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IEditableObject Interface
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The IEditableObject interface is used by data binding to interact with an object. As I discuss earlier in the chapter, data binding expects a single level of undo and that each object is independent of other objects (so no cascading of undo calls from parent to children). The BeginEdit() method must detect whether the developer has disabled IEditableObject support and must also only honor the first call to BeginEdit() by data binding. It is important to realize that data binding may make many calls to BeginEdit() and that only the first one should have any effect: Private Sub IEditableObject_BeginEdit() Implements _ System.ComponentModel.IEditableObject.BeginEdit If Not _disableIEditableObject AndAlso Not BindingEdit Then BindingEdit = True BeginEdit() End If End Sub The BindingEdit property is set to True to indicate that the object is now bound to a UI. As you ve already seen, this property is used to alter the behavior of other undo features and most importantly prevents cascading of method calls to child objects. The CancelEdit() method is more complex because it supports the automatic removal of certain new objects when they are a child of a list:
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Private Sub IEditableObject_CancelEdit() Implements _ System.ComponentModel.IEditableObject.CancelEdit If Not _disableIEditableObject AndAlso BindingEdit Then CancelEdit() If IsNew AndAlso _neverCommitted AndAlso EditLevel <= EditLevelAdded Then ' we're new and no EndEdit or ApplyEdit has ever been ' called on us, and now we've been cancelled back to ' where we were added so we should have ourselves ' removed from the parent collection If Parent IsNot Nothing Then Parent.RemoveChild(Me) End If End If End If End Sub This call to Parent.RemoveChild() is useful only for Windows Forms 1.0 data binding when using the Windows Forms 1.0 grid control. When using modern data binding and grid controls, this code is unused because BindingList(Of T) handles removing child objects by itself. The simplest of the three methods is ApplyEdit(): Private Sub IEditableObject_EndEdit() Implements _ System.ComponentModel.IEditableObject.EndEdit If Not _disableIEditableObject AndAlso BindingEdit Then ApplyEdit() End If End Sub Assuming IEditableObject isn t disabled and that the object is currently data bound, this method simply delegates to the ApplyEdit() method I discussed earlier. As you can see, the BusinessBase class exposes the undo functionality in three ways: Through public methods for manual invocation Through the ISupportUndo interface for manual invocation Through the IEditableObject interface for data binding It also interacts with its UndoableBase base class to do most of the work, including interacting with child objects, such as editable collections. This means BusinessListBase must also participate in the undo functionality.
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The BusinessListBase class combines some of what UndoableBase does and some of what you ve seen in BusinessBase. Like UndoableBase, the BusinessListBase class implements IUndoableObject, and like BusinessBase, it implements ISupportUndo and the three public undo methods. The implementation of this functionality in a collection isn t as complex as it is for editable objects, however, because a collection is primarily responsible for cascading the method calls to all the child objects it contains.
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The hardest part of implementing n-level undo functionality is that not only can child objects be added or deleted but they can be undeleted or unadded in the case of an undo operation. Csla. Core.BusinessBase and UndoableBase use the concept of an edit level. The edit level allows the object to keep track of how many BeginEdit() calls have been made to take a snapshot of its state without corresponding CancelEdit() or ApplyEdit() calls. More specifically, it tells the object how many states have been stacked up for undo operations. BusinessListBase needs the same edit-level tracking as in BusinessBase. However, a collection won t actually stack its states. Rather, it cascades the call to each of its child objects so that they can stack their own states. Because of this, the edit level can be tracked using a simple numeric counter. It merely counts how many unpaired BeginEdit() calls have been made: ' keep track of how many edit levels we have Private _editLevel As Integer The implementations of CopyState(), UndoChanges(), and AcceptChanges() alter this value accordingly.
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