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September 30, 2022
Business Design Google Microsoft Tech

React Context: How to Use the useContext Hook

The components in React are the product’s building blocks. These components are organized in a tree hierarchy, with one component serving as the parent of another.

The data flow in react is down the hierarchy, from the parent component to the child component and so on. If the component is far down the hierarchy from the root component, the data must first transit through all of the middle components before reaching the component far down the hierarchy.

To prevent this type of design some hire react developer use redux as a library that works on the global store that stores the data or state globally and is accessible to all the components directly without passing data or state down the hierarchy.

Context is another very curious and awesome way introduced by react to store the data or state globally. 

To use react context in your project, follow these steps:

1. Create a file to create a context object and export that context object from that file.

theme-context.js


import React from ‘react’

export const ThemeContext = React.createContext( ‘dark’ );

We’ve built and exported a context object in this example. A default parameter or value (default props) can be supplied to the context object. If the parent component does not pass any props, the default props defined in the context object are used.

A react context object always comes with a provider function. In the next step, we will see what a provider is.

2. Now in a parent component whose values are to be passed to child component.  

We will wrap any child component that is going down the hierarchy in a react context provider.

Note: If any child component wants to access the state or value from ancestor than the parent or grandparent of that child component should be wrapped inside a react context provider.

Component A  —>  Component B  —->   Component C ——> Component D

  (ancestor)     (child 1)     (child 2)         (child 3)

Here Component A is the root component, component B is the child of component A, component C is the child of component B, component D is the child of component C.

If child 3 wants to access the data from the ancestor or component A than any of component B, C or D should be wrapped in provider.

The Child who wants to access the data, its any parent, or itself should be wrapped inside the context provider of the ancestor that pass the data. 

app.js

import {ThemeContext, themes} from './theme-context';

import ThemedButton from './themed-button';


// An intermediate component that uses the ThemedButton

function Toolbar(props) {

  return (

    <ThemedButton onClick={props.changeTheme}>

       Change Theme

    </ThemedButton>

  );

}

class App extends React.Component {

  constructor(props) {

    super(props);

    this.state = {

      theme: themes.light,

    };


    this.toggleTheme = () => {

      this.setState(state => ({

        theme:

          state.theme === themes.dark

            ? themes.light

            : themes.dark,

      }));

    };

  }


  render() {


   // The ThemedButton button inside the ThemeProvider


   // uses the theme from the state while the one outside uses


   // the default dark theme


   return (

      <Page>


       <ThemeContext.Provider value={this.state.theme}>


          <Toolbar changeTheme={this.toggleTheme} />


       </ThemeContext.Provider>


       <Section>


          <ThemedButton />


       </Section>

      </Page>

    );

  }

}


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

Here class App is the ancestor. The Toolbar is a child functional component and ThemedButton is also a child component.

The context provider contains the Toolbar. The context provider sends the values that Toolbar and any child components can access. Because ThemedButton is not wrapped in any of the context providers, it will use the context’s default value.

The value passed by the provider to the child component can be accessible by it or any child of that child component.

Note: Any child component wrapped inside the provider will take the values passed and if the child component is not wrapped inside the provider it will use the default value of the react context provider.

So we are done with parent component, now let’s move on to the child component definition.

3. In child components, we can access the values in multiple ways. 

Let’s discuss the first way to access the value in child component (contextType).

Themed-button.js


Before Context


import {ThemeContext} from './theme-context';


class ThemedButton extends React.Component {

  render() {

    let props = this.props;


   let theme = this.props; // this.props used


   return (

      <button

        {...props}

        style={{backgroundColor: theme.background}}

      />

    );

  }

}


ThemedButton.contextType = ThemeContext;

export default ThemedButton;


After Context


import {ThemeContext} from './theme-context';

import React from ‘react’


class ThemedButton extends React.Component {

  render() {

    let props = this.props;


   let theme = this.context; //Replaced by this.context


   return (

      <button

        {...props}

        style={{backgroundColor: theme.background}}

      />

    );

  }

}


ThemedButton.contextType = ThemeContext;


export default ThemedButton;

Here ThemedButton is the child component. If you want to access the value then make use contextType before exporting the component. After that everywhere you have used this.props can be replaced by this.context.  

Another way of accessing values is by using react hooks.

import {ThemeContext} from './theme-context';

import React, {useContext} from ‘react’


class ThemedButton extends React.Component {

  render() {

  
   let theme = useContext(ThemeContext);


   return (

      <button

        {...props}

        style={{backgroundColor: theme.background}}

      />

    );

  }

}

export default ThemedButton;

Here we made use of hooks useContext. After this, we can directly access the properties as shown above.

This was all about Context.

 Hope you all the understood it!!!

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