MSDN Magazine - December 2008 - (Page 25)

that default view of the collection within the ListBox controls. You could create multiple CollectionView instances based on the same collection and display the data in different ways (sorted, for example) within one of the ListBox controls. Although it’s outside the scope of this discussion, it’s worth investigating the CollectionView class if you need to display multiple views of the same collection. For more information, please look at msdn.microsoft.com/library/system.winFigure 4 Capturing Change Events for Data in the Collection dows.data.collectionview. Follow these steps to give the sample application a workout. 1. Within Visual Studio 2008, load and run the sample application. You’ll see two instances of the same window. 2. Click to open the combobox on the left side of the window. Note that the control contains the numbers 0 through 2, corresponding to the three current customers. Choose 1, which selects Ana Trujillo and copies her name into the textbox. 3. In one window, click Delete. Ana Trujillo disappears from both windows, because the two ListBox controls are bound to the same ObservableCollection instance, and the binding causes updates to appear immediately. Open the combobox again, and note that it now indicates that there are only two customers. Try it in both windows to verify that both instances are up-to-date. 4. Click Delete two more times, removing all the customers. Click Reset Data to refill the lists in both windows. 5. In the textbox next to the Add New Item button, enter your own name and click Add New Item. The new name appears in both ListBox controls. Click to open the combobox and verify that it now contains numbers from 0 to 3 (one for each of the four customers). Verify that the combobox has changed on both windows—obviously, both windows’ classes received the event that indicated that the collection had changed. 6. In the ListBox in one window, select a name. In the lower textbox on the left, modify the name and click Change. First, you’ll see an alert indicating that you have changed the property, and once you dismiss it, you’ll see that the name has changed in both windows (see Figure 4). Except for the alert that appeared when you changed the customer name and the ComboBox control whose items change to match the number of customers in the collection, all the code in the sample window deals with the window’s user interface. That is, all the work done to keep the ListBox control in sync with the ObservableCollection instance happens for “free” and is managed by WPF. When you add a new item, the ListBox automatically displays the full list. When you change an item, the ListBox automatically displays the modified list. When you delete items, the ListBox maintains full fidelity with the underlying collection. In other words, in terms of binding the ObservableCollection class to controls in WPF, you can easily say “It just works.” Actually, there is some “magic” going on under the covers. When you bind the ListBox to the ObservableCollection, WPF actually creates a CollectionView instance, which presents a view of the data that handles grouping, sorting, filtering, and so on. You’re seeing msdnmagazine.com Examining the Code The MainWindow class defines the CustomerList instance using the WithEvents keyword so that the code can handle events of the ObservableCollection list without requiring the code to add handlers manually: Public WithEvents Data As CustomerList = CustomerList.GetList() In the Change Event Handlers region of the code, you’ll find the CollectionChanged event handler, which verifies that you have either added or removed an item from the collection. If so, the code sets the combobox’s data source and enables buttons on the window accordingly, as you see in Figure 5. The important part of this simple code lies in the NotifyCollecFigure 5 Check for Collection Changed Private Sub Data_CollectionChanged( _ ByVal sender As Object, _ ByVal e As NotifyCollectionChangedEventArgs) _ Handles Data.CollectionChanged ' ' ' ' Because the collection raises this event, you can modify your user interface on any window that displays controls bound to the data. On both windows, if you add or remove an item, all the controls update to indicate the new collection! ' Did you add or remove an item in the collection? If e.Action = NotifyCollectionChangedAction.Add Or _ e.Action = NotifyCollectionChangedAction.Remove Then ' Set the list of integers in the combo box: SetComboDataSource() ' Enable buttons as necessary: EnableButtons() End If End Sub Figure 6 NotifyCollectionChangedEventArgs Parameters Parameter Action Description Retrieves information about the action that caused the event. This property contains a NotifyCollectionChangedAction value, which can be either Add, Remove, Replace, Move, or Reset. Retrieves a list of the new items that were involved in the change to the collection. Retrieves the index of the collection at which the change occurred. Retrieves a list of the old items that were affected by the Replace, Remove, or Move action. Retrieves the index of the collection at which the Replace, Remove, or Move action occurred. December 2008 25 NewItems NewStartingIndex OldItems OldStartingIndex http://msdn.microsoft.com/library/system.windows.data.collectionview http://msdn.microsoft.com/library/system.windows.data.collectionview 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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