MSDN Magazine - December 2008 - (Page 22)

AdvAnced BAsics Ken GeTz The ObservableCollection Class Imagine that you’re creating a Windows Forms application, and you have bound a DataGridView control to a standard List(Of Customer) data structure. You’d like to be able to keep the items in the grid synchronized with the values in the underlying data source. That is, if some other code or some other form changes a customer’s data in the List, you’d like the grid to update and display the modified data. Using Windows Forms, this feat is possible, in a very general sense. You can make the update work, but it will have severe limitations. For example, you can, under the right circumstances, see the updates immediately in the grid, but there’s no easy way to add a new row to the grid when someone adds a new row to the data source. Windows Presentation Foundation (WPF) adds functionality to the Microsoft .NET Framework so that you actually can reliably keep bound controls synchronized with their data sources. Here I’ll demonstrate how to use the ObservableCollection class provided by WPF. Although the ObservableCollection class makes it possible for WPF applications to keep bound controls in sync with underlying data sources, it provides even more useful information—specifically, the ObservableCollection class also raises the CollectionChanged event when you add, remove, move, refresh, or replace an item in the collection. This feature makes it possible to react when code outside your window modifies the underlying data. You’ll see how you might use this information in this month’s sample application, which I’ll describe next. Figure 1 CustomerList System.Collections.ObjectModel Imports System.ComponentModel Public Class CustomerList Inherits ObservableCollection(Of Customer) Private Shared list As New CustomerList Public Shared Function GetList() As CustomerList Return list End Function Private Sub New() ' Make the constructor private, enforcing the "factory" concept ' the only way to create an instance of this class is by calling ' the GetList method. AddItems() End Sub Public Shared Sub Reset() list.ClearItems() list.AddItems() End Sub Private Sub AddItems() Add(New Customer("Maria Anders")) Add(New Customer("Ana Trujillo")) Add(New Customer("Antonio Moreno")) End Sub End Class Introducing the ObservableCollection Class The System.Collections.ObjectModel.ObservableCollection(Of T) class inherits from Collection(Of T), which is the base class for generic collections, and implements both the INotifyCollectionChanged and INotifyPropertyChanged interfaces. It’s the INotifyCollectionChanged interface that makes the collection interesting—this is the interface that allows bound objects (and your code) to determine if the collection has changed. It’s important to note that although the ObservableCollection class broadcasts information about changes to its elements, it doesn’t know or care about changes to the properties of its elements. In Send your questions and comments for Ken to basics@microsoft.com. Code download: code.msdn.microsoft.com/mag200812Basics. 22 msdn magazine other words, it doesn’t watch for property change notification on the items within its collection. If you need to know if someone has changed a property of one of the items within the collection, you’ll need to ensure that the items in the collection implement the INotifyPropertyChanged interface, and you’ll need to manually attach property changed event handlers for those objects. No matter how you change properties of objects within the collection, the collection’s PropertyChanged event will not fire. As a matter of fact, the ObservableCollection’s PropertyChanged event handler is protected—you can’t even react to it unless you inherit from the class and expose it yourself. You could, of course, handle the PropertyChanged event for each item within the collection from your inherited collection, although in the sample application I take a simpler approach, allowing the client application to handle individual items’ change events. If you disregard inherited and protected members (assuming that you are already familiar with the base Collection class, from which all of the members of the ObservableCollection class derive), the only interesting members left are the Move method (which allows http://code.msdn.microsoft.com/mag200812Basics

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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