Implement useIntersection hook.

README.md

@@ -49,6 +49,7 @@
   - [`useGeolocation`](./docs/useGeolocation.md) — tracks geo location state of user's device. [![][img-demo]](https://streamich.github.io/react-use/?path=/story/sensors-usegeolocation--demo)
   - [`useHover` and `useHoverDirty`](./docs/useHover.md) — tracks mouse hover state of some element. [![][img-demo]](https://codesandbox.io/s/zpn583rvx)
   - [`useIdle`](./docs/useIdle.md) — tracks whether user is being inactive.
+  - [`useIntersection`](./docs/useIntersection.md) — tracks an HTML element's intersection. [![][img-demo]](https://streamich.github.io/react-use/?path=/story/sensors-useintersection--demo)
   - [`useKey`](./docs/useKey.md), [`useKeyPress`](./docs/useKeyPress.md), [`useKeyboardJs`](./docs/useKeyboardJs.md), and [`useKeyPressEvent`](./docs/useKeyPressEvent.md) — track keys. [![][img-demo]](https://streamich.github.io/react-use/?path=/story/sensors-usekeypressevent--demo)
   - [`useLocation`](./docs/useLocation.md) and [`useSearchParam`](./docs/useSearchParam.md) — tracks page navigation bar location state.
   - [`useMedia`](./docs/useMedia.md) — tracks state of a CSS media query. [![][img-demo]](https://streamich.github.io/react-use/?path=/story/sensors-usemedia--demo)
@@ -104,7 +105,7 @@
   - [`useTitle`](./docs/useTitle.md) — sets title of the page.
   - [`usePermission`](./docs/usePermission.md) — query permission status for browser APIs.
     <br/>
-    <br/> 
+    <br/>
 - [**Lifecycles**](./docs/Lifecycles.md)
   - [`useEffectOnce`](./docs/useEffectOnce.md) &mdash; a modified [`useEffect`](https://reactjs.org/docs/hooks-reference.html#useeffect) hook that only runs once.
   - [`useEvent`](./docs/useEvent.md) &mdash; subscribe to events.
@@ -134,7 +135,6 @@
   - [`useList`](./docs/useList.md) &mdash; tracks state of an array.
   - [`useMap`](./docs/useMap.md) &mdash; tracks state of an object.
 
-
 <br />
 <br />
 <br />
@@ -159,7 +159,6 @@
 
 [img-demo]: https://img.shields.io/badge/demo-%20%20%20%F0%9F%9A%80-green.svg
 
-
 <div align="center">
   <h1>Contributors</h1>
 </div>

docs/useIntersection.md

@@ -0,0 +1,36 @@
+# `useIntersection`
+
+React sensor hook that tracks the changes in the intersection of a target element with an ancestor element or with a top-level document's viewport. Uses the [Intersection Observer API](https://developer.mozilla.org/en-US/docs/Web/API/Intersection_Observer_API) and returns a [IntersectionObserverEntry](https://developer.mozilla.org/en-US/docs/Web/API/IntersectionObserverEntry).
+
+## Usage
+
+```jsx
+import * as React from 'react';
+import { useIntersection } from 'react-use';
+
+const Demo = () => {
+  const intersectionRef = React.useRef(null);
+  const intersection = useIntersection(intersectionRef, {
+    root: null,
+    rootMargin: '0px',
+    threshold: 1
+  });
+
+  return (
+    <div ref={intersectionRef}>
+      {intersection && intersection.intersectionRatio < 1
+        ? 'Obscured'
+        : 'Fully in view'}
+    </div>
+  );
+};
+```
+
+## Reference
+
+```ts
+useIntersection(
+  ref: RefObject<HTMLElement>,
+  options: IntersectionObserverInit,
+): IntersectionObserverEntry | null;
+```

package.json

@@ -69,6 +69,7 @@
     "@semantic-release/changelog": "3.0.4",
     "@semantic-release/git": "7.0.16",
     "@semantic-release/npm": "5.1.13",
+    "@shopify/jest-dom-mocks": "^2.8.2",
     "@storybook/addon-actions": "5.1.11",
     "@storybook/addon-knobs": "5.1.11",
     "@storybook/addon-notes": "5.1.11",

src/__stories__/useIntersection.story.tsx

@@ -0,0 +1,53 @@
+import { storiesOf } from '@storybook/react';
+import * as React from 'react';
+import { useIntersection } from '..';
+import ShowDocs from './util/ShowDocs';
+
+const Spacer = () => (
+  <div
+    style={{
+      width: '200px',
+      height: '300px',
+      backgroundColor: 'whitesmoke',
+    }}
+  />
+);
+
+const Demo = () => {
+  const intersectionRef = React.useRef(null);
+  const intersection = useIntersection(intersectionRef, {
+    root: null,
+    rootMargin: '0px',
+    threshold: 1,
+  });
+
+  return (
+    <div
+      style={{
+        width: '400px',
+        height: '400px',
+        backgroundColor: 'whitesmoke',
+        overflow: 'scroll',
+      }}
+    >
+      Scroll me
+      <Spacer />
+      <div
+        ref={intersectionRef}
+        style={{
+          width: '100px',
+          height: '100px',
+          padding: '20px',
+          backgroundColor: 'palegreen',
+        }}
+      >
+        {intersection && intersection.intersectionRatio < 1 ? 'Obscured' : 'Fully in view'}
+      </div>
+      <Spacer />
+    </div>
+  );
+};
+
+storiesOf('Sensors/useIntersection', module)
+  .add('Docs', () => <ShowDocs md={require('../../docs/useIntersection.md')} />)
+  .add('Demo', () => <Demo />);

src/__tests__/useIntersection.test.tsx

@@ -0,0 +1,119 @@
+import React, { createRef } from 'react';
+import ReactDOM from 'react-dom';
+import TestUtils from 'react-dom/test-utils';
+import TestRenderer from 'react-test-renderer';
+import { intersectionObserver } from '@shopify/jest-dom-mocks';
+import { renderHook } from '@testing-library/react-hooks';
+import { useIntersection } from '..';
+
+beforeEach(() => {
+  intersectionObserver.mock();
+});
+
+afterEach(() => {
+  intersectionObserver.restore();
+});
+
+describe('useIntersection', () => {
+  const container = document.createElement('div');
+  let targetRef;
+
+  it('should be defined', () => {
+    expect(useIntersection).toBeDefined();
+  });
+
+  it('should setup an IntersectionObserver targeting the ref element and using the options provided', () => {
+    TestUtils.act(() => {
+      targetRef = createRef();
+      ReactDOM.render(<div ref={targetRef} />, container);
+    });
+
+    expect(intersectionObserver.observers).toHaveLength(0);
+    const observerOptions = { root: null, threshold: 0.8 };
+
+    renderHook(() => useIntersection(targetRef, observerOptions));
+
+    expect(intersectionObserver.observers).toHaveLength(1);
+    expect(intersectionObserver.observers[0].target).toEqual(targetRef.current);
+    expect(intersectionObserver.observers[0].options).toEqual(observerOptions);
+  });
+
+  it('should return null if a ref without a current value is provided', () => {
+    targetRef = createRef();
+
+    const { result } = renderHook(() => useIntersection(targetRef, { root: null, threshold: 1 }));
+    expect(result.current).toBe(null);
+  });
+
+  it('should return the first IntersectionObserverEntry when the IntersectionObserver registers an intersection', () => {
+    TestUtils.act(() => {
+      targetRef = createRef();
+      ReactDOM.render(<div ref={targetRef} />, container);
+    });
+
+    const { result } = renderHook(() => useIntersection(targetRef, { root: container, threshold: 0.8 }));
+
+    const mockIntersectionObserverEntry = {
+      boundingClientRect: targetRef.current.getBoundingClientRect(),
+      intersectionRatio: 0.81,
+      intersectionRect: container.getBoundingClientRect(),
+      isIntersecting: true,
+      rootBounds: container.getBoundingClientRect(),
+      target: targetRef.current,
+      time: 300,
+    };
+    TestRenderer.act(() => {
+      intersectionObserver.simulate(mockIntersectionObserverEntry);
+    });
+
+    expect(result.current).toEqual(mockIntersectionObserverEntry);
+  });
+
+  it('should setup a new IntersectionObserver when the ref changes', () => {
+    let newRef;
+    TestUtils.act(() => {
+      targetRef = createRef();
+      newRef = createRef();
+      ReactDOM.render(
+        <div ref={targetRef}>
+          <span ref={newRef} />
+        </div>,
+        container
+      );
+    });
+
+    const observerOptions = { root: null, threshold: 0.8 };
+    const { rerender } = renderHook(({ ref, options }) => useIntersection(ref, options), {
+      initialProps: { ref: targetRef, options: observerOptions },
+    });
+
+    expect(intersectionObserver.observers[0].target).toEqual(targetRef.current);
+
+    TestRenderer.act(() => {
+      rerender({ ref: newRef, options: observerOptions });
+    });
+
+    expect(intersectionObserver.observers[0].target).toEqual(newRef.current);
+  });
+
+  it('should setup a new IntersectionObserver when the options change', () => {
+    TestUtils.act(() => {
+      targetRef = createRef();
+      ReactDOM.render(<div ref={targetRef} />, container);
+    });
+
+    const initialObserverOptions = { root: null, threshold: 0.8 };
+    const { rerender } = renderHook(({ ref, options }) => useIntersection(ref, options), {
+      initialProps: { ref: targetRef, options: initialObserverOptions },
+    });
+
+    expect(intersectionObserver.observers[0].options).toEqual(initialObserverOptions);
+
+    const newObserverOptions = { root: container, threshold: 1 };
+    TestRenderer.act(() => {
+      rerender({ ref: targetRef, options: newObserverOptions });
+    });
+
+    expect(intersectionObserver.observers[0].options).toEqual(newObserverOptions);
+  });
+});

src/index.ts

@@ -27,6 +27,7 @@ export { default as useHarmonicIntervalFn } from './useHarmonicIntervalFn';
 export { default as useHover } from './useHover';
 export { default as useHoverDirty } from './useHoverDirty';
 export { default as useIdle } from './useIdle';
+export { default as useIntersection } from './useIntersection';
 export { default as useInterval } from './useInterval';
 export { default as useIsomorphicLayoutEffect } from './useIsomorphicLayoutEffect';
 export { default as useKey } from './useKey';

src/useIntersection.ts

@@ -0,0 +1,30 @@
+import { RefObject, useEffect, useState } from 'react';
+
+const useIntersection = (
+  ref: RefObject<HTMLElement>,
+  options: IntersectionObserverInit
+): IntersectionObserverEntry | null => {
+  const [intersectionObserverEntry, setIntersectionObserverEntry] = useState<IntersectionObserverEntry | null>(null);
+
+  useEffect(() => {
+    if (ref.current) {
+      const handler = (entries: IntersectionObserverEntry[]) => {
+        setIntersectionObserverEntry(entries[0]);
+      };
+
+      const observer = new IntersectionObserver(handler, options);
+      observer.observe(ref.current);
+
+      return () => {
+        if (ref.current) {
+          observer.disconnect();
+        }
+      };
+    }
+    return () => {};
+  }, [ref, options.threshold, options.root, options.rootMargin]);
+
+  return intersectionObserverEntry;
+};
+
+export default useIntersection;