set_header_func

Sets a header function.

By setting a header function on the this one can dynamically add headers in front of rows, depending on the contents of the row and its position in the list.

For instance, one could use it to add headers in front of the first item of a new kind, in a list sorted by the kind.

The update_header can look at the current header widget using [method@Gtk.ListBoxRow.get_header] and either update the state of the widget as needed, or set a new one using [method@Gtk.ListBoxRow.set_header]. If no header is needed, set the header to null.

Note that you may get many calls update_header to this for a particular row when e.g. changing things that don’t affect the header. In this case it is important for performance to not blindly replace an existing header with an identical one.

The update_header function will be called for each row after the call, and it will continue to be called each time a row changes ( via [method@Gtk.ListBoxRow.changed]) and when the row before changes (either by [method@Gtk.ListBoxRow.changed] on the previous row, or when the previous row becomes a different row). It is also called for all rows when [method@Gtk.ListBox.invalidate_headers] is called.

Parameters:

this	a `GtkListBox`
update_header	callback that lets you add row headers
destroy	destroy notifier for user_data
user_data	user data passed to update_header