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Of course, the GetValue() method returns everything as type Object, so the value is cast to IUndoableObject in order to call the CopyState() method.
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I want to call your attention to the BindingEdit property that is passed as a parameter to the child s CopyState() method. The BindingEdit property indicates whether this object is currently data bound to a UI. If BindingEdit is True, the object is currently bound to the UI, and data binding has called BeginEdit() through the IEditableObject interface. Because this parameter is included, the code is calling the following overload of CopyState(): Private Sub CopyState(ByVal parentEditLevel As Integer, _ ByVal parentBindingEdit As Boolean) Implements IUndoableObject.CopyState If Not parentBindingEdit Then CopyState(parentEditLevel) End If End Sub
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CHAPTER 13 N-LEV EL UNDO
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This overload is obviously not complex. It simply ensures that the child object takes a snapshot of its own state only when the parent is not using data binding. The idea is to provide the behaviors required by data binding and the behaviors for manual invocation, as I discussed earlier in the chapter. You might wonder why the parent even tries to cascade the call to the child if BindingEdit is True. The reason is that it is the child object s decision whether it should take a snapshot of its state. This approach preserves encapsulation by letting the child object determine its own behavior. Some child objects, such as a BusinessBase child, will ignore the call. But other child objects such as FieldDataManager always take a snapshot of their state. In the case of FieldDataManager this is important, because it contains field values that are directly part of the containing object, so it isn t really a child in the same sense as an editable child that inherits from BusinessBase. Later in this chapter you ll see that the methods to undo or accept any changes will work the same way that is, they ll cascade the calls to any child objects. This way, all objects handle undo without breaking encapsulation.
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Taking a Snapshot of a Regular Field
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With a regular field, the code simply stores the field value into the HybridDictionary object, associating that value with the combined class name and field name: ' this is a normal field, simply trap the value state.Add(GetFieldName(field), value) Note that these regular fields might actually be complex types in and of themselves. All that is known is that the field doesn t reference an editable business object because the value didn t implement IUndoableObject. It could be a simple value such as an Integer or a String, or it could be a complex object (as long as that object is marked as Serializable). Having gone through every field for every class in the object s inheritance hierarchy, the HybridDictionary contains a complete snapshot of all the data in the business object.
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This snapshot includes some fields put into the BusinessBase class to keep track of the object s status (such as whether it s new, dirty, deleted, and so on). The snapshot also includes the collection of broken rules that will be implemented later. An undo operation restores the object to its previous state in every way.
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Serializing and Stacking the HybridDictionary
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At this point, the object s field values are recorded but the snapshot is in a complex data type: a HybridDictionary. To further complicate matters, some of the elements contained in the HybridDictionary might be references to more complex objects. In that case, the HybridDictionary just has a reference to the existing object, not a copy or a snapshot at all. Fortunately, there s an easy answer to both issues. The BinaryFormatter or NetDataContractSerializer can be used to convert the HybridDictionary to a byte stream, reducing it from a complex data type to a very simple one for storage. Better yet, the very process of serializing the HybridDictionary automatically serializes any objects to which it has references. This does require that all objects referenced by any business object must be marked as Serializable so that they can be included in the byte stream. If referenced objects aren t serializable, the serialization attempt results in a runtime error. Alternatively, any nonserializable object references can be marked as NotUndoable so that the undo process simply ignores them. The code to do the serialization is fairly straightforward:
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