[scribus-dev] Some notes/questions about Scribus internals and my project

[Elvis Stansvik]

Hi Cezary,

2011/5/23 Cezary Grabski <czarek at oferuje.pl>:
>> 2) How does the current undo/redo system work? How do I implement
>> undoable actions? Will I need to implement one (or several?) new
>> UndoState classes?
>>
> As I made undo for text last time I can answer you what I know about undo
> system in Scribus.
> It is quite simple in idea - in UndoState are saved changed parameteres of
> item before and after changing. For example for undow moving PageItem in undo
> state is saved its start position and end position. On undo start position of
> PageItem will be restored. On redo end position will be restored again.

Ah, I see. After looking a little closer I see that it is actually a
variant of the command pattern after all.

> In implementation undo can be more complicated because in undostate only
> simple variables types can be saved - numbers and strings.
> So for text undo, where in undostate must be saved text with all styles
> information, I prepare function which return all that information in string by
> "saxing" (like in SLA file, when all information is in human readable strings
> in tagged fashion, lets say semi-XML manner).

Okay, so it's serialized. Makes sense. Undo/redo really is a
complicated business :)

> BTW I think you can dont care about undo at this state of developpment.
> We were live with Scribus without undo for text by years, so we can live
> without undo for tables at the beginning. Undo for tables can be added later
> when all new tables code will be "stable", even by someone else.

Okay. I think that the table operations should not be _that_ hard to
make undoable this way. I think they will lend themselves quite well
to being encapsulated as a set of properties in an undo state. But
we'll see whether I'll have time for undo/redo then.

BTW, I _very_ much appreciate how well commented/documented the
undo/redo code is. Good job there!

Elvis