Intersection Observer is a practical API provided by H5 for lazy loading and animation triggering. 1. Lazy loading pictures: By monitoring whether the element enters the viewport, assign the real image address from data-src to src to reduce the loading pressure on the first screen; 2. Animation trigger: When the element enters the field of view, add class names to start CSS animation, save resources and improve experience; 3. Usage tips: reuse Observer instances, cancel monitoring in time, pay attention to compatibility and mobile adaptation issues. Compared to scroll events, Intersection Observer has better performance and concise code.
Intersection Observer is a very practical API provided by H5, which is especially suitable for lazy loading and animation triggering things related to "whether elements appear in the viewport". Compared with the previous method of using scroll event to listen to getBoundingClientRect, it performs better and has a simpler code.
Lazy loading pictures: Reduce the loading pressure on the first screen
The most common application scenario for lazy loading is pictures. When the page is just opened, it does not need to load all images. It only starts loading when the user scrolls near a certain image, which can significantly increase the speed of the first screen.
The implementation method is very simple: first put the real image address in data-src attribute, and set src to a placeholder (such as a transparent gif). Then listen for these image elements through Intersection Observer. Once it enters the viewport range, data-src is assigned to src and cancels the listening to avoid repeated triggers.
const images = document.querySelectorAll('img[data-src]');
const observer = new IntersectionObserver((entries, observer) => {
entries.forEach(entry => {
if (entry.isIntersecting) {
entry.target.src = entry.target.dataset.src;
observer.unobserve(entry.target);
}
});
}, {
rootMargin: '0px 0px 200px 0px' // Can load in advance to improve the experience});
images.forEach(img => observer.observe(img)); Note that rootMargin is used here to expand the detection area down by 200px, so that the image can be loaded in advance before it has been completely rolled to the visual area, making the user experience smoother.
Animation trigger: enter the field of view and play it
In addition to lazy loading of images, Intersection Observer is also very suitable for controlling the playback timing of animations. For example, if you want to make an animation effect of "the element fades in after entering the viewport", you can add a class name to the target element when you hear it appear, and the CSS animation will automatically start.
const sections = document.querySelectorAll('.animate-on-scroll');
const observer = new IntersectionObserver((entries) => {
entries.forEach(entry => {
if (entry.isIntersecting) {
entry.target.classList.add('in-view');
}
});
}, {
threshold: 0.1
});
sections.forEach(section => observer.observe(section));Cooperate with CSS:
.animate-on-scroll {
opacity: 0;
transition: opacity 0.6s ease-out;
}
.animate-on-scroll.in-view {
opacity: 1;
} The advantage of this method is that the animation will not be executed from the beginning, nor will it waste resources to run the animation when the user does not see it. And you can flexibly control the trigger conditions, such as setting threshold to decide "how much you see is considered a trigger".
A few tips for using
- Try to reuse Observer instances : If you want to listen to multiple elements, you don’t need to new one Observer for each, just pass the configuration uniformly.
- Timely unobserve : There is no need to continue listening after the picture is loaded. Tweaking
unobservecan reduce performance overhead. - Pay attention to compatibility : Modern browsers basically support it, but if you want to be compatible with IE, you may have to find polyfill or downgrade solutions.
- Pay attention to resize and orientationchange on mobile terminals : Sometimes the device rotates or zooms will change the element position, remember to recalculate the observation status.
Basically that's it. The core idea of Intersection Observer is "wait until you see me". It is more efficient and easier to maintain than listening to scroll events frequently. Whether it is lazy loading or animation triggering, it is a basic skill worth mastering.
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