feat(context-selector): add initial implementation (#2292)

* feat(context-selector): add initial implementation

* fix review comments

* remove todo

* bump version to align with other packages

Author: layershifter

packages/react-context-selector/.gulp.js

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+module.exports = require('../../.gulp')

packages/react-context-selector/README.md

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+# `@fluentui/react-context-selector`
+
+React `useContextSelector()` and `useContextSelectors()` hooks in userland.
+
+## Introduction
+
+[React Context](https://reactjs.org/docs/context.html) and [`useContext()`](https://reactjs.org/docs/hooks-reference.html#usecontext) is often used to avoid prop drilling,
+however it's known that there's a performance issue. When a context value is changed, all components that are subscribed with `useContext()` will re-render.
+
+[useContextSelector](https://github.com/reactjs/rfcs/pull/119) is recently proposed. While waiting for the process, this library provides the API in userland.
+
+# Installation
+
+**NPM**
+
+```bash
+npm install --save @fluentui/react-context-selector
+```
+
+**Yarn**
+
+```bash
+yarn add @fluentui/react-context-selector
+```
+
+## Technical memo
+
+React context by nature triggers propagation of component re-rendering if a value is changed. To avoid this, this library uses undocumented feature of `calculateChangedBits`. It then uses a subscription model to force update when a component needs to re-render.
+
+## Limitations
+
+- In order to stop propagation, `children` of a context provider has to be either created outside of the provider or memoized with `React.memo`.
+- `<Consumer />` components are not supported.
+- The [stale props](https://react-redux.js.org/api/hooks#stale-props-and-zombie-children) issue can't be solved in userland. (workaround with try-catch)
+
+## Related projects
+
+The implementation is heavily inspired by:
+
+- [use-context-selector](https://github.com/dai-shi/use-context-selector)
+- [react-tracked](https://github.com/dai-shi/react-tracked)

packages/react-context-selector/babel.config.js

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+module.exports = api => require('@fluentui/internal-tooling/babel')(api)

packages/react-context-selector/gulpfile.ts

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+import '../../gulpfile'

packages/react-context-selector/jest.config.js

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+module.exports = {
+  ...require('@fluentui/internal-tooling/jest'),
+  name: 'react-context-selector',
+  moduleNameMapper: require('lerna-alias').jest(),
+}

packages/react-context-selector/package.json

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+{
+  "name": "@fluentui/react-context-selector",
+  "description": "React useContextSelector & useContextSelectors hooks in userland",
+  "version": "0.43.0",
+  "author": "Oleksandr Fediashov <[email protected]>",
+  "bugs": "https://github.com/microsoft/fluent-ui-react/issues",
+  "dependencies": {
+    "@babel/runtime": "^7.7.6"
+  },
+  "devDependencies": {
+    "@fluentui/internal-tooling": "^0.44.0",
+    "@types/react-is": "^16.7.1",
+    "lerna-alias": "^3.0.3-0",
+    "react": "^16.8.0",
+    "react-is": "^16.6.3"
+  },
+  "peerDependencies": {
+    "react": "^16.8.0"
+  },
+  "files": [
+    "dist"
+  ],
+  "homepage": "https://github.com/microsoft/fluent-ui-react/tree/master/packages/react-context-selector",
+  "jsnext:main": "dist/es/index.js",
+  "license": "MIT",
+  "main": "dist/commonjs/index.js",
+  "module": "dist/es/index.js",
+  "publishConfig": {
+    "access": "public"
+  },
+  "repository": "microsoft/fluent-ui-react.git",
+  "scripts": {
+    "build": "gulp bundle:package:no-umd",
+    "clean": "gulp bundle:package:clean",
+    "test": "gulp test",
+    "test:watch": "gulp test:watch"
+  },
+  "sideEffects": false,
+  "types": "dist/es/index.d.ts"
+}

packages/react-context-selector/src/createContext.ts

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+import * as React from 'react'
+import { Context, ContextListener, ContextValue, CreateContextOptions } from './types'
+
+// Stops React Context propagation
+// https://github.com/facebook/react/blob/95bd7aad7daa80c381faa3215c80b0906ab5ead5/packages/react-reconciler/src/ReactFiberBeginWork.js#L2656
+const calculateChangedBits = () => 0
+
+const createProvider = <Value>(Original: React.Provider<ContextValue<Value>>) => {
+  const Provider: React.FC<React.ProviderProps<Value>> = props => {
+    const listeners = React.useRef<ContextListener<Value>[]>([])
+
+    // We call listeners in render intentionally. Listeners are not technically pure, but
+    // otherwise we can't get benefits from concurrent mode.
+    //
+    // We make sure to work with double or more invocation of listeners.
+    listeners.current.forEach(listener => listener(props.value))
+
+    // Disables updates propogation for React Context as `value` is always shallow equal
+    const subscribe = React.useCallback((listener: ContextListener<Value>) => {
+      listeners.current.push(listener)
+
+      const unsubscribe = () => {
+        const index = listeners.current.indexOf(listener)
+        listeners.current.splice(index, 1)
+      }
+
+      return unsubscribe
+    }, [])
+
+    return React.createElement(
+      Original,
+      { value: { subscribe, value: props.value } },
+      props.children,
+    )
+  }
+
+  /* istanbul ignore else */
+  if (process.env.NODE_ENV !== 'production') {
+    Provider.displayName = 'ContextSelector.Provider'
+  }
+
+  return Provider
+}
+
+const createContext = <Value>(
+  defaultValue: Value,
+  options: CreateContextOptions = {},
+): Context<Value> => {
+  const { strict = true } = options
+
+  const context = React.createContext<ContextValue<Value>>(
+    {
+      get subscribe() {
+        if (strict) {
+          /* istanbul ignore next */
+          throw new Error(
+            process.env.NODE_ENV === 'production'
+              ? ''
+              : `Please use <Provider /> component from "@fluentui/react-context-selector"`,
+          )
+        }
+
+        /* istanbul ignore next */
+        return () => () => {}
+      },
+      value: defaultValue,
+    },
+    calculateChangedBits,
+  )
+  context.Provider = createProvider<Value>(context.Provider) as any
+
+  // We don't support Consumer API
+  delete context.Consumer
+
+  return (context as unknown) as Context<Value>
+}
+
+export default createContext

packages/react-context-selector/src/index.ts

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+export { default as createContext } from './createContext'
+export { default as useContextSelector } from './useContextSelector'
+export { default as useContextSelectors } from './useContextSelectors'
+export * from './types'

packages/react-context-selector/src/types.ts

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+import * as React from 'react'
+
+export type Context<Value> = React.Context<Value> & {
+  Provider: React.FC<React.ProviderProps<Value>>
+  Consumer: never
+}
+
+export type CreateContextOptions = {
+  strict?: boolean
+}
+
+export type ContextListener<Value> = (value: Value) => void
+
+export type ContextSelector<Value, SelectedValue> = (value: Value) => SelectedValue
+
+export type ContextValue<Value> = {
+  subscribe: (listener: ContextListener<Value>) => any
+  value: Value
+}

packages/react-context-selector/src/useContextSelector.ts

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+import * as React from 'react'
+
+import { Context, ContextSelector, ContextValue } from './types'
+import { useIsomorphicLayoutEffect } from './utils'
+
+type UseSelectorRef<Value, SelectedValue> = {
+  selector: ContextSelector<Value, SelectedValue>
+  selected: SelectedValue
+  value: Value
+}
+
+/**
+ * This hook returns context selected value by selector.
+ * It will only accept context created by `createContext`.
+ * It will trigger re-render if only the selected value is referencially changed.
+ */
+const useContextSelector = <Value, SelectedValue>(
+  context: Context<Value>,
+  selector: ContextSelector<Value, SelectedValue>,
+): SelectedValue => {
+  const { subscribe, value } = React.useContext(
+    (context as unknown) as Context<ContextValue<Value>>,
+  )
+  const [, forceUpdate] = React.useReducer((c: number) => c + 1, 0) as [never, () => void]
+
+  const ref = React.useRef<UseSelectorRef<Value, SelectedValue>>()
+  const selected = selector(value)
+
+  useIsomorphicLayoutEffect(() => {
+    ref.current = {
+      selector,
+      value,
+      selected,
+    }
+  })
+  useIsomorphicLayoutEffect(() => {
+    const callback = (nextState: Value) => {
+      try {
+        const reference: UseSelectorRef<Value, SelectedValue> = ref.current as NonNullable<
+          UseSelectorRef<Value, SelectedValue>
+        >
+
+        if (
+          reference.value === nextState ||
+          Object.is(reference.selected, reference.selector(nextState))
+        ) {
+          // not changed
+          return
+        }
+      } catch (e) {
+        // ignored (stale props or some other reason)
+      }
+
+      forceUpdate()
+    }
+
+    return subscribe(callback)
+  }, [subscribe])
+
+  return selected
+}
+
+export default useContextSelector