.net qr code generator N-LEV EL UNDO in VB.NET

Generate QR-Code in VB.NET N-LEV EL UNDO

CHAPTER 13 N-LEV EL UNDO
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' serialize the state and stack it Using buffer As New MemoryStream() Dim formatter = SerializationFormatterFactory.GetFormatter() formatter.Serialize(buffer, state) _stateStack.Push(buffer.ToArray()) End Using 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. 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 Overridable 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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CHAPTER 13 N-LEV EL UNDO
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It is possible for a business developer to accidentally trigger a call to UndoChanges() when there is no state to restore. If this condition isn t caught, it will cause a runtime error. To avoid such a scenario, the first thing the UndoChanges() method does is to get the edit level of the object by retrieving the Count property from the stack object. If the edit level is 0, there s no state to restore, and UndoChanges() just exits without doing any work. This edit level concept is even more important in the implementation of BusinessListBase, so you ll notice that the value is implemented as a property. Also notice that the edit level is checked to make sure it is in sync with this object s parent object (if any): If Me.EditLevel - 1 < parentEditLevel Then Throw New UndoException(String.Format( _ My.Resources.EditLevelMismatchException, "UndoChanges")) End If All three of the undo methods do this check, and these exceptions help the business developer debug her code when using n-level undo. The most common place where these exceptions occur is when using Windows Forms data binding because it is very easy for a UI developer to forget to properly unbind an object from the UI, leaving the object in a partially edited state. These exceptions help identify those situations so the developer can fix the UI code.
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