How Do You UNDO?

[Mark Wieder]

Igor-

Saturday, May 26, 2012, 7:42:31 PM, you wrote:

> How do you do it? How do you implement UNDO in your applications?
> Is there a generic mechanism or technique that you use, or do you
> re-invent the wheel with every project?

Depends on what you want to undo. Here's what I do in an upcoming
change to the glx2 script editor. Here what I'm interested in saving
for later undos is the contents of the script editor field. I'm not
concerned about changes to other objects. This creates a LIFO stack by
setting up a new array element in the undo stack every time I save a
change. The state of the script field is recovered by popping the last
saved off the top of the stack. Note that not deleting the top element
after popping it off the stack allows a redo as well as an undo.

local sUndoPointArray

/**
* UndoPointer
*
* Return the index of the last-saved state
*/
private function UndoPointer
  local tCount
    
  put sUndoPointArray["count"] into tCount
  if tCount is empty then
    put 0 into tCount
  end if
  return tCount
end UndoPointer

/**
* InsertUndoScript
*
* Save the current script for undoing later on
*/
private command InsertUndoScript
  local tCount
    
  put UndoPointer()+1 into tCount
  put the htmltext of field kCodeField into sUndoPointArray[tCount]
  put tCount into sUndoPointArray["count"]
end InsertUndoScript

/**
* UndoRefactor
*
* undo the last-saved state of the text
*/
command UndoChange
  local tCount
    
  lock screen
  put UndoPointer() into tCount
  if tCount > 0 then
    set the htmltext of field kCodeField to sUndoPointArray[tCount]
    -- decrement the stack pointer
    put tCount-1 into sUndoPointArray["count"]
  end if
  unlock screen
end UndoChange

Hope this helps.

-- 
-Mark Wieder
 mwieder at ahsoftware.net