《react-intersection-observer 源码》

import React from "react";
import { useInView } from "react-intersection-observer";

const Component = () => {
  const [ref, inView] = useInView();

  return (
    <div ref={ref}>
      <h2>{`Header inside viewport ${inView}.`}</h2>
    </div>
  );
};

export function useInView(
  options: IntersectionOptions = {},
): InViewHookResponse {
  const ref = React.useRef<Element>()
  const [state, setState] = React.useState<State>(initialState)

  // 中间部分..

  return [setRef, state.inView, state.entry]
}

// 中间部分 1
const setRef = React.useCallback(
  (node) => {
    if (ref.current) {
      unobserve(ref.current);
    }

    if (node) {
      observe(
        node,
        (inView, intersection) => {
          setState({ inView, entry: intersection });

          if (inView && options.triggerOnce) {
            // If it should only trigger once, unobserve the element after it's inView
            unobserve(node);
          }
        },
        options
      );
    }

    // Store a reference to the node, so we can unobserve it later
    ref.current = node;
  },
  [options.threshold, options.root, options.rootMargin, options.triggerOnce]
);

// 中间部分 2
useEffect(() => {
  if (!ref.current && state !== initialState && !options.triggerOnce) {
    // If we don't have a ref, then reset the state (unless the hook is set to only `triggerOnce`)
    // This ensures we correctly reflect the current state - If you aren't observing anything, then nothing is inView
    setState(initialState);
  }
});

export function observe(
  element: Element,
  callback: ObserverInstanceCallback,
  options: IntersectionObserverInit = {}
) {
  // IntersectionObserver needs a threshold to trigger, so set it to 0 if it's not defined.
  // Modify the options object, since it's used in the onChange handler.
  if (!options.threshold) options.threshold = 0;
  const { root, rootMargin, threshold } = options;
  // Validate that the element is not being used in another <Observer />
  invariant(
    !INSTANCE_MAP.has(element),
    "react-intersection-observer: Trying to observe %s, but it's already being observed by another instance.\nMake sure the `ref` is only used by a single <Observer /> instance.\n\n%s"
  );
  /* istanbul ignore if */
  if (!element) return;
  // Create a unique ID for this observer instance, based on the root, root margin and threshold.
  // An observer with the same options can be reused, so lets use this fact
  let observerId: string =
    getRootId(root) +
    (rootMargin
      ? `${threshold.toString()}_${rootMargin}`
      : threshold.toString());

  let observerInstance = OBSERVER_MAP.get(observerId);
  if (!observerInstance) {
    observerInstance = new IntersectionObserver(onChange, options);
    /* istanbul ignore else  */
    if (observerId) OBSERVER_MAP.set(observerId, observerInstance);
  }

  const instance: ObserverInstance = {
    callback,
    element,
    inView: false,
    observerId,
    observer: observerInstance,
    // Make sure we have the thresholds value. It's undefined on a browser like Chrome 51.
    thresholds:
      observerInstance.thresholds ||
      (Array.isArray(threshold) ? threshold : [threshold]),
  };

  INSTANCE_MAP.set(element, instance);
  observerInstance.observe(element);

  return instance;
}

function onChange(changes: IntersectionObserverEntry[]) {
  changes.forEach((intersection) => {
    const { isIntersecting, intersectionRatio, target } = intersection;
    const instance = INSTANCE_MAP.get(target);

    // Firefox can report a negative intersectionRatio when scrolling.
    /* istanbul ignore else */
    if (instance && intersectionRatio >= 0) {
      // If threshold is an array, check if any of them intersects. This just triggers the onChange event multiple times.
      let inView = instance.thresholds.some((threshold) => {
        return instance.inView
          ? intersectionRatio > threshold
          : intersectionRatio >= threshold;
      });

      if (isIntersecting !== undefined) {
        // If isIntersecting is defined, ensure that the element is actually intersecting.
        // Otherwise it reports a threshold of 0
        inView = inView && isIntersecting;
      }

      instance.inView = inView;
      instance.callback(inView, intersection);
    }
  });
}

export function unobserve(element: Element | null) {
  if (!element) return;
  const instance = INSTANCE_MAP.get(element);

  if (instance) {
    const { observerId, observer } = instance;
    const { root } = observer;

    observer.unobserve(element);

    // Check if we are still observing any elements with the same threshold.
    let itemsLeft = false;
    // Check if we still have observers configured with the same root.
    let rootObserved = false;
    /* istanbul ignore else  */
    if (observerId) {
      INSTANCE_MAP.forEach((item, key) => {
        if (key !== element) {
          if (item.observerId === observerId) {
            itemsLeft = true;
            rootObserved = true;
          }
          if (item.observer.root === root) {
            rootObserved = true;
          }
        }
      });
    }
    if (!rootObserved && root) ROOT_IDS.delete(root);
    if (observer && !itemsLeft) {
      // No more elements to observe for threshold, disconnect observer
      observer.disconnect();
    }

    // Remove reference to element
    INSTANCE_MAP.delete(element);
  }
}

const Component = () => {
  const [ref, inView] = useInView();

  return <Child ref={ref} />;
};

const Child = ({ loading, ref }) => {
  if (loading) {
    // 这一步会判定为 inView：false
    return <Spin />;
  }

  return <div ref={ref}>Child</div>;
};