MSDN Magazine - December 2008 - (Page 23)

you to move a member to a new location within the collection) and the CollectionChanged event (which broadcasts information about changes to the contents of the collection). Before reading further, you might want to download and install the sample WPF application that demonstrates these features. Examining the Sample The sample solution, ObservableCollectionTest, contains a class named CustomerList, which inherits from ObservableCollection (see Figure 1). As you might imagine, the CustomerList class exposes an ObservableCollection instance containing Customer objects. If you examine the code, however, you’ll see that the class exposes only one list, so that multiple consumers of the class each retrieve a reference to the same collection. (That’s the point of this particular demonstration, and isn’t necessary for other applications.) The class provides a private constructor, so the only way to retrieve an instance of the class is to call the shared GetList method, which hands out the existing collection instance: Private Shared list As New CustomerList Public Shared Function GetList() As CustomerList Return list End Function The ObservableCollection class broadcasts information about changes to its elements. setter calls the OnPropertyChanged method if the new value is different from the current value of the property: Private _name As String Public Property Name() As String Get Return _name End Get Set(ByVal value As String) If _name value Then _name = value OnPropertyChanged("Name") End If End Set End Property The private constructor calls the AddItems method; the public shared Reset method clears the list and then calls the AddItems method. Either way, the result is three customers in the collection: Private Sub AddItems() Add(New Customer("Maria Anders")) Add(New Customer("Ana Trujillo")) Add(New Customer("Antonio Moreno")) End Sub Given the PropertyChanged event that the class raises, it’s possible for code that uses the class, or a collection of instances of the class, to react to the PropertyChanged event and take action based on the fact that the property changed. (Please note that the PropertyChangedEventArgs class adds only a PropertyName property to the standard event arguments and doesn’t provide any information on either the old or the new value of the property. The sample application works around this limitation, as you’ll see, to at least determine the new value of the changed property.) The sample also contains a single WPF window named MainWindow, as shown in Figure 3. The only important detail in the markup for the window lies in the definition for the ListBox control, which includes declarative data binding for the ItemsSource propFigure 2 The Customer Class with the PropertyChanged Event Imports System.ComponentModel Public Class Customer Implements INotifyPropertyChanged Public Event PropertyChanged( _ ByVal sender As Object, _ ByVal e As PropertyChangedEventArgs) _ Implements INotifyPropertyChanged.PropertyChanged Protected Overridable Sub OnPropertyChanged( _ ByVal PropertyName As String) ' Raise the event, and make this procedure ' overridable, should someone want to inherit from ' this class and override this behavior: RaiseEvent PropertyChanged( _ Me, New PropertyChangedEventArgs(PropertyName)) End Sub Public Sub New(ByVal Name As String) ' Set the backing field so that you don't raise the ' PropertyChanged event when you first create the Customer. _name = Name End Sub Private _name As String Public Property Name() As String Get Return _name End Get Set(ByVal value As String) If _name value Then _name = value OnPropertyChanged("Name") End If End Set End Property End Class The Customer class, for this example, is extremely simple (that is, just complex enough to demonstrate the necessary features). The class, shown in Figure 2, contains only a Name property and wouldn’t even be worth discussing if it weren’t for the fact that the class implements the INotifyPropertyChanged interface, which enables consumers of instances of the class (including data-bound controls) to be notified when property values change. When a class implements the INotifyPropertyChanged interface, it must supply the PropertyChanged event: Public Event PropertyChanged( _ ByVal sender As Object, _ ByVal e As PropertyChangedEventArgs) _ Implements INotifyPropertyChanged.PropertyChanged In order to raise the event using the standard .NET design pattern, the Customer class includes a protected, overridable OnPropertyChanged procedure, which raises the event: Protected Overridable Sub OnPropertyChanged( _ ByVal PropertyName As String) ' Raise the event, and make this procedure ' overridable, should someone want to inherit from ' this class and override this behavior: RaiseEvent PropertyChanged( _ Me, New PropertyChangedEventArgs(PropertyName)) End Sub Then, within the definition for the Name property, the property msdnmagazine.com December 2008 23 http://www.msdnmagazine.com

Table of Contents Feed for the Digital Edition of MSDN Magazine - December 2008

MSDN Magazine - December 2008 Contents Toolbox CLR Inside Out Advanced Basics Cutting Edge Patterns In Practice Team System Real-World WF Visual Studio OBA Tools SOA Data Access Geneva Framework Test Run Foundations Windows With C++ Going Places End Bracket

MSDN Magazine - December 2008

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