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CHAPTER 13 N-LEV EL UNDO
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On the surface, this doesn t seem too complicated but look at the code that deals with the child s EditLevelAdded property in the UnDeleteChild() method. In the InsertItem() method I discussed earlier, the assumption is that any child being added to the collection is a new object, and therefore InsertItem() sets its edit level value to the collection s current value. However, the InsertItem() method is run when this preexisting object is reinserted into the collection, altering its edit level. That would leave the child object with an incorrect edit level value. The problem is that, in this case, the child object isn t a new object; it is a preexisting object that is just being restored to the collection. To solve this, the object s edit level value is stored in a temporary field, the child object is re-added to the collection, and then the child object s edit level value is reset to the original value, effectively leaving it unchanged.
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Everything has so far laid the groundwork for the n-level undo functionality. All the pieces now exist to make it possible to implement the CopyState(), UndoChanges(), and AcceptChanges() methods and then the BeginEdit(), CancelEdit() and ApplyEdit() methods. The CopyState() method needs to take a snapshot of the collection s current state. It is invoked when the BeginEdit() method is called on the root object (either the collection itself or the collection s parent object). At that time, the root object takes a snapshot of its own state and calls CopyState() on any child objects or collections so they can take snapshots of their states as well: Private Sub CopyState(ByVal parentEditLevel As Integer, _ ByVal parentBindingEdit As Boolean) Implements Core.IUndoableObject.CopyState If Not parentBindingEdit Then CopyState(parentEditLevel) End If End Sub Private Sub CopyState(ByVal parentEditLevel As Integer) If Me.EditLevel + 1 > parentEditLevel Then Throw New Core.UndoException(String.Format( _ My.Resources.EditLevelMismatchException, "CopyState")) End If ' we are going a level deeper in editing _editLevel += 1 ' cascade the call to all child objects For Each child As C In Me child.CopyState(_editLevel, False) Next ' cascade the call to all deleted child objects For Each child As C In DeletedList child.CopyState(_editLevel, False) Next End Sub There are technically two CopyState() methods one for the Csla.Core.IUndoableObject interface and the other a Private implementation for use within BusinessListBase. The interface implementation merely delegates to the Private implementation. As CopyState() takes a snapshot of the collection s state, it increases the edit level by one. Remember that UndoableBase relies on the Stack object to track the edit level, but this code just uses a simple numeric counter. A collection has no state of its own, so there s nothing to add to a stack of states. Instead, a collection is responsible only for ensuring that all the objects it contains
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CHAPTER 13 N-LEV EL UNDO
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take snapshots of their states. All it needs to do is keep track of how many times CopyState() has been called so the collection can properly implement the adding and removing of child objects, as described earlier. Notice that the CopyState() call is also cascaded to the objects in DeletedList. This is important because those objects might at some point get restored as active objects in the collection. Even though they re not active at the moment (because they re marked for deletion), they need to be treated the same as regular nondeleted objects. Overall, this process is fairly straightforward: the CopyState() call is just cascaded down to the child objects. The same can t be said for UndoChanges() or AcceptChanges().
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The UndoChanges() method is more complex than the CopyState() method. It too cascades the call down to the child objects, deleted or not, but it also needs to find any objects added since the latest snapshot. Those objects must be removed from the collection and discarded because an undo operation means that it must be as though they were never added. Furthermore, it needs to find any objects deleted since the latest snapshot. Those objects must be re-added to the collection. Here s the complete method: Private Sub UndoChanges(ByVal parentEditLevel As Integer, _ ByVal parentBindingEdit As Boolean) Implements Core.IUndoableObject.UndoChanges If Not parentBindingEdit Then UndoChanges(parentEditLevel) End If End Sub Private _completelyRemoveChild As Boolean Private Sub UndoChanges(ByVal parentEditLevel As Integer) Dim child As C If Me.EditLevel - 1 < parentEditLevel Then Throw New Core.UndoException(String.Format( _ My.Resources.EditLevelMismatchException, "UndoChanges")) End If ' we are coming up one edit level _editLevel -= 1 If _editLevel < 0 Then _editLevel = 0 End If Dim oldRLCE As Boolean = Me.RaiseListChangedEvents Me.RaiseListChangedEvents = False Try ' Cancel edit on all current items For index As Integer = Count - 1 To 0 Step -1 child = Me(index) DeferredLoadIndexIfNotLoaded() _indexSet.RemoveItem(child) child.UndoChanges(_editLevel, False) _indexSet.InsertItem(child) ' if item is below its point of addition, remove
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