By setting a header function on the box 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.
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.
Sets a header function.
By setting a header function on the box 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 gtk.list_box_row.ListBoxRow.getHeader and either update the state of the widget as needed, or set a new one using gtk.list_box_row.ListBoxRow.setHeader. 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 gtk.list_box_row.ListBoxRow.changed) and when the row before changes (either by gtk.list_box_row.ListBoxRow.changed on the previous row, or when the previous row becomes a different row). It is also called for all rows when gtk.list_box.ListBox.invalidateHeaders is called.