Re: [Pythoncad-developer] Undo functionality

[Matteo B. <mat...@bo...>]

Hi Gertwing,
I like this solution very clever solution  .,....

regars,
Matteo


On Sat, 2010-02-06 at 18:21 +0000, ger...@gm... wrote:
> Hi All,
> 
> I was thinking about implementing undo functionality in combination
> with the database.
> I am not aware how this is solved in the current version but with a
> database we can make the undo recording persistent (you can undo
> changes from a previous drawing session). 
> 
> The idea is not to modify entities in the database but mark the old
> entity state as deleted and insert a new record for the modified
> entity state.
> Then there are two records in the database, one is the previous state
> and one is the the current state of the entity.
> Of course the entities that are marked as deleted (previous state) are
> not drawn.
> 
> When the user does undo the current state is marked as deleted and the
> previous state is marked as not deleted.
> A redo does this marking in the opposite direction.
> 
> If we add a column "state" and a column "undo_mark" to our entity
> table we can achieve this.
> The column "state" holds the delete (or not) state.
> The column "undo_mark" holds the value of the undo id.
> Because the user can modify many entities at once all those entities
> get the same "undo_mark".
> 
> This way the history of the drawing is kept in the database and the
> user can undo and redo actions from previous sessions.
> If we also add a time stamp we even can provide functionality that
> will show how the drawing looked at a certain date.
> 
> I think this functionality can have some use and is something other
> CAD programs does not always provide.
> Undo and redo are simple database queries that are easy to implement.
> We must keep the display list in sync with the state of the database
> but I think this also it not difficult.
> 
> We also need a function that compresses the drawing by removing the
> history.
> 
> If it needs clarification, let me know I will try to explain.
> 
> Do you think it is useful?
> 
> Regards,
> Gertwin
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