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CHAPTER 13 N-LEV EL UNDO
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if (value == null) { // variable has no value - store that fact state.Add(GetFieldName(field), null); } else { // this is a child object, cascade the call ((Core.IUndoableObject)value). CopyState(this.EditLevel + 1, BindingEdit); } } else { // this is a normal field, simply trap the value state.Add(GetFieldName(field), value); } } } } currentType = currentType.BaseType; } while (currentType != typeof(UndoableBase)); // serialize the state and stack it using (MemoryStream buffer = new MemoryStream()) { ISerializationFormatter formatter = SerializationFormatterFactory.GetFormatter(); formatter.Serialize(buffer, state); _stateStack.Push(buffer.ToArray()); } CopyStateComplete(); } [EditorBrowsable(EditorBrowsableState.Advanced)] protected virtual void CopyingState() { } [EditorBrowsable(EditorBrowsableState.Advanced)] protected virtual void CopyStateComplete() { } The CopyState() method is scoped as protected because BusinessBase subclasses UndoableBase, and the BeginEdit() method in BusinessBase will need to call CopyState(). To take a snapshot of data, there needs to be somewhere to store the various field values before they are pushed onto the stack. A HybridDictionary is ideal for this purpose, as it stores name-value pairs. It also provides high-speed access to values based on their names, which is important for the undo implementation. Finally, the HybridDictionary object supports .NET serialization, which means that it can be serialized and passed by value across the network as part of a business object. The CopyState() routine is essentially a big loop that starts with the outermost class in the object s inheritance hierarchy and walks back up through the chain of classes until it gets to UndoableBase. At that point, it can stop it knows that it has a snapshot of all the business data.
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CHAPTER 13 N-LEV EL UNDO
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At the start and end of the process, methods are called so a subclass can do pre- and post-processing. Notice that CopyingState() and CopyStateComplete() are virtual methods with no implementation. The idea is that a subclass can override these methods if additional actions should be taken before or after the object s state is copied. They provide an extensibility point for advanced business developers.
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It s inside the loop where the real work occurs. The first step is to get a list of all the fields corresponding to the current class: // get the list of fields in this type fields = currentType.GetFields( BindingFlags.NonPublic | BindingFlags.Instance | BindingFlags.Public); It doesn t matter whether the fields are public they all need to be recorded regardless of scope. What s more important is to only record instance fields, not those declared as static. The result of this call is an array of FieldInfo objects, each of which corresponds to a field in the business object.
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As discussed earlier, the FieldInfo array could include fields from the base classes of the current class. Due to the way the JIT compiler optimizes code within the same assembly, if some base classes are in the same assembly as the actual business class, the same field name may be listed in multiple classes. As the code walks up the inheritance hierarchy, it could end up processing those fields twice. To avoid this, the code only looks at the fields that directly belong to the class currently being processed: foreach(FieldInfo field in fields) { // make sure we process only our variables if(field.DeclaringType == currentType)
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At this point in the proceedings, it is established that the current FieldInfo object refers to a field within the object that s part of the current class in the inheritance hierarchy. However, a snapshot of the field should only be taken if it doesn t have the NotUndoable attribute: // see if this field is marked as not undoable if(!NotUndoableField(field)) Having reached this point, it is clear that the field value needs to be part of the snapshot, so there are two possibilities: this may be a regular field or it may be a reference to a child object that implements Csla.Core.IUndoableObject.
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