wp-calypso/client/reader/components/reader-main/index.jsx

import { Component } from 'react';
import SyncReaderFollows from 'calypso/components/data/sync-reader-follows';
import Main from 'calypso/components/main';
import { ReaderPendingActionHandler } from './pending-action-handler';
import './style.scss';

/*
 * We ref-count number of ReaderMains on screen in order to avoid a race condition
 *
 * If two pages in a row have a ReaderMain there is no guarantee as to the order of dismounting
 * and mounting. If we naively toggled the readerPage within willMount and willDismount
 * we could run into a weird state.
 *
 * A problem sequence would be:
 * 1. land on reader (mount, 1 ref)
 * 2. navigate to another reader page (mount new ReaderMain, 2 ref)
 * 3. dismount old ReaderMain from the first step (dismount, 1 ref)
 */
let activeReaderMainRefCount = 0;
const setIsReaderPage = ( add ) => {
	if ( add ) {
		document.querySelector( 'body' ).classList.add( 'is-reader-page' );
	} else if ( activeReaderMainRefCount === 0 ) {
		document.querySelector( 'body' ).classList.remove( 'is-reader-page' );
	}
};

/**
 * A specialization of `Main` that adds a class to the body of the document
 *
 * This class is used by pieces of the Reader to indicate they want "editorial" styles.
 * Notably, this overrides the background color of the document and is used as a hook by other parts to override styles.
 */
export default class ReaderMain extends Component {
	componentDidMount() {
		activeReaderMainRefCount++;
		setIsReaderPage( true );
	}

	componentWillUnmount() {
		activeReaderMainRefCount--;
		setIsReaderPage( false );
	}

	render() {
		const { children, forwardRef, ...props } = this.props;
		return (
			<div ref={ forwardRef }>
				<Main { ...props }>
					<SyncReaderFollows key="syncReaderFollows" />
					<ReaderPendingActionHandler />
					{ children }
				</Main>
			</div>
		);
	}
}