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CHAPTER 13 N-LEV EL UNDO
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CopyState
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The CopyState() method takes a snapshot of the object s current data and stores it in a Stack object.
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Since UndoableBase is an implementation of n-level undo capability, each object could end up storing a number of snapshots. As each undo or accept operation occurs, it gets rid of the most recently stored snapshot; this is the classic behavior of a stack data structure. Fortunately, the .NET Framework includes a prebuilt Stack(Of T) class that implements the required functionality. It is declared as follows: <NotUndoable()> _ Private _stateStack As New Stack(Of Byte())() This field is marked as NotUndoable to prevent taking a snapshot of previous snapshots. CopyState() should just record the fields that contain actual business data. Once a snapshot has been taken of the object s data, the snapshot is serialized into a single byte stream. That byte stream is then put on the stack. From there, it can be retrieved and deserialized to perform an undo operation if needed.
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The process of taking a snapshot of each field value in an object is a bit tricky. Reflection is used to walk through all the fields in the object. During this process, each field is checked to determine whether it has the NotUndoable attribute. If so, the field is ignored. The big issue is that field names may not be unique within an object. To see what I mean, consider the following two classes: Namespace Test Public Class BaseClass Private _id As Integer End Class Public Class SubClass Inherits BaseClass Private _id As Integer End Class End Namespace Here, each class has its own field named _id, and in most circumstances it s not a problem. However, when using reflection to walk through all the fields in a SubClass object, it will return two _id fields: one for each of the classes in the inheritance hierarchy. To get an accurate snapshot of an object s data, CopyState() needs to accommodate this scenario. In practice, this means prefixing each field name with the full name of the class to which it belongs. Instead of two _id fields, the result is Test.BaseClass!_id and Test.SubClass!_id. The use of an exclamation point for a separator is arbitrary, but some character is necessary to separate the class name from the field name. As if this weren t complex enough, reflection works differently with classes that are subclassed from other classes in the same assembly than with classes that are subclassed from classes in a different assembly. If in the previous example, BaseClass and SubClass are in the same assembly, one technique can be used; but if they re in different assemblies, a different technique is necessary. Of course, CopyState() should deal with both scenarios so the business developer doesn t have to worry about these details.
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CHAPTER 13 N-LEV EL UNDO
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Note Not all the code for UndoableBase is listed in this book. I cover only the key parts of the algorithm. For the rest of the code, refer to the download at www.apress.com/book/view/1430210192 or www.lhotka.net/ cslanet/download.aspx.
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The following method deals with all of the preceding issues. I walk through how it works after the listing: <EditorBrowsable(EditorBrowsableState.Never)> _ Protected Sub CopyState() CopyingState() Dim currentType As Type = Me.GetType() Dim state As New HybridDictionary() Dim fields As FieldInfo() If Me.EditLevel + 1 > parentEditLevel Then Throw New UndoException(String.Format( _ My.Resources.EditLevelMismatchException, "CopyState")) End If Do ' get the list of fields in this type fields = currentType.GetFields(_ BindingFlags.NonPublic Or _ BindingFlags.Instance Or _ BindingFlags.Public) For Each field As FieldInfo In fields ' make sure we process only our variables If field.DeclaringType = currentType Then ' see if this field is marked as not undoable If Not NotUndoableField(field) Then ' the field is undoable, so it needs to be processed. Dim value As Object = field.GetValue(Me) If GetType(Csla.Core.IUndoableObject).IsAssignableFrom( _ field.FieldType) Then ' make sure the variable has a value If value Is Nothing Then ' variable has no value - store that fact state.Add(GetFieldName(field), Nothing) Else ' this is a child object, cascade the call DirectCast(value, Core.IUndoableObject).CopyState( _ Me.EditLevel + 1, BindingEdit) End If Else ' this is a normal field, simply trap the value state.Add(GetFieldName(field), value) End If End If End If Next currentType = currentType.BaseType Loop While currentType <> GetType(UndoableBase) ' serialize the state and stack it Using buffer As New MemoryStream() Dim formatter = SerializationFormatterFactory.GetFormatter()
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