# useContext

## 基本

```jsx
const value = useContext(MyContext)
```

接收一个 context 对象（`React.createContext` 的返回值）并返回该 context 的当前值。当前的 context 值由上层组件中距离当前组件最近的 `<MyContext.Provider>` 的 `value` prop 决定。

当组件上层最近的 `<MyContext.Provider>` 更新时，该 Hook 会触发重渲染，并使用最新传递给 `MyContext` provider 的 context `value` 值。

因此调用了 `useContext` 的组件总会在 context 值变化时重新渲染。

{% hint style="info" %}
`useContext(MyContext)` 相当于 class 组件中的 `static contextType = MyContext` 或者 `<MyContext.Consumer>`
{% endhint %}

## 示例

```jsx
const themes = {
  light: {
    foreground: "#000000",
    background: "#eeeeee"
  },
  dark: {
    foreground: "#ffffff",
    background: "#222222"
  }
};

const ThemeContext = React.createContext(themes.light);

function App() {
  return (
    <ThemeContext.Provider value={themes.dark}>
      <Toolbar />
    </ThemeContext.Provider>
  );
}

function Toolbar(props) {
  return (
    <div>
      <ThemedButton />
    </div>
  );
}

function ThemedButton() {
  const theme = useContext(ThemeContext);
  return (
    <button style={{ background: theme.background, color: theme.foreground }}>
      I am styled by theme context!
    </button>
  );
}
```


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