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CHAPTER 13 N-LEV EL UNDO
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// the field is undoable so see if it is a child object if (typeof(Csla.Core.IUndoableObject). IsAssignableFrom(field.FieldType)) { object value = field.GetValue(this); // make sure the variable has a value if (value != null) { // it is a child object so cascade the call ((Core.IUndoableObject)value).AcceptChanges( this.EditLevel, BindingEdit); } } Simple field values don t need any processing. Remember that the idea is that the current values have been accepted so there s no need to change those current values at all. You should now understand how the three undo methods in UndoableBase are able to take and restore a snapshot of an object s state, and how the calls are cascaded to child objects in a way that preserves encapsulation while supporting the needs of both data binding and manual invocation of the undo functionality.
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The UndoableBase class does the majority of the work to support the undo functionality. However, BusinessBase does include some methods that allow an editable object to participate in the undo process. In particular, it is BusinessBase that implements ISupportUndo, and so it is BusinessBase that implements methods such as BeginEdit(). These methods are public to enable the manual invocation of the undo functionality.
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I ll start by discussing the simplest of the three methods. Here s the BeginEdit() method: public void BeginEdit() { CopyState(this.EditLevel + 1); } When the business or UI developer explicitly calls BeginEdit() the object takes a snapshot of its state and cascades that call to its child objects. You ve already seen how this is done in the CopyState() method implemented in UndoableBase, and BeginEdit() relies on that preexisting behavior. Remember that the undo methods in UndoableBase throw an exception if the object s edit level gets out of sync with its parent object. The parent s edit level is passed in as a parameter to each undo method, such as CopyState(). When manually invoking CopyState(), it is necessary to pass in a parameter indicating the future state of the edit level. When taking a snapshot, the future edit level is one higher than the current edit level. In other words, this would cause an exception: CopyState(this.EditLevel); The reason is that CopyState() would see that it is about to raise the object s edit level above the value passed in as a parameter, so it would throw an exception. By passing in EditLevel + 1, the BeginEdit() method is effectively giving permission for the object to take a snapshot of its state.
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CHAPTER 13 N-LEV EL UNDO
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The CancelEdit() method is a little more complex. Actually, it isn t CancelEdit() that s more complex but the required post-processing implemented in an override of UndoChangesComplete() that is complex: public void CancelEdit() { UndoChanges(this.EditLevel - 1); } protected override void UndoChangesComplete() { BindingEdit = false; ValidationRules.SetTarget(this); InitializeBusinessRules(); OnUnknownPropertyChanged(); base.UndoChangesComplete(); } Like BeginEdit(), the CancelEdit() method lets UndoableBase do the hard work. But when the UndoChanges() method is complete, there is some housekeeping that must be done by BusinessBase, and that is handled by the UndoChangesComplete() override. When either CancelEdit() or ApplyEdit() is called, that call ends any data binding edit currently in effect, so the BindingEdit property is set to false. If data binding calls BeginEdit() through IEditableObject, BindingEdit will be set to true again, but any cancel or accept operation ends that edit process. An UndoChanges() operation effectively deserializes some data that is stored in the Stack object, as discussed earlier in the chapter. This means it is necessary to ensure that all the internal references between objects are correct. For example, the ValidationRules.SetTarget() method ensures that this object s ValidationRules object has the correct reference as the target for all business rules. It is also the case that an undo operation probably changed one or more property values. Most likely, the user has changed some property values and the undo reset them to some previous values. The call to OnUnknownPropertyChanged() raises a PropertyChanged event so data binding knows that the UI needs to be refreshed to reflect the changes.
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