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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, etc.). 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: // serialize the state and stack it using (MemoryStream buffer = new MemoryStream()) { ISerializationFormatter formatter = SerializationFormatterFactory.GetFormatter(); formatter.Serialize(buffer, state); _stateStack.Push(buffer.ToArray()); } The SerializationFormatterFactory uses the CslaSerializationFormatter config setting to determine whether to use the BinaryFormatter (the default) or the NetDataContractSerializer. This is set in the app.config or web.config file in the appSettings element: <add key="CslaSerializationFormatter" value="NetDataContractSerializer" /> Either formatter works with Serializable objects, but only NetDataContractSerializer works with DataContract objects.
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CHAPTER 13 N-LEV EL UNDO
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Regardless of which formatter is used, the formatter object serializes the HybridDictionary (and any objects to which it refers) into a stream of bytes in an in-memory buffer. The byte stream is simply extracted from the in-memory buffer and pushed onto the stack: _stateStack.Push(buffer.ToArray()); Converting a MemoryStream to a byte array is not an issue because the MemoryStream is implemented to store its data in a byte array. The ToArray() method simply returns a reference to that existing array, so no data is copied. The act of conversion to a byte array is important, however, because a byte array is serializable, while a MemoryStream object is not. If the business object is passed across the network by value while it is being edited, the stack of states needs to be serializable.
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Tip Passing objects across the network while they re being edited is not anticipated, but since business objects are Serializable, you can t prevent the business developer from doing just that. If the stack were to reference a MemoryStream, the business application would get a runtime error as the serialization fails, and that s not acceptable. Converting the data to a byte array avoids accidentally crashing the application on the off chance that the business developer does decide to pass an object across the network as it s being edited.
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At this point, we re a third of the way through implementing n-level undo support. It is now possible to create a stack of snapshots of an object s data. It is time to move on and discuss the undo and accept operations.
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UndoChanges
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The UndoChanges() method is the reverse of CopyState(). It takes a snapshot of data off the stack, deserializes it back into a HybridDictionary, and then takes each value from the HybridDictionary and restores it into the appropriate object field. Like CopyState(), there are virtual methods called before and after the process to allow subclasses to take additional actions. The hard issues of walking through the types in the object s inheritance hierarchy and finding all the fields in the object are solved in the implementation of CopyState(). The structure of UndoChanges() is virtually identical, except that it restores field values rather than takes a snapshot of them. Since the overall structure of UndoChanges() is essentially the reverse of CopyState(), I won t show the entire code here. Rather, I ll focus on the key functionality.
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