6.3.2 Undo histories

As mentioned before, every page has its own undo history; additionally, there is a special page tlo_universe for storing undo history items of actions performed on objs. Every undo history item corresponds to one top-level transaction, the nested transactions are included in it. Creating an undo history item for one page does in no way affect the undo histories of other pages; the undo histories are independent.

One undo item has two levels – the outer one for the GUI and the inner one for the kernel. The GUI level contains the name of the item and a list of elementary kernel operations, such as object link, anchor rehang or a property change. When undo is called, a whole GUI undo action is undone.

To manipulate the undo histories, these functions can be used:

     trans_undo(struct obj_tlo* tlo, char *error_string);
     trans_redo(struct obj_tlo* tlo, char *error_string);
     
     trans_clear_undo(struct obj_tlo* tlo); // clear the whole undo history
     trans_clear_one_redo(struct obj_tlo* tlo);
     trans_clear_all_redo(struct obj_tlo* tlo);
     
     trans_merge_undo(struct obj_tlo *tlo, struct undo_gui *first, string name);
     trans_rename_undo(struct obj_tlo *tlo, struct undo_gui *ug, string name);

They must not be called when a transaction is active (for obvious reasons). The following functions enable the GUI to navigate through the undo histories:

     trans_get_first_gui(struct obj_tlo* t);
     trans_get_next_gui(struct obj_tlo* t, struct undo_gui *ug);
     trans_get_last_gui(struct obj_tlo* t);
     trans_get_prev_gui(struct obj_tlo* t, struct undo_gui *ug);