context.js

// eslint-disable-next-line no-unused-vars
import { createElement, Component, createContext, Fragment } from "preact";
const { Provider, Consumer } = createContext();

class ThemeProvider extends Component {
	state = {
		value: this.props.value
	};

	onClick = () => {
		this.setState(prev => ({
			value: prev.value===this.props.value ? this.props.next : this.props.value
		}));
	}

	render() {
		return (
			<div>
				<button onClick={this.onClick}>Toggle</button>
				<Provider value={this.state.value}>{this.props.children}</Provider>
			</div>
		);
	}
}

class Child extends Component {
	shouldComponentUpdate() {
		return false;
	}

	render() {
		return (
			<>
				<p>(blocked update)</p>
				{this.props.children}
			</>
		);
	}
}

export default class ContextDemo extends Component {
	render() {
		return (
			<ThemeProvider value="blue" next="red">
				<Child>
					<Consumer>
						{data => (
							<div>
								<p>current theme: <b>{data}</b></p>
								<ThemeProvider value="black" next="white">
									<Consumer>
										{data => <p>current sub theme: <b>{data}</b></p>}
									</Consumer>
								</ThemeProvider>
							</div>
						)}
					</Consumer>
				</Child>
			</ThemeProvider>
		);
	}
}