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filter is not described

is_active_sidebar filter-hook . WP 3.9.0

Filters whether a dynamic sidebar is considered "active".

Usage

add_filter( 'is_active_sidebar', 'filter_function_name_5629', 10, 2 );
function filter_function_name_5629( $is_active_sidebar, $index ){
	// filter...

	return $is_active_sidebar;
}
$is_active_sidebar(true/false)
Whether or not the sidebar should be considered "active". In other words, whether the sidebar contains any widgets.
$index(int/string)
Index, name, or ID of the dynamic sidebar.

Where the hook is called

is_active_sidebar()
wp-includes/widgets.php 881
return apply_filters( 'is_active_sidebar', $is_active_sidebar, $index );

Where the hook is used (in WP core)

wp-includes/class-wp-customize-widgets.php 123
add_filter( 'is_active_sidebar', array( $this, 'tally_sidebars_via_is_active_sidebar_calls' ), 10, 2 );