Stub/example of reliably detecting if month view is empty, even if filtered via Filter Bar, early enough to perform a redirect (TEC 4.2.x/4.3)

detect-empty-filtered-month-view.php

<?php
/**
 * Stub: take action if month view contains no events.
 *
 * Currently (in TEC 4.2/4.3) the month view template object does not
 * reliably determine if there are any events available until late in the request
 * (and if a Filter Bar filter is applied, it will be too late to perform a
 * redirect).
 *
 * This function works around this to detect if month view is empty nice and
 * early; this regrettably involves some undesirable hacks at present however
 * the overhead of this approach should be fairly neglible: it simply changes when
 * some queries run.
 */
add_action( 'template_redirect', function() {
	global $wp_query,
	       $wp_filter;

	$month_view = false;

	// We're only interested in month view
	if ( 'month' !== $wp_query->get( 'eventDisplay' ) ) {
		return;
	}

	// Template objects are anonymous: if nothing is set up on the following hook we'll be
	// unable to obtain a reference to the current template object
	if ( ! isset( $wp_filter['tribe_events_before_view'] ) ) {
		return;
	}

	// Obtain a reference to the month view template class
	foreach ( $wp_filter['tribe_events_before_view'] as $priority => $callbacks ) {
		foreach ( $callbacks as $single_callback ) {
			if ( ! is_array( $single_callback['function'] ) ) {
				continue;
			}

			if ( is_a( current( $single_callback['function'] ), 'Tribe__Events__Template__Month' ) ) {
				$month_view = current( $single_callback['function'] );
				break;
			}
		}
	}

	// Bail if we were unable to extract the reference to the month view class
	if ( ! $month_view ) {
		return;
	}

	// Run setup_view early and stop it from running a second time later in the request
	$month_view->setup_view();
	remove_action( 'tribe_events_before_view', array( $month_view, 'setup_view' ) );

	// Count the true number of events per day
	while ( Tribe__Events__Template__Month::have_days() ) {
		Tribe__Events__Template__Month::the_day();
		$days_events = Tribe__Events__Template__Month::get_current_day();

		// If non-zero/non-empty then we have at least one event, no need to do anything
		if ( $days_events['total_events'] ) {
			return;
		}
	}

	/**
	 * @todo DO SOMETHING HERE!
	 *
	 * If execution reaches this point, we know that the (possibly filtered) month view does
	 * not contain any events; so we can redirect to another page/view or take some other
	 * action.
	 */
	return;
}, 50 );