[lazarus] Form editing: Undo/Redo

Mattias Gaertner nc-gaertnma at netcologne.de
Thu Jun 20 11:30:14 EDT 2002

On Thu, 20 Jun 2002 15:57:00 +0200 (W. Europe Daylight Time)
Michael Van Canneyt <michael.vancanneyt at wisa.be> wrote:

> > > >I want to implement Undo/Redo for the form editing, but I'm not sure how
> > > >this should be done.
> > > >Normally an undolist just keeps tracks of the changes. But for example
> > > >moving a control can result in changing other controls and many other
> > > >things. So, for a correct undo, one have to stream the form after every
> > > >move/resize/add/delete/change property operation. Or is there a better way?
> [...]
> > Forms can contain images and I want undo/redo for at least 10 open forms and 100 steps and, as Martyn mentioned, for almost all kinds of form editing, not just like Delphi.
> > The needed memory should be reduced by saving only the diffs between the streams. Does anybody know a unit for diffing/patching?
> The streaming method contains some functionality for streaming diffs.
> The problem is that you need 2 instances of one object for this to work.
> The 'root' object and the 'child' object.
> To produce a diff stream, it should be sufficient to have the 'root'
> object as the form before the operation, and the child object as the
> form after the operation. The streaming method should then only record
> differences. (this is, BTW, how form inheritance works: root is the
> parent form, and the child form is the descendent form).

AFAIK this is only done for standard types. The big data (e.g. images, nodes) is stored via DefineProperties, which seldom support ancestor comparison. And I'm not sure if the ancestor technique can be used at all. We would need a copy to compare. And creating a copy means stream writing, stream reading, component creating and destroying the copy.


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