First of all please specify what do you use to draw. Do you use any API or Core-Graphics Framework for drawing.
Here are some of the approaches from the link I found.
Approach:
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on touchesEnded, save the image to app sandbox with a name like.. 1.png, 2.png etc. then put a count on “undo” variable. when user press undo, just loads the old image base on the count. (this is what I can think of roughly, but haven’t gone through the coding of it yet, but sounds pretty simple to code).
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NSUndoManager.
Hope this helps for anyone else looking to do this.
Reference Link:
http://www.iphonedevsdk.com/forum/iphone-sdk-development/46996-drawing-app-how-do-undo.html
Cheers
solved How to “undo” when we draw in iPhone SDK? [closed]