Form.js

import React from 'react'

const FormContext = React.createContext({
	formState: {},
	update: () => {},
})

export class Form extends React.PureComponent {
	state = {
		formState: {},
		update: (name, value) =>
			this.setState(state => ({
				formState: {
					...state.formState,
					[name]: value,
				}
			}))
	}

	onSubmit = () =>
		this.props.onSubmit(this.state.formState)

	render () {
		return <FormContext.Provider value={this.state}>
			<form onSubmit={this.onSubmit}>
				{this.props.children}
			</form>
		</FormContext.Provider>
	}
}

export const Input = ({ name }) =>
	<FormContext.Consumer>
		{({ formState, update }) =>
			<input onChange={({ target }) => update(name, target.value)} value={formState[name]} />
		}
	</FormContext.Consumer>