cron-example-plugin.php

<?php
/*
 * Plugin Name: Cron Example for https://core.trac.wordpress.org/ticket/42660
 */

class Cron_example_42660 {
	function __construct() {
		add_action( 'admin_menu', array( $this, 'admin_menu' ) );
		add_action( 'cron_single_event', array( $this, 'cron_callback' ) );
	}
	function admin_menu() {
		add_menu_page( 'Cron Example', 'Cron Example', 'read', 'cron-example-42660', array( $this, 'render' ) );

		add_action( 'admin_init', array( $this, 'maybe_schedule_cron' ) );
		add_action( 'admin_init', array( $this, 'maybe_delete_cron_runs' ) );
	}
	function render() {
		echo '<div class="wrap">';
		echo '<h1>Cron Example</h1>';

		echo '<div>
			<form action="admin.php?page=cron-example-42660" method="post"><input type="submit" name="schedule_cron" value="Schedule the cron to run once." /></form>
			<form action="admin.php?page=cron-example-42660" method="post"><input type="submit" name="delete_cron_runs" value="Delete the logs" /></form>
			</div>';

		if ( $runs = (array) get_option( __CLASS__, array() ) ) {
			echo '<div>Times the cron has run:<ol>';
			foreach ( $runs as $i => $time ) {
				echo '<li>' . date_format( DateTime::createFromFormat( 'U.u', $time ), 'Y-m-d H:i:s.u' ) . '</li>';
			}
			echo '</div>';
		} else {
			echo "<div>The cron hasn't yet run.</div>";
		}

		echo '</div>';
	}

	function maybe_delete_cron_runs() {
		if ( isset( $_POST['delete_cron_runs'] ) ) {
			delete_option( __CLASS__ );

			// Redirect so that if we reload the page, it doesn't re-delete it again.
			wp_safe_redirect( 'admin.php?page=cron-example-42660' );
			die();
		}
	}

	function maybe_schedule_cron() {
		if ( ! isset( $_POST['schedule_cron'] ) ) {
			return; // Didn't request the cron run.
		}

		$when_to_run = time(); // Run now.
		if ( wp_next_scheduled( 'cron_single_event' ) ) {
			// It's already schedule to run, lets just leave that be.
			return;
		}
		wp_schedule_single_event( $when_to_run, 'cron_single_event' );

		// Redirect so that if we reload the page, it doesn't re-schedule it again.
		wp_safe_redirect( 'admin.php?page=cron-example-42660' );
		die();
	}

	function cron_callback() {
		$option_value = (array) get_option( __CLASS__, array() );
		$option_value[] = microtime(1);
		update_option( __CLASS__, $option_value );
	}
}
new Cron_example_42660();