Add useTheme hook for emotion-theming (#1499)

* Add useTheme hook for emotion-theming

* Add changeset

* Small fixes to imports and docs

* Update theming.mdx

.changeset/weak-spies-help/changes.json

@@ -0,0 +1,4 @@
+{
+  "releases": [{ "name": "emotion-theming", "type": "patch" }],
+  "dependents": []
+}

.changeset/weak-spies-help/changes.md

@@ -0,0 +1 @@
+Add useTheme React hook to emotion-theming

docs/theming.mdx

@@ -10,6 +10,33 @@ npm install -S emotion-theming
 
 Add `ThemeProvider` to the top level of your app and access the theme with `props.theme` in a styled component or provide a function that accepts the theme as the css prop. The api is laid out in detail [in the documentation](https://emotion.sh/docs/emotion-theming).
 
+## Examples
+
+### css prop
+
+```jsx
+// @live
+/** @jsx jsx */
+import { jsx } from '@emotion/core'
+import { ThemeProvider } from 'emotion-theming'
+
+const theme = {
+  colors: {
+    primary: 'hotpink'
+  }
+}
+
+render(
+  <ThemeProvider theme={theme}>
+    <div css={theme => ({ color: theme.colors.primary })}>
+      some other text
+    </div>
+  </ThemeProvider>
+)
+```
+
+### styled
+
 ```jsx
 // @live
 /** @jsx jsx */
@@ -30,9 +57,38 @@ const SomeText = styled.div`
 render(
   <ThemeProvider theme={theme}>
     <SomeText>some text</SomeText>
-    <div css={theme => ({ color: theme.colors.primary })}>
-      some other text
-    </div>
   </ThemeProvider>
 )
 ```
+
+### useTheme hook
+
+```jsx
+// @live
+/** @jsx jsx */
+import { jsx } from '@emotion/core'
+import { ThemeProvider, useTheme } from 'emotion-theming'
+
+const theme = {
+  colors: {
+    primary: 'hotpink'
+  }
+}
+
+function SomeText (props) {
+  const theme = useTheme()
+  return (
+    <div
+      css={{ color: theme.colors.primary }}
+      {...props}
+    />
+  )
+}
+
+render(
+  <ThemeProvider theme={theme}>
+    <SomeText>some text</SomeText>
+  </ThemeProvider>
+)
+```
+

packages/emotion-theming/README.md

@@ -11,6 +11,7 @@ _`emotion-theming` is a theming library inspired by [styled-components](https://
 - [API](#api)
   - [ThemeProvider](#themeprovider-reactcomponenttype)
   - [withTheme](#withthemecomponent-reactcomponenttype-reactcomponenttype)
+  - [useTheme](#usetheme)
 - [Credits](#credits)
 - [License](#license)
 
@@ -149,6 +150,40 @@ TellMeTheColor.propTypes = {
 const TellMeTheColorWithTheme = withTheme(TellMeTheColor)
 ```
 
+### useTheme
+
+A React hook that provides the current theme as its value. If the theme is updated, the child component will be re-rendered accordingly.
+
+```jsx
+// @live
+/** @jsx jsx */
+import { jsx } from '@emotion/core'
+import styled from '@emotion/styled'
+import { ThemeProvider, useTheme } from 'emotion-theming'
+
+const theme = {
+  colors: {
+    primary: 'hotpink'
+  }
+}
+
+function SomeText (props) {
+  const theme = useTheme()
+  return (
+    <div
+      css={{ color: theme.colors.primary }}
+      {...props}
+    />
+  )
+}
+
+render(
+  <ThemeProvider theme={theme}>
+    <SomeText>some text</SomeText>
+  </ThemeProvider>
+)
+```
+
 ## Credits
 
 Thanks goes to the [styled-components team](https://github.com/styled-components/styled-components) and [their contributors](https://github.com/styled-components/styled-components/graphs/contributors) who designed this API.

packages/emotion-theming/__tests__/__snapshots__/use-theme.js.snap

@@ -0,0 +1,46 @@
+// Jest Snapshot v1, https://goo.gl/fbAQLP
+
+exports[`Nested useTheme works 1`] = `
+.emotion-1 {
+  color: green;
+}
+
+.emotion-1:hover {
+  color: darkgreen;
+}
+
+.emotion-0 {
+  color: lawngreen;
+}
+
+.emotion-0:hover {
+  color: seagreen;
+}
+
+<div
+  className="emotion-1"
+>
+  Should be green
+  <div
+    className="emotion-0"
+  >
+    Should be lawngreen
+  </div>
+</div>
+`;
+
+exports[`useTheme works 1`] = `
+.emotion-0 {
+  color: green;
+}
+
+.emotion-0:hover {
+  color: darkgreen;
+}
+
+<div
+  className="emotion-0"
+>
+  Should be green
+</div>
+`;

packages/emotion-theming/__tests__/use-theme.js

@@ -0,0 +1,77 @@
+// @flow
+/** @jsx jsx */
+import 'test-utils/next-env'
+import * as renderer from 'react-test-renderer'
+import { jsx } from '@emotion/core'
+import { useTheme, ThemeProvider } from 'emotion-theming'
+
+test('useTheme works', () => {
+  function TestComponent(props) {
+    const theme = useTheme()
+    return (
+      <div
+        css={{ color: theme.lightGreen, '&:hover': { color: theme.darkGreen } }}
+      >
+        Should be green
+      </div>
+    )
+  }
+
+  expect(
+    renderer
+      .create(
+        <ThemeProvider theme={{ lightGreen: 'green', darkGreen: 'darkgreen' }}>
+          <TestComponent />
+        </ThemeProvider>
+      )
+      .toJSON()
+  ).toMatchSnapshot()
+})
+
+test('Nested useTheme works', () => {
+  function TestComponent1(props) {
+    const theme = useTheme()
+    return (
+      <div
+        css={{ color: theme.lightGreen, '&:hover': { color: theme.darkGreen } }}
+        {...props}
+      />
+    )
+  }
+
+  function NestedComponent(props) {
+    const theme = useTheme()
+    return (
+      <div
+        css={{
+          color: theme.lightGreen,
+          '&:hover': { color: theme.darkGreen }
+        }}
+        {...props}
+      />
+    )
+  }
+
+  function TestComponent2(props) {
+    return (
+      <TestComponent1>
+        Should be green
+        <ThemeProvider
+          theme={{ lightGreen: 'lawngreen', darkGreen: 'seagreen' }}
+        >
+          <NestedComponent>Should be lawngreen</NestedComponent>
+        </ThemeProvider>
+      </TestComponent1>
+    )
+  }
+
+  expect(
+    renderer
+      .create(
+        <ThemeProvider theme={{ lightGreen: 'green', darkGreen: 'darkgreen' }}>
+          <TestComponent2 />
+        </ThemeProvider>
+      )
+      .toJSON()
+  ).toMatchSnapshot()
+})

packages/emotion-theming/src/index.js

@@ -1,3 +1,4 @@
 // @flow
 export { default as ThemeProvider } from './theme-provider'
 export { default as withTheme } from './with-theme'
+export { default as useTheme } from './use-theme'

packages/emotion-theming/src/use-theme.js

@@ -0,0 +1,7 @@
+// @flow
+import React from 'react'
+import { ThemeContext } from '@emotion/core'
+
+export default function useTheme() {
+  return React.useContext(ThemeContext)
+}