feat: add usePreviousDistinct

docs/usePreviousDistinct.md

@@ -0,0 +1,51 @@
+# `usePreviousDistinct`
+
+Just like `usePrevious` but it will only update once the value actually changes. This is important when other
+hooks are involved and you aren't just interested in the previous props version, but want to know the previous
+distinct value
+
+## Usage
+
+```jsx
+import {usePreviousDistinct, useCounter} from 'react-use';
+
+const Demo = () => {
+  const [count, { inc: relatedInc }] = useCounter(0);
+  const [unrelatedCount, { inc }] = useCounter(0);
+  const prevCount = usePreviousDistinct(count);
+
+  return (
+    <p>
+      Now: {count}, before: {prevCount}
+      <button onClick={() => relatedInc()}>Increment</button>
+      Unrelated: {unrelatedCount}
+      <button onClick={() => inc()}>Increment Unrelated</button>
+    </p>
+  );
+};
+```
+
+You can also provide a way of identifying the value as unique. By default, a strict equals is used.
+
+```jsx
+import {usePreviousDistinct} from 'react-use';
+
+const Demo = () => {
+  const [str, setStr] = React.useState("something_lowercase");
+  const [unrelatedCount] = React.useState(0);
+  const prevStr = usePreviousDistinct(str, (prev, next) => (prev && prev.toUpperCase()) === next.toUpperCase());
+
+  return (
+    <p>
+      Now: {count}, before: {prevCount}
+      Unrelated: {unrelatedCount}
+    </p>
+  );
+};
+```
+
+## Reference
+
+```ts
+const prevState = usePreviousDistinct = <T>(state: T, compare?: (prev: T | undefined, next: T) => boolean): T;
+```

src/__stories__/usePreviousDistinct.story.tsx

@@ -0,0 +1,23 @@
+import { storiesOf } from '@storybook/react';
+import * as React from 'react';
+import { usePreviousDistinct, useCounter } from '..';
+import ShowDocs from './util/ShowDocs';
+
+const Demo = () => {
+  const [count, { inc: relatedInc }] = useCounter(0);
+  const [unrelatedCount, { inc }] = useCounter(0);
+  const prevCount = usePreviousDistinct(count);
+
+  return (
+    <p>
+      Now: {count}, before: {prevCount}
+      <button onClick={() => relatedInc()}>Increment</button>
+      Unrelated: {unrelatedCount}
+      <button onClick={() => inc()}>Increment Unrelated</button>
+    </p>
+  );
+};
+
+storiesOf('State|usePreviousDistinct', module)
+  .add('Docs', () => <ShowDocs md={require('../../docs/usePreviousDistinct.md')} />)
+  .add('Demo', () => <Demo />);

src/__tests__/usePreviousDistinct.test.tsx

@@ -0,0 +1,65 @@
+import { renderHook } from '@testing-library/react-hooks';
+import usePreviousDistinct from '../usePreviousDistinct';
+
+describe('usePreviousDistinct with default compare', () => {
+  const hook = renderHook(props => usePreviousDistinct(props), { initialProps: 0 });
+
+  it('should return undefined on initial render', () => {
+    expect(hook.result.current).toBe(undefined);
+  });
+
+  it('should return previous state only after a different value is rendered', () => {
+    expect(hook.result.current).toBeUndefined();
+    hook.rerender(1);
+    expect(hook.result.current).toBe(0);
+    hook.rerender(2);
+    hook.rerender(2);
+    expect(hook.result.current).toBe(1);
+
+    hook.rerender(3);
+    expect(hook.result.current).toBe(2);
+  });
+});
+
+describe('usePreviousDistinct with complex comparison', () => {
+  const exampleObjects = [
+    {
+      id: 'something-unique',
+      name: 'Nancy',
+    },
+    {
+      id: 'something-unique2',
+      name: 'Fred',
+    },
+    {
+      id: 'something-unique3',
+      name: 'Bill',
+    },
+    {
+      id: 'something-unique4',
+      name: 'Alice',
+    },
+  ];
+  const hook = renderHook(
+    props => usePreviousDistinct(props, (prev, next) => (prev && prev.id) === (next && next.id)),
+    {
+      initialProps: exampleObjects[0],
+    }
+  );
+
+  it('should return undefined on initial render', () => {
+    expect(hook.result.current).toBe(undefined);
+  });
+
+  it('should return previous state only after a different value is rendered', () => {
+    expect(hook.result.current).toBeUndefined();
+    hook.rerender(exampleObjects[1]);
+    expect(hook.result.current).toMatchObject(exampleObjects[0]);
+    hook.rerender(exampleObjects[2]);
+    hook.rerender(exampleObjects[2]);
+    expect(hook.result.current).toMatchObject(exampleObjects[1]);
+
+    hook.rerender(exampleObjects[3]);
+    expect(hook.result.current).toMatchObject(exampleObjects[2]);
+  });
+});

src/index.ts

@@ -54,6 +54,7 @@ export { default as useOrientation } from './useOrientation';
 export { default as usePageLeave } from './usePageLeave';
 export { default as usePermission } from './usePermission';
 export { default as usePrevious } from './usePrevious';
+export { default as usePreviousDistinct } from './usePreviousDistinct';
 export { default as usePromise } from './usePromise';
 export { default as useRaf } from './useRaf';
 export { default as useRafLoop } from './useRafLoop';

src/usePreviousDistinct.ts

@@ -0,0 +1,19 @@
+import { useRef } from 'react';
+
+function strictEquals<T>(prev: T | undefined, next: T) {
+  return prev === next;
+}
+
+export default function usePreviousDistinct<T>(
+  value: T,
+  compare: (prev: T | undefined, next: T) => boolean = strictEquals
+) {
+  const prevRef = useRef<T>();
+  const curRef = useRef<T>();
+  if (!compare(curRef.current, value)) {
+    prevRef.current = curRef.current;
+    curRef.current = value;
+  }
+
+  return prevRef.current;
+}