools to see that there's no initial request for these files).
This happens because they each start off with the active
property set to false
,
and have a click
event listener, that
sets the <lazy-image>
's active
attribute to true
:
<lazy-image src="..." alt="..." id="i"></lazy-image>
<script>
i.addEventListener('click', function() {
if (!this.active)
this.active = true;
});
</script>
If you want to have some global setting that controls all <lazy-images>
on the page (i.e. activates or deactivates all of them), you can set the window.LazyImageSiteDefaultActive
global before loading the lazy-image.js
script.
Intersection Observer
Intersection observers let you figure out when an element enters into view.
Combined with a <lazy-image>
, this lets you only load
images that are scrolled into view, while leaving images at the bottom
of the page that haven't been seen inactive.
// Create an observer.
var observer = new IntersectionObserver(onChange, {
threshold: [0.5] // rootMargin: '50% 0%'
});
// That observes all the random images we've created.
els.forEach(el => observer.observe(el));
// Whenever we scroll...
function onChange(changes) {
changes.forEach(change => {
var el = change.target;
if (!el.active)
el.active = true;
observer.unobserve(el); // Don't care anymore.
});
}