Tips for rolling your own lazy loading

Efficiently implement lazy loading of web page images: taking into account both responsiveness and performance

Have you ever considered lazy loading of images to optimize web page performance? Lazy loading technology can effectively reduce the load weight of the initial page and improve the loading speed of the web page. This article will explore an efficient image lazy loading method, while taking into account responsive images and the best user experience.

Lazy loading is not a browser native function, and it currently needs to be implemented with the help of JavaScript. Although Chrome 75 and subsequent versions plan to natively support lazy loading of images and iframes, we still need to use JavaScript solutions in order to be compatible with other browsers. Many static website generators, libraries, and frameworks provide lazy loading capabilities out of the box, but introducing a full library or framework just to implement this functionality can cause performance overhead. Therefore, we chose to implement lazy loading by ourselves.

Core mechanism:

Most lazy loading methods follow the following pattern:

1. HTML tags: Use <img src="/static/imghw/default1.png" data-src="https://img.php.cn/upload/article/000/000/000/174502705448368.jpg" class="lazy" alt="Tips for rolling your own lazy loading">

2. Loading time: Use Intersection Observer API to detect whether the image enters the viewport. Intersection Observer provides efficient element visibility detection, avoiding frequent scroll and resize event listening.

 // Create Intersection Observer
const observer = new IntersectionObserver(lazyLoad, {
  rootMargin: '100px',
  threshold: 1.0
});

function lazyLoad(elements) {
  elements.forEach(image => {
    if (image.intersectionRatio > 0) {
      image.src = image.dataset.src;
      observer.unobserve(image);
    }
  });
}

// Observe all img elements with "lazy" class const lazyImages = document.querySelectorAll('img.lazy');
lazyImages.forEach(img => {
  observer.observe(img);
});

Improvement and optimization:

There are some shortcomings in the above basic methods:

1. JavaScript dependency: The image cannot be displayed until JavaScript is loaded and executed.
2. Visual beating: The page is blank before the picture is loaded, which may cause visual dissatisfaction.

In response to the above issues, we have made the following improvements:

1. Native lazy load detection: Use feature detection to determine whether the browser supports the native loading="lazy" attribute. If supported, use it directly without JavaScript intervention.

 if ('loading' in HTMLImageElement.prototype) {
  document.querySelectorAll('img.lazy').forEach(img => img.src = img.dataset.src);
} else {
  // Use Intersection Observer to implement lazy loading}

2. Placeholder map and CSS transition: Use a small-size placeholder map as the initial state, and use CSS to scale and blur, remove the blur effect after the image is loaded to achieve a smooth transition.

<picture>
  <img src="/static/imghw/default1.png" data-src="https://img.php.cn/upload/article/000/000/000/174502705554652.jpg" class="lazy" alt="Tips for rolling your own lazy loading">
</picture>

picture { width: 100%; overflow: hidden; }
picture img { width: 100%; transition: filter 0.5s; }
picture.lazy img { filter: blur(20px); }

3. Responsive picture: Use<picture></picture> Elements and<source></source> Elements implement responsive pictures, loading pictures of different sizes according to different viewport sizes.

Auxiliary tools:

To simplify the development process, we can use the following auxiliary tools:

1. HTML generator: custom function or template engine assisted in generation<picture></picture> HTML code for the element.

2. Image batch processing: Use tools such as gulp-image-resize to batch generate pictures of different sizes, or use cloud services (such as Netlify and Cloudinary) to convert pictures on demand.

Summarize:

This article introduces an efficient image lazy loading implementation method. By combining native lazy loading feature detection, placeholder map, CSS transition, responsive pictures and auxiliary tools, it can improve the user experience while ensuring performance. Choosing the right image processing method (build or on-demand conversion) depends on the project size and requirements. This method can significantly improve web page loading speed and provide smoother visual effects.

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