s BUSINESS FRAMEWORK IMPLEMENTATION in VB.NET

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CHAPTER 3 s BUSINESS FRAMEWORK IMPLEMENTATION
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Else mNonSerializableHandlers = DirectCast( _ System.Delegate.Remove( _ mNonSerializableHandlers, value), PropertyChangedEventHandler) End If End RemoveHandler RaiseEvent(ByVal sender As Object, ByVal e As PropertyChangedEventArgs) Dim nonSerializableHandlers As PropertyChangedEventHandler = _ mNonSerializableHandlers If nonSerializableHandlers IsNot Nothing Then nonSerializableHandlers.Invoke(sender, e) End If Dim serializableHandlers As PropertyChangedEventHandler = _ mSerializableHandlers If serializableHandlers IsNot Nothing Then serializableHandlers.Invoke(sender, e) End If End RaiseEvent End Event <EditorBrowsable(EditorBrowsableState.Advanced)> _ Protected Overridable Sub OnIsDirtyChanged() OnUnknownPropertyChanged() End Sub <EditorBrowsable(EditorBrowsableState.Advanced)> _ Protected Overridable Sub OnUnknownPropertyChanged() Dim properties() As PropertyInfo = _ Me.GetType.GetProperties(BindingFlags.Public Or BindingFlags.Instance) For Each item As PropertyInfo In properties RaiseEvent PropertyChanged( _ Me, New PropertyChangedEventArgs(item.Name)) Next End Sub <EditorBrowsable(EditorBrowsableState.Advanced)> _ Protected Overridable Sub OnPropertyChanged(ByVal propertyName As String) RaiseEvent PropertyChanged( _ Me, New PropertyChangedEventArgs(propertyName)) End Sub End Class End Namespace It s important that this class is marked as <Serializable()>. Ultimately, all business objects will be serializable, and that means that any classes they inherit from must also be marked as such. Also, the class is declared as MustInherit. This means that an instance of this class can t be created directly. Before declaring the event itself, the code declares two delegate fields. These fields will hold delegate references to all event handlers registered to receive the PropertyChanged event:
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<NonSerialized()> _ Private mNonSerializableHandlers As PropertyChangedEventHandler Private mSerializableHandlers As PropertyChangedEventHandler Notice that one is declared with the <NonSerialized()> attribute, while the other is not. The BinaryFormatter will ignore the first one and all objects referenced by that delegate field. Objects referenced by the second field will be serialized as normal. The event declaration uses a block structure, including AddHandler, RemoveHandler, and RaiseEvent sections. Notice how the code in the AddHandler and RemoveHandler sections checks to see if the event handler is contained within a serializable object: If value.Method.IsPublic AndAlso _ (value.Method.DeclaringType.IsSerializable OrElse _ value.Method.IsStatic) Then If the event handler is contained in a serializable object, it is added or removed from the serializable delegate; otherwise it is added or removed from the non-serialized delegate. The thing about events and inheritance is that an event can only be raised by code in the class in which it is declared. This is because the event member can only be accessed directly from the class in which it is defined. It can t be raised by code in classes that inherit from this class. This means that business objects can t raise the PropertyChanged event directly, even though that is the goal. To solve this problem, the code follows a standard .NET design pattern by creating a Protected method that in turn raises the event: <EditorBrowsable(EditorBrowsableState.Advanced)> _ Protected Overridable Sub OnPropertyChanged(ByVal propertyName As String) RaiseEvent PropertyChanged( _ Me, New PropertyChangedEventArgs(propertyName)) End Sub Any classes that inherit from the base class can call this method when they want to raise the event. This method is marked with the <EditorBrowsable()> attribute, indicating that this is an advanced method. In VB, this means that the method appears in the All tab in IntelliSense, and won t appear in the Common tab. In C#, this means that the method won t appear in IntelliSense unless the IDE is set to show advanced members. The OnUnknownPropertyChanged() method covers a special case, different from the OnPropertyChanged() method. Where OnPropertyChanged() raises the PropertyChanged event for a single property, OnUnknownPropertyChanged() raises the event for all properties of the object: <EditorBrowsable(EditorBrowsableState.Advanced)> _ Protected Overridable Sub OnUnknownPropertyChanged() Dim properties() As PropertyInfo = _ Me.GetType.GetProperties(BindingFlags.Public Or BindingFlags.Instance) For Each item As PropertyInfo In properties RaiseEvent PropertyChanged( _ Me, New PropertyChangedEventArgs(item.Name)) Next End Sub There are a number of cases in which the object s state will change in such a way that it isn t possible to know which properties actually changed. In that case, this blanket notification approach ensures that data binding is aware that something changed, so the UI updates as needed.
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