javascript - 如何使用 useContext 改變 Context 的值？

在 React 16.8 以上版本使用 useContext 鉤子運作良好。你可以創建一個組件，使用該鉤子，並且可以毫無問題地使用上下文的值。

我不確定的是如何將更改應用到 Context Provider 的值。

1) useContext 鉤子是否僅僅是用來消費上下文值的手段？

2) 有沒有推薦的方法，使用 React hooks，從子組件更新值，這將觸發使用此 context 的任何組件使用 useContext hook 重新渲染？

const ThemeContext = React.createContext({
  style: 'light',
  visible: true
});

function Content() {
  const { style, visible } = React.useContext(ThemeContext);

  const handleClick = () => {
    // change the context values to
    // style: 'dark'
    // visible: false
  }

  return (
    <div>
      <p>
        The theme is <em>{style}</em> and state of visibility is 
        <em> {visible.toString()}</em>
      </p>
      <button onClick={handleClick}>Change Theme</button>
    </div>
  )
};

function App() {
  return <Content />
};

const rootElement = document.getElementById('root');
ReactDOM.render(<App />, rootElement);

<div id="root"></div>
<script src="https://cdnjs.cloudflare.com/ajax/libs/react/16.8.2/umd/react.production.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/react-dom/16.8.2/umd/react-dom.production.min.js"></script>

答案＃1

如何使用 hooks 更新上下文在 Hooks 常見問題的「如何避免向下傳遞回調？」部分有所討論。

傳遞給 createContext 的參數只有在使用 useContext 的元件上方沒有任何 Provider 時，才會作為預設值。你也可以創建一個 Provider，提供 style 和 visibility，以及切換它們的函數。

範例

const { createContext, useContext, useState } = React;

const ThemeContext = createContext(null);

function Content() {
  const { style, visible, toggleStyle, toggleVisible } = useContext(
    ThemeContext
  );

  return (
    <div>
      <p>
        The theme is <em>{style}</em> and state of visibility is
        <em> {visible.toString()}</em>
      </p>
      <button onClick={toggleStyle}>Change Theme</button>
      <button onClick={toggleVisible}>Change Visibility</button>
    </div>
  );
}

function App() {
  const [style, setStyle] = useState("light");
  const [visible, setVisible] = useState(true);

  function toggleStyle() {
    setStyle(style => (style === "light" ? "dark" : "light"));
  }
  function toggleVisible() {
    setVisible(visible => !visible);
  }

  return (
    <ThemeContext.Provider
      value={{ style, visible, toggleStyle, toggleVisible }}
    >
      <Content />
    </ThemeContext.Provider>
  );
}

ReactDOM.render(<App />, document.getElementById("root"));

<script src="https://unpkg.com/react@16/umd/react.development.js"></script>
<script src="https://unpkg.com/react-dom@16/umd/react-dom.development.js"></script>

<div id="root"></div>

答案＃2

你可以使用這種方法，無論你有多少個嵌套的組件，它都能正常運作。

// Settings Context - src/context/Settings
import React, { useState } from "react";

const SettingsContext = React.createContext();

const defaultSettings = {
  theme: "light",
};

export const SettingsProvider = ({ children, settings }) => {
  const [currentSettings, setCurrentSettings] = useState(
    settings || defaultSettings
  );

  const saveSettings = (values) => {
   setCurrentSettings(values)
  };

  return (
    <SettingsContext.Provider
      value={{ settings: currentSettings, saveSettings }}
    >
      {children}
    </SettingsContext.Provider>
  );
};

export const SettingsConsumer = SettingsContext.Consumer;

export default SettingsContext;

// Settings Hook - src/hooks/useSettings
import { useContext } from "react";
import SettingsContext from "src/context/SettingsContext";

export default () => {
  const context = useContext(SettingsContext);

  return context;
};

// src/index
ReactDOM.render(
  <SettingsProvider settings={settings}>
    <App />
  </SettingsProvider>,
  document.getElementById("root")
);

// Any component do you want to toggle the theme from
// Example: src/components/Navbar
const { settings, saveSettings } = useSettings();

const handleToggleTheme = () => {
  saveSettings({ theme: "light" });
};