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Figure 3-6. A more complex example with nested edit method calls
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Figure 3-7. The result after calling ApplyEdit()
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Both objects remain marked for deletion because the changes made at edit level 2 were applied. Were CancelEdit() called now, the collection would return to the same state as when the first BeginEdit() was called, meaning that only child A (not marked for deletion) would be left. Alternatively, a call to ApplyEdit() would commit all changes made at edit level 1: child A would continue to be marked for deletion, and child B would be totally discarded since it was added and deleted at edit level 1. Both of these possible outcomes are illustrated in Figure 3-8.
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Figure 3-8. Result after calling either CancelEdit() or ApplyEdit()
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Having gone through all that, let s take a look at the code that will implement these behaviors. The DeleteChild() and UnDeleteChild() methods deal with marking the child objects as deleted and moving them between the active items in the collection and the DeletedList object: Private Sub DeleteChild(ByVal child As C) ' mark the object as deleted child.DeleteChild() ' and add it to the deleted collection for storage DeletedList.Add(child) End Sub Private Sub UnDeleteChild(ByVal child As C) ' we are inserting an _existing_ object so ' we need to preserve the object's editleveladded value ' because it will be changed by the normal add process Dim SaveLevel As Integer = child.EditLevelAdded Add(child) child.EditLevelAdded = SaveLevel ' since the object is no longer deleted, remove it from ' the deleted collection DeletedList.Remove(child) End Sub 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. Recall the InsertItem() method implemented earlier. That method assumes that any child being added to the collection is a new object, and therefore sets its edit level value to the collection s current value. However, the InsertItem() method will be run when this preexisting object is reinserted into the collection, altering its edit level. This 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() Implements Core.IEditableCollection.CopyState Dim Child As C ' we are going a level deeper in editing mEditLevel += 1 ' cascade the call to all child objects For Each Child In Me Child.CopyState() Next ' cascade the call to all deleted child objects For Each Child In DeletedList Child.CopyState() Next End Sub As CopyState() takes a snapshot of the collection s state, it increases the edit level by one. Remember that UndoableBase relied on the Stack object to track the edit level, but this code just uses a simple numeric counter. Remember, a collection has no state of its own, so there s nothing to add to a stack of states. Instead, a collection is only responsible for ensuring that all the objects it contains 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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