Indeed the events are fired twice but only when sorted column visibility is being toggled.
I see no point in showing loading indicator for client-side processing, column visibility changes occur very fast. For server-side processing, there is processing option already available.
You can do something like this. But I had to put an alert() because columns are toggled very fast and Processing... message disappears quickly.
$(document).ready(function() {
$('#example').DataTable( {
dom: 'C<"clear">lfrtip',
processing: true,
drawCallback : function() {
$('.dataTables_processing', $('#example').DataTable().table().container()).hide();
}
} );
} );
$('#example').on( 'column-visibility.dt', function ( e, settings, column, state ) {
$('.dataTables_processing', $('#example').DataTable().table().container()).show();
alert('Column visibility is toggled');
} );
See this jsFiddle for code and demonstration.
