score:501

Accepted answer

Using hooks

Hooks were introduced in 16.8.0 so the following code requires a minimum version of 16.8.0 (scroll down for the class components example). CodeSandbox Demo

1. Setting parent state for dynamic context

Firstly, in order to have a dynamic context which can be passed to the consumers, I'll use the parent's state. This ensures that I've a single source of truth going forth. For example, my parent App will look like this:

const App = () => {
  const [language, setLanguage] = useState("en");
  const value = { language, setLanguage };

  return (
    ...
  );
};

The language is stored in the state. We will pass both language and the setter function setLanguage via context later.

2. Creating a context

Next, I created a language context like this:

// set the defaults
const LanguageContext = React.createContext({
  language: "en",
  setLanguage: () => {}
});

Here I'm setting the defaults for language ('en') and a setLanguage function which will be sent by the context provider to the consumer(s). These are only defaults and I'll provide their values when using the provider component in the parent App.

Note: the LanguageContext remains same whether you use hooks or class based components.

3. Creating a context consumer

In order to have the language switcher set the language, it should have the access to the language setter function via context. It can look something like this:

const LanguageSwitcher = () => {
  const { language, setLanguage } = useContext(LanguageContext);
  return (
    <button onClick={() => setLanguage("jp")}>
      Switch Language (Current: {language})
    </button>
  );
};

Here I'm just setting the language to 'jp' but you may have your own logic to set languages for this.

4. Wrapping the consumer in a provider

Now I'll render my language switcher component in a LanguageContext.Provider and pass in the values which have to be sent via context to any level deeper. Here's how my parent App look like:

const App = () => {
  const [language, setLanguage] = useState("en");
  const value = { language, setLanguage };

  return (
    <LanguageContext.Provider value={value}>
      <h2>Current Language: {language}</h2>
      <p>Click button to change to jp</p>
      <div>
        {/* Can be nested */}
        <LanguageSwitcher />
      </div>
    </LanguageContext.Provider>
  );
};

Now, whenever the language switcher is clicked it updates the context dynamically.

CodeSandbox Demo

Using class components

The latest context API was introduced in React 16.3 which provides a great way of having a dynamic context. The following code requires a minimum version of 16.3.0. CodeSandbox Demo

1. Setting parent state for dynamic context

Firstly, in order to have a dynamic context which can be passed to the consumers, I'll use the parent's state. This ensures that I've a single source of truth going forth. For example, my parent App will look like this:

class App extends Component {
  setLanguage = language => {
    this.setState({ language });
  };

  state = {
    language: "en",
    setLanguage: this.setLanguage
  };

  ...
}

The language is stored in the state along with a language setter method, which you may keep outside the state tree.

2. Creating a context

Next, I created a language context like this:

// set the defaults
const LanguageContext = React.createContext({
  language: "en",
  setLanguage: () => {}
});

Here I'm setting the defaults for language ('en') and a setLanguage function which will be sent by the context provider to the consumer(s). These are only defaults and I'll provide their values when using the provider component in the parent App.

3. Creating a context consumer

In order to have the language switcher set the language, it should have the access to the language setter function via context. It can look something like this:

class LanguageSwitcher extends Component {
  render() {
    return (
      <LanguageContext.Consumer>
        {({ language, setLanguage }) => (
          <button onClick={() => setLanguage("jp")}>
            Switch Language (Current: {language})
          </button>
        )}
      </LanguageContext.Consumer>
    );
  }
}

Here I'm just setting the language to 'jp' but you may have your own logic to set languages for this.

4. Wrapping the consumer in a provider

Now I'll render my language switcher component in a LanguageContext.Provider and pass in the values which have to be sent via context to any level deeper. Here's how my parent App look like:

class App extends Component {
  setLanguage = language => {
    this.setState({ language });
  };

  state = {
    language: "en",
    setLanguage: this.setLanguage
  };

  render() {
    return (
      <LanguageContext.Provider value={this.state}>
        <h2>Current Language: {this.state.language}</h2>
        <p>Click button to change to jp</p>
        <div>
          {/* Can be nested */}
          <LanguageSwitcher />
        </div>
      </LanguageContext.Provider>
    );
  }
}

Now, whenever the language switcher is clicked it updates the context dynamically.

CodeSandbox Demo

score:0

Just wanted to add to Divyanshu Maithani's answer that it's generally safer to use useMemo when wrapping the consumer in a provider.

const App = () => {
  const [language, setLanguage] = useState("en");

  const value = useMemo(
    () => ({ language, setLanguage }),
    [language, setLanguage ],
  );

  return (
    <LanguageContext.Provider value={value}>
      <h2>Current Language: {language}</h2>
      <p>Click button to change to jp</p>
      <div>
        {/* Can be nested */}
        <LanguageSwitcher />
      </div>
    </LanguageContext.Provider>
  );
};

from react/jsx-no-constructed-context-values rule :

React Context, and all its child nodes and Consumers are rerendered whenever the value prop changes. Because each Javascript object carries its own identity, things like object expressions ({foo: 'bar'}) or function expressions get a new identity on every run through the component. This makes the context think it has gotten a new object and can cause needless rerenders and unintended consequences.

This can be a pretty large performance hit because not only will it cause the context providers and consumers to rerender with all the elements in its subtree, the processing for the tree scan react does to render the provider and find consumers is also wasted.

score:4

One quite simple solution is to set state on your context by including a setState method in your provider like so:

return ( 
            <Context.Provider value={{
              state: this.state,
              updateLanguage: (returnVal) => {
                this.setState({
                  language: returnVal
                })
              }
            }}> 
              {this.props.children} 
            </Context.Provider>
        )

And in your consumer, call updateLanguage like so:

// button that sets language config
<Context.Consumer>
{(context) => 
  <button onClick={context.updateLanguage({language})}> 
    Set to {language} // if you have a dynamic val for language
  </button>
<Context.Consumer>

score:8

I personally like this pattern:

File: context.jsx

import React from 'react';

// The Context 
const TemplateContext = React.createContext({});

// Template Provider
const TemplateProvider = ({children}) => {

    const [myValue, setMyValue] = React.useState(0);

    // Context values passed to consumer
    const value = {
        myValue,    // <------ Expose Value to Consumer
        setMyValue  // <------ Expose Setter to Consumer
    };

    return (
        <TemplateContext.Provider value={value}>
            {children}
        </TemplateContext.Provider>
    )
}

// Template Consumer
const TemplateConsumer = ({children}) => {
    return (
        <TemplateContext.Consumer>
            {(context) => {
                if (context === undefined) {
                    throw new Error('TemplateConsumer must be used within TemplateProvider');
                }
                return children(context)
            }}
        </TemplateContext.Consumer>
    )
}

// useTemplate Hook
const useTemplate = () => {
    const context = React.useContext(TemplateContext);
    if(context === undefined)
        throw new Error('useTemplate must be used within TemplateProvider');
    return context;
}

export {
    TemplateProvider,
    TemplateConsumer,
    useTemplate
}

Then you can create a functional component, if it is a child in the tree of the provider:

File: component.jsx

import React            from 'react';
import {useTemplate}    from 'context.jsx';
const MyComponent = () => {

    // Get the value and setter from the consumer hook
    const {myValue, setMyValue} = useTemplate();

    // Demonstrate incrementing the value
    React.useEffect(() => {

        // Increment, set in context
        const increment = () => setMyValue(prev => prev + 1); 

        // Increment every second
        let interval = setInterval(increment, 1000);

        // Cleanup, kill interval when unmounted
        return () => clearInterval(interval);

    },[]) // On mount, no dependencies

    // Render the value as it is pulled from the context
    return (
        <React.Fragment>
            Value of MyValue is: {myValue}
        </React.Fragment>
    )
}

score:88

Since it is recommended by React to use functional components and hooks so I will implement it with useContext and useState hooks. Here is how you can update the context from within a child component.

LanguageContextMangement.js

import React, { useState } from 'react'

export const LanguageContext = React.createContext({
  language: "en",
  setLanguage: () => {}
})

export const LanguageContextProvider = (props) => {

  const setLanguage = (language) => {
    setState({...state, language: language})
  }

  const initState = {
    language: "en",
    setLanguage: setLanguage
  } 

  const [state, setState] = useState(initState)

  return (
    <LanguageContext.Provider value={state}>
      {props.children}
    </LanguageContext.Provider>
  )
}

App.js

import React, { useContext } from 'react'
import { LanguageContextProvider, LanguageContext } from './LanguageContextManagement'

function App() {

  const state = useContext(LanguageContext)

  return (
    <LanguageContextProvider>
      <button onClick={() => state.setLanguage('pk')}>
        Current Language is: {state.language}
      </button>
    </LanguageContextProvider>
  )
}

export default App

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