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I ll start by discussing the simplest of the three methods. Here s the BeginEdit() method: Public Sub BeginEdit() CopyState(Me.EditLevel + 1) End Sub 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.
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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(Me.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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CancelEdit Method
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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 Sub CancelEdit() UndoChanges(Me.EditLevel - 1) End Sub Protected Overrides Sub UndoChangesComplete() BindingEdit = False ValidationRules.SetTarget(Me) InitializeBusinessRules() OnUnknownPropertyChanged() MyBase.UndoChangesComplete() End Sub 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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ApplyEdit Method
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The ApplyEdit() method is also a little complex. It also does some processing beyond that provided by UndoableBase:
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Public Sub ApplyEdit() _neverCommitted = False AcceptChanges(Me.EditLevel - 1) BindingEdit = False End Sub Protected Overrides Sub AcceptChangesComplete() If Parent IsNot Nothing Then Parent.ApplyEditChild(Me) End If MyBase.AcceptChangesComplete() End Sub Again, 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. There s also a _neverCommitted flag that is used to track whether the ApplyEdit() method on this object has ever been called. This field is used in the IEditableObject implementation to support the automatic removal of child objects from a list but is used only for Windows Forms 1.0 style data binding (so is nearly obsolete at this point in time). The most interesting bit of code here is in AcceptChangesComplete(), where a Parent. ApplyEditChild() method is called. This occurs if this object is a child of some other object and the ApplyEditChild() method is used to tell the parent object that its child s ApplyEdit() is invoked. The EditableRootListBase class uses this to determine that the user has completed editing a particular child object in the list, so it knows to trigger an immediate save of that child object s data. I discuss EditableRootListBase in more detail in 16. At this point you ve seen the code for the Public methods that support the undo functionality. There are also three similar methods that support data binding through the IEditableObject interface.
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