Source: my-calendar-api-client/ui-admin.php

<?php
/**
 * REST API Client Admin UI
 *
 * @category API
 * @package  My Calendar Pro
 * @author   Joe Dolson
 * @license  GPLv2 or later
 * @link     https://www.joedolson.com/my-calendar-pro/
 */

if ( ! defined( 'ABSPATH' ) ) {
	exit;
}

// add checkbox in event creation.
// admin UI to show calendars to display on central.

/**
 * Add custom fields under event title.
 *
 * @since 3.0. Filter adds tools directly under event title.
 *
 * @param string $fields Existing fields.
 * @param bool   $has_data Does this event have data.
 * @param array  $data Array of event data.
 *
 * @return string new field data.
 */
function mcs_api_choose_targets( $fields, $has_data, $data ) {
	$sites  = mcs_api_endpoints();
	$output = '';
	$shared = ( $has_data ) ? get_post_meta( $data->event_post, '_log_endpoint' ) : array();
	if ( ! is_array( $shared ) ) {
		$shared = array();
	}
	if ( ! empty( $sites ) ) {
		foreach ( $sites as $endpoint => $site ) {
			// verify and regularize URL.
			$label    = $site['label'];
			$verified = mcs_api_verify_endpoint( $endpoint );
			$checked  = '';
			// checked if either in extant endpoints *or* creating a new event.
			if ( $has_data && in_array( $endpoint, $shared, true ) ) {
				$checked = "checked='checked'";
			}

			// toggle default state using mcapi_default_state.
			if ( ! isset( $_GET['event_id'] ) ) {
				/**
				 * Filter the checked state of an API endpoint in the admin.
				 *
				 * @hook mcapi_default_state
				 *
				 * @param {string} $checked Default checked state for an endpoint. E.g 'checked="checked"'.
				 * @param {string} $endpoint The endpoint being displayed. URL from API settings.
				 *
				 * @return {string}
				 */
				$checked = apply_filters( 'mcapi_default_state', $checked, $endpoint );
			}

			$id = 'mc_api_' . sanitize_title( $endpoint );
			if ( $verified ) {
				$output .= "<li><input type='checkbox' name='mc_api_endpoints[]' id='" . esc_attr( $id ) . "' value='" . esc_url( $endpoint ) . "' $checked /> <label for='" . esc_attr( $id ) . "'>" . esc_html( $label ) . '</label></li>';
			}
		}

		if ( '' !== $output ) {
			// The filter this is attached to doesn't echo itself.
			?>
			<div class='mc_api_endpoints selector'>
				<fieldset>
					<legend><?php esc_html_e( 'Cross Post Event:', 'my-calendar-pro' ); ?></legend>
					<ul class='checkboxes'>
						<?php echo wp_kses( $output, mc_kses_elements() ); ?>
					</ul>
				</fieldset>
			</div>
			<?php
		}
	}

	return $fields;
}
add_filter( 'mc_insert_custom_fields', 'mcs_api_choose_targets', 10, 3 );

/**
 * Save response from API for display
 *
 * @param array $responses Log of responses to this API event.
 * @param int   $event_id Event ID.
 */
function mcs_api_log_responses( $responses, $event_id ) {
	$event_post = mc_get_event_post( $event_id );
	update_post_meta( $event_post, '_log_response', $responses );
	foreach ( $responses as $ep => $response ) {
		// record sites shared with.
		update_post_meta( $event_post, '_log_endpoint', $ep );
	}
}

/**
 * Add filter to display responses
 *
 * @param array $response Response from server.
 *
 * @return array $response.
 */
function mcs_api_display_responses( $response ) {
	$event_post = mc_get_event_post( $response['event_id'] );
	$log        = get_post_meta( $event_post, '_log_response', true );

	$original_message = $response['message'];
	$new_message      = '';

	if ( ! is_array( $log ) ) {
		return $response;
	}

	foreach ( $log as $endpoint => $resp ) {
		if ( ! is_string( $resp['body'] ) ) {
			continue;
		}
		$r     = json_decode( $resp['body'] );
		$label = mcs_api_id_endpoint( $endpoint );

		if ( is_object( $r ) ) {
			$event_id = ( property_exists( $r, 'event_id' ) ) ? $r->event_id : false;
			if ( ! $event_id ) {
				$error = trim( $r->message );
				// Translators: Link to source of error.
				$new_message .= "<div class='error'><p>" . sprintf( __( 'Error at %s.', 'my-calendar-pro' ), "<a href='" . esc_url( $endpoint ) . "'>" . esc_html( $label ) . '</a>' ) . '</p></div>';
				$new_message .= $error;
			} else {
				// Translators: 1) Link to site being shared to, 2) Event ID on other site.
				$new_message .= "<div class='updated'><p>" . sprintf( __( 'Event shared to %1$s. (Event ID: %2$d)', 'my-calendar-pro' ), "<a href='" . esc_url( $endpoint ) . "'>" . esc_html( $label ) . '</a>', $event_id ) . '</p></div>';
			}
		} else {
			// Translators: Link to remote site.
			$new_message .= "<div class='error'><p>" . sprintf( __( 'There was an error posting that event to the remote site (%s).', 'my-calendar-pro' ), "<a href='" . esc_url( $endpoint ) . "'>" . esc_html( $label ) . '</a>' ) . '</p></div>';
		}
		$new_message = apply_filters( 'mcs_api_message', $new_message, $r );
	}
	$response['message'] = $original_message . $new_message;

	return $response;
}
add_filter( 'mc_event_saved_message', 'mcs_api_display_responses', 10, 1 );

/**
 * Take endpoint URL and verify that it is a permitted endpoint & get title.
 *
 * @param string $endpoint URL of API endpoint.
 *
 * @return Label of valid endpoint.
 */
function mcs_api_id_endpoint( $endpoint ) {
	$endpoints = mcs_api_endpoints();
	// endpoints should be considered identical whether they're https or http, slashed or unslashed.
	// this way, if a endpoint is changed from one to the other, references aren't lost.
	$endpoint1 = str_replace( 'http:', 'https:', $endpoint );
	$endpoint2 = str_replace( 'https:', 'http:', $endpoint );
	$endpoint3 = trailingslashit( $endpoint1 );
	$endpoint4 = untrailingslashit( $endpoint1 );
	$endpoint5 = trailingslashit( $endpoint2 );
	$endpoint6 = untrailingslashit( $endpoint2 );
	$id        = ( isset( $endpoints[ $endpoint1 ] ) ) ? $endpoints[ $endpoint1 ] : false;

	if ( ! $id ) {
		$id = ( isset( $endpoints[ $endpoint2 ] ) ) ? $endpoints[ $endpoint2 ] : false;
	}

	if ( ! $id ) {
		$id = ( isset( $endpoints[ $endpoint3 ] ) ) ? $endpoints[ $endpoint3 ] : false;
	}

	if ( ! $id ) {
		$id = ( isset( $endpoints[ $endpoint4 ] ) ) ? $endpoints[ $endpoint4 ] : false;
	}

	if ( ! $id ) {
		$id = ( isset( $endpoints[ $endpoint5 ] ) ) ? $endpoints[ $endpoint5 ] : false;
	}

	if ( ! $id ) {
		$id = ( isset( $endpoints[ $endpoint6 ] ) ) ? $endpoints[ $endpoint6 ] : false;
	}

	if ( $id ) {
		$label = $id['label'];
	} else {
		$label = __( 'Untitled Endpoint', 'my-calendar-pro' );
	}

	return $label;
}

/**
 * Add filter to show where event is saved.
 *
 * @param array $data Event data.
 * @param int   $event_id Event ID.
 */
function mcs_api_shared_events( $data, $event_id ) {
	if ( $event_id ) {
		if ( is_object( $event_id ) ) {
			$data = $event_id;
		}
		// given Event ID, fetch site where event has been shared/where event came from.
		$post     = $data->event_post;
		$response = get_post_meta( $post, '_log_endpoint', true );
	}
}
add_filter( 'mc_event_notices', 'mcs_api_shared_events', 10, 2 );