A way to make a Multi-level Undo / Redo

[Loïc]

Hi!

There are many ways to add Undo / Redo feature using GdPicture Toolkits.

I give one which is simple and fast to implement.

The idea is to make an array of cloned image. Each entry of this array contains a state of the image on which we are working:


Create a public array containing the handle of the cloned image and a value defining the current array position

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Private Const UNDO_LEVEL = 10 'You can change this value to have an other undo/redo level
Dim arImagesUndo(0 To UNDO_LEVEL - 1) As Long
Dim nUndoIDX As Long = -1 '-1 means that we haven't yet saved image state

Create a SaveGraphicState procedure which saves the current image state. Call this procedure when you want to allow the user to undo the image modifications

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Private Sub SaveGraphicState()
   Dim i As Integer
   
   nUndoIDX = nUndoIDX + 1
   If nUndoIDX > UNDO_LEVEL - 1 Then
      Imaging1.CloseImage (arImagesUndo(0))
      For i = 0 To UNDO_LEVEL - 2
          arImagesUndo(i) = arImagesUndo(i + 1)
          nUndoIDX = UNDO_LEVEL - 1
      Next i
   Else
      For i = nUndoIDX + 1 To UNDO_LEVEL - 1
         If arImagesUndo(i) <> 0 Then
            Imaging1.CloseImage (arImagesUndo(i))
            arImagesUndo(i) = 0
         End If
      Next i
   End If
   arImagesUndo(nUndoIDX) = Imaging1.CreateClonedImage(Imaging1.GetNativeImage)  
End Sub

Undo the to the previous saved state

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Private Sub Undo()
   nUndoIDX = nUndoIDX - 1
   Imaging1.ClearImage (White)
   Call Imaging1.DrawImage(arImagesUndo(nUndoIDX), 0, 0, Imaging1.GetWidth, Imaging1.GetHeight, InterpolationModeNearestNeighbor)
End Sub

Redo to the next saved state

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Private Sub Redo()
   nUndoIDX = nUndoIDX + 1
   Imaging1.ClearImage (White)
   Call Imaging1.DrawImage(arImagesUndo(nUndoIDX), 0, 0, Imaging1.GetWidth, Imaging1.GetHeight, InterpolationModeNearestNeighbor)
End Sub

You can find an application of this method into the following samples:

- FreehandDraw vb6 sample of GdPicture Pro Imaging SDK
- FreehandDraw vb.net sample of GdPicture Pro Imaging SDK


Let me know any suggestion or question.


Loïc

[bdcsoho]

Hello Loïc,

I was sick over the last week therefore I did not have access to my computer...

I will go and look into this sample, to see how I can use it for the benefit of the program I am composing,

Again, thank you for posting the sample.

Sven

[Loïc]

Hi Sven,

Let me know if I was not clear enough or if you encounter problems to include this method on your application.

Best regards,

Loïc

[mercdave]

I am using GDPicture to scan drawings and save as pdf's. I want to give users the ability of dropping inspection symbols (tri-angle) on the drawing and save the amended version. I have been able to drop the required shape on the drawing but want to give the user the ability to undo and re-drop the shape. I am using Delphi 7 and the relevant samples are in VB. How do I do it in Delphi?

Thanks


Dave

[Loïc]

Dave, I can't give the same sample for all the programming languages.

The sample above can be converted in Delphi very easily.

Thanks for your comprehension.

Best regards,

Loïc