These docs are for an old release. Info on upgrading to v5
edit doc

loading

Triggered when event or resource fetching starts/stops.

function( isLoading )

Triggered with a true argument when the calendar begins fetching events via AJAX. Triggered with false when done.

This function is often used to show/hide a loading indicator.

When one of the resource plugins are loaded, this callback is also fired when resources are fetched.