context-component

context-component is aimed at reducing the boilerplate of writing flexible centralized state management with React context.

context-component provides extendable React class that automatically assigns its state and methods to context and provides it to its children. It also exposes api to easily consume contexts by connect HOC (high order component) or by React regular context api methods - Consumer, contextType and useContext.

To learn more about context visit - react context documentation.

For example of context-component visit - context-component/example.

install

npm install context-component -S

Usage

Creating ContextComponent

In order to create a shared state create a component that extends ContextComponent with state and methods you want to share in your app:

ThemeContext.jsx

import ContextComponent from 'context-component';

export default class ThemeContext extends ContextComponent {

    state = {theme: 'dark'}

    toggleTheme = () => {
        this.setState(state => (state.theme === 'dark' ? {theme: 'light'} : {theme: 'dark'}));
    }

}

The ContextComponent implements for you a render method that renders the this.componentContext.Provider with the component state and instance methods as value.

Methods defined on the ContextComponent are provided by the context automatically, except for React lifecycle methods and methods starting with '_'. You can override this behavior by adding actions property to the class with the methods you want to expose.

You can use React lifecycle methods in ContextComponent to initialize and manage the state, they will run as the lifecycle of the context provider.

Providing ContextComponent

To provide the context to the React tree you render the component:

App.jsx

import React from 'react';
import ThemeContext from './ThemeContext';

export const App = () => (
    <ThemeContext>
        <otherComponent />
    </ThemeContext>
);

Consuming ContextComponent

To consume the context you can use the component static connect HOC method:

otherComponent.jsx

import React from 'react';
import ThemeContext from './ThemeContext';

const otherComponent = ({toggleTheme, theme}) =>
    <div className={theme} onClick={toggleTheme} />;

const mapContextToProps = context =>
    ({theme: context.theme, toggleTheme: context.toggleTheme});

export default ThemeContext.connect(otherComponent, mapContextToProps);

The component connect HOC method takes three parameters:

• WrappedComponent - The component to connect.
• mapContextsToProps - Callback with two parameters context and ownProps (props assigned by the parent component), and returns new object of props, enabling you to transform, rename and pick the relevant values from the context.
• options - Optional object with the keys:
  • memo - Memorizes the WrappedComponent to not re-render if there aren't changes to the value returned from mapContextsToProps. Boolean type for whether or not to memorizes with shallow check, or function type for memorizes with a custom equality check, defaulted to true.
  • forwardRef - Forwards the ref prop to the WrappedComponent ref. Boolean value, defaulted to false.

---

Or consume the context by rendering the ContextComponent.Consumer:

import React from 'react';
import ThemeContext from './ThemeContext';

export const otherComponent = () => (
    <ThemeContext.Consumer>
        {({theme, toggleTheme}) =>
            <div className={theme} onClick={toggleTheme} />}
    </ThemeContext.Consumer>
);

---

Or by using the React class component contextType property:

import React, {Component} from 'react';
import ThemeContext from './ThemeContext';

export class otherComponent extends Component {

    static contextType = ThemeContext.componentContext;

    render() {
        const {theme, toggleTheme} = this.context;

        return <div className={theme} onClick={toggleTheme} />;
    }

};

---

Or by using the useContext() hook:

import React, {useContext} from 'react';
import ThemeContext from './ThemeContext';

export const otherComponent = () => {
    const {theme, toggleTheme} = useContext(ThemeContext.componentContext);

    return <div className={theme} onClick={toggleTheme} />;
};

Multiple contexts

You can use the Provider component to provide multiple contexts together:

App.jsx

import React from 'react';
import {Provider} from 'context-component';
import ThemeContext from './ThemeContext';
import CounterContext from './CounterContext';

export const App = () => (
    <Provider ContextComponents={[CounterContext, ThemeContext]}>
        <otherComponent />
    </Provider>
);

The Provider requires ContextComponents prop - the ContextComponent classes array.

---

You can consume multiple contexts together with the connect HOC function:

otherComponent.js

import React from 'react';
import {connect} from 'context-component';
import ThemeContext from './ThemeContext';
import CounterContext from './CounterContext';

const otherComponent = ({counter, increase, theme, toggleTheme}) =>
    <div>
        <div className={theme} onClick={toggleTheme} />
        <div onClick={increase}>{counter}</div>
    </div>;

const mapContextsToProps = ([counterContext, themeContext]) => ({
    counter: counterContext.counter,
    increase: counterContext.increase,
    theme: themeContext.theme,
    toggleTheme: themeContext.toggleTheme
});

export default connect(otherComponent, [CounterContext, ThemeContext], mapContextsToProps);

The connect HOC function takes four parameters:

• WrappedComponent - The component to connect.
• ContextComponents - Array of contextComponent classes.
• mapContextsToProps - Callback with two parameters contexts[] and ownProps (props assigned by the parent component), and returns new object of props, enabling you to transform, rename and pick the relevant values from the context.
• options - Optional object with the keys:
  • memo - Memorizes the WrappedComponent to not re-render if there aren't changes to the value returned from mapContextsToProps. Boolean type for whether or not to memorizes with shallow check, or function type for memorizes with a custom equality check, defaulted to true.
  • forwardRef - Forwards the ref prop to the WrappedComponent ref. Boolean value, defaulted to false.

Optimization warning

In connect HOC the mapContextToProps callback shouldn't return new references for the same input (context and ownProps). If you compute a new value like this:

const mapContextToProps = context =>
    ({theme: {color: context.theme}});

The theme value will always return a new object reference for every function call and the React.memo shallow equality check will fail, which means the component will re-render for the same props.

In order to solve this you can set a custom equality function on the memo option:

const areEqual = (prevProps, nextProps) =>
    prevProps.theme.color === nextProps.theme.color;

export default ThemeContext.connect(otherComponent, mapContextToProps, {memo: areEqual});

or set the connect options.memo to 'false' and not memorize the component.

export default ThemeContext.connect(otherComponent, mapContextToProps, {memo: false});