s BUSINESS FRAMEWORK IMPLEMENTATION in VB.NET

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CHAPTER 3 s BUSINESS FRAMEWORK IMPLEMENTATION
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IEditableObject is most important when an object is being edited within a grid control. In that case, this interface is the only way to get the editing behavior that s expected by users. Clearly, implementing the interface requires understanding of how it is used. The interface defines three methods, as described in Table 3-6. Table 3-6. IEditableObject Interface Methods
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This is called by data binding to indicate the start of an edit process. However, it may be called by the Windows Forms data binding infrastructure many times during the same edit process, and only the first call should be honored. This is called by data binding to indicate that any changes since the first BeginEdit() call should be undone. However, it may be called by the Windows Forms data binding infrastructure many times during the same edit process, and only the first call should be honored. This is called by data binding to indicate that the edit process is complete, and that any changes should be kept intact. However, it may be called by the Windows Forms data binding infrastructure many times during the same edit process, and only the first call should be honored.
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CancelEdit()
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The official Microsoft documentation on these methods is somewhat inconsistent with their actual behavior. In the documentation, only BeginEdit() is noted for being called multiple times, but experience has shown that any of these methods may be called multiple times.
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While these methods are certainly similar to the edit methods implemented earlier, there are some key differences in the way these new methods work. Consider BeginEdit(), for example. Every call to the existing BeginEdit() method will result in a new snapshot of the object s state, while only the first call to IEditableObject.BeginEdit() should be honored. Any subsequent calls (and they do happen during data binding) should be ignored. The same is true for the other two methods. Remember, data binding only uses a single level of undo. By definition, this means that only the first call to BeginEdit() through the IEditableObject interface has any meaning. To implement the behavior of the IEditableObject methods properly, the object needs to keep track of whether the edit process has been started and when it ends. At the same time, though, it is important to preserve the existing BeginEdit() functionality. This means implementing separate methods for IEditableObject, which will call the preexisting n-level undo methods when appropriate. There is one other complication to deal with as well. When a collection of objects is bound to a Windows Forms grid control, the user can dynamically add and remove child objects in the collection by using the grid control. When an object is removed in this manner, the grid control does not notify the collection object. Instead, it notifies the child object, and it s up to the child object to remove itself from the collection. It is then up to the child to interact with its parent collection to be removed from the collection itself. For this to happen, the child object needs a reference to its parent collection. This is expressed through a Protected property named Parent, which is discussed later in the chapter, in the Root, Parent, and Child Behaviors section. A flag is used to ignore multiple calls to the IEditableObject methods:
CHAPTER 3 s BUSINESS FRAMEWORK IMPLEMENTATION
<NotUndoable()> _ Private mBindingEdit As Boolean Private mNeverCommitted As Boolean = True Notice that mBindingEdit is declared with the <NotUndoable()> attribute. This field controls interaction with the UI, not internal object state; and because of this, there s no reason to make it part of the object s snapshot data, as that would just waste memory. A second flag is also declared, and is used to track whether ApplyEdit() has been called on the object. This value was set to False in the ApplyEdit() implemented earlier, and will be used to control whether a child object should remove itself from its parent collection. The three interface methods are implemented as follows: Private Sub IEditableObject_BeginEdit() _ Implements System.ComponentModel.IEditableObject.BeginEdit If Not mBindingEdit Then BeginEdit() End If End Sub Private Sub IEditableObject_CancelEdit() _ Implements System.ComponentModel.IEditableObject.CancelEdit If mBindingEdit Then CancelEdit() If IsNew AndAlso mNeverCommitted AndAlso _ EditLevel <= EditLevelAdded Then If Not Parent Is Nothing Then Parent.RemoveChild(Me) End If End If End If End Sub Private Sub IEditableObject_EndEdit() _ Implements System.ComponentModel.IEditableObject.EndEdit If mBindingEdit Then ApplyEdit() End If End Sub Notice that the methods are declared using syntax to explicitly implement the IEditableObject interface. This is required because BeginEdit() and CancelEdit() are already public methods in the class, and this avoids any naming conflict. All three methods call the corresponding edit methods implemented earlier. The mBindingEdit field is used to determine whether the BeginEdit() method has been called already so any subsequent method calls can be ignored. The mBindingEdit field is set to True when an edit process is started, and to False when either CancelEdit() or ApplyEdit() is called. The mNeverCommitted field tracks whether the ApplyEdit() method has ever been called. If it hasn t ever been called, and data binding attempts to cancel the edit operation, this flag is used to control whether the object should remove itself from its parent collection. The mNeverCommitted field starts out True and is set to False if ApplyEdit() is called. With this mechanism in place, the implementation of IEditableObject.BeginEdit() calls only the real BeginEdit() method if no edit session is currently underway. With the implementation of the n-level undo methods and System.ComponentModel.IEditableObject, business objects now provide full control over editing and undo capabilities, both to the UI developer and to Windows Forms data binding.
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