30秒学会 React 片段 – Lazy-loading image

Renders an image that supports lazy loading.

• Use the useState() hook to create a stateful value that indicates if the image has been loaded.
• Use the useEffect() hook to check if the HTMLImageElement.prototype contains 'loading', effectively checking if lazy loading is supported natively. If not, create a new IntersectionObserver and use IntersectionObserver.observer() to observer the <img> element. Use the return value of the hook to clean up when the component unmounts.
• Use the useCallback() hook to memoize a callback function for the IntersectionObserver. This callback will update the isLoaded state variable and use IntersectionObserver.disconnect() to disconnect the IntersectionObserver instance.
• Use the useRef() hook to create two refs. One will hold the <img> element and the other the IntersectionObserver instance, if necessary.
• Finally, render the <img> element with the given attributes. Apply loading='lazy' to make it load lazily, if necessary. Use isLoaded to determine the value of the src attribute.

代码实现

const LazyLoadImage = ({
  alt,
  src,
  className,
  loadInitially = false,
  observerOptions = { root: null, rootMargin: '200px 0px' },
  ...props
}) => {
  const observerRef = React.useRef(null);
  const imgRef = React.useRef(null);
  const [isLoaded, setIsLoaded] = React.useState(loadInitially);

  const observerCallback = React.useCallback(
    entries => {
      if (entries[0].isIntersecting) {
        observerRef.current.disconnect();
        setIsLoaded(true);
      }
    },
    [observerRef]
  );

  React.useEffect(() => {
    if (loadInitially) return;

    if ('loading' in HTMLImageElement.prototype) {
      setIsLoaded(true);
      return;
    }

    observerRef.current = new IntersectionObserver(
      observerCallback,
      observerOptions
    );
    observerRef.current.observe(imgRef.current);
    return () => {
      observerRef.current.disconnect();
    };
  }, []);

  return (
    <img
      alt={alt}
      src={isLoaded ? src : ''}
      ref={imgRef}
      className={className}
      loading={loadInitially ? undefined : 'lazy'}
      {...props}
    />
  );
};

使用样例

ReactDOM.createRoot(document.getElementById('root')).render(
  <LazyLoadImage
    src="https://picsum.photos/id/1080/600/600"
    alt="Strawberries"
  />
);

翻译自：https://www.30secondsofcode.org/react/s/lazy-load-image