当我更新 DataGrid 控件中的数据时，实现 INotifyPropertyChanged 的​​ ViewModel 的属性是否应该自动更新它们？

Question

我可能对 INotifyPropertyChanged 和 Silverlight 背后的目的感到困惑...

我有在设计器中创建的 XAML：

<UserControl.Resources>
    <CollectionViewSource x:Key="theViewSource" d:DesignSource="{d:DesignInstance my:transferObject, CreateList=True}" />
</UserControl.Resources>

以及这个

            <Canvas DataContext="{StaticResource theViewSource}">
                <sdk:DataGrid ItemsSource="{Binding}" Name="theDataGrid">
                    <sdk:DataGrid.Columns>
                     <!-- columns omitted from post -->
                    </sdk:DataGrid.Columns>
                </sdk:DataGrid>
            </Canvas>

我还有一些代码隐藏：

Private Sub control_Loaded(ByVal sender As System.Object, ByVal e As System.Windows.RoutedEventArgs) Handles MyBase.Loaded
    If Not (System.ComponentModel.DesignerProperties.GetIsInDesignMode(Me)) Then
        Dim myCollectionViewSource As System.Windows.Data.CollectionViewSource =
            CType(Me.Resources("theViewSource"), System.Windows.Data.CollectionViewSource)

        Dim myViewModel As New ViewModels.ViewModelThatHasAnObservableCollectionProperty()

        myCollectionViewSource.Source = myViewModel.TheObservableCollection()
    End If
End Sub

和一个看起来有点像这样的 ViewModel：

Public Class ViewModelBase
    Implements ComponentModel.INotifyPropertyChanged

    Protected Sub RaisePropertyChanged(ByVal propertyName As String)
        RaiseEvent PropertyChanged(Me, New PropertyChangedEventArgs(propertyName))
    End Sub

    Public Event PropertyChanged(ByVal sender As Object, ByVal e As System.ComponentModel.PropertyChangedEventArgs) Implements System.ComponentModel.INotifyPropertyChanged.PropertyChanged
End Class

Public Class ViewModelThatHasAnObservableCollectionProperty
    Inherits ViewModelBase

    Public Sub RetrieveCollectionCompletedHandler(ByVal sender As Object, ByVal e As SilverlightConsumableService.RetrieveMyCollectionCompletedEventArgs)
        Dim unobservable As List(Of TransferObjects.TheTransferObject) = e.Result

        'I was hoping for this next line of code to force a refresh of the DataGrid
        TheObservableCollection = New ObservableCollection(Of TransferObjects.transferObject)(unobservable)
    End Sub

    Public Sub New()
        Dim theClient As New SilverlightConsumableService.SilverlightConsumableSoapClient

        AddHandler theClient.RetrieveMyCollectionCompleted, AddressOf RetrieveCollectionCompletedHandler

        theClient.RetrieveMyCollectionAsync()
    End Sub

    Private _theObservableCollection As ObservableCollection(Of TransferObjects.transferObject)
    Public Property TheObservableCollection() As ObservableCollection(Of TransferObjects.transferObject)
        Get
            Return _theObservableCollection
        End Get
        Set(ByVal value As ObservableCollection(Of TransferObjects.transferObject))
            _theObservableCollection = value
            RaisePropertyChanged("TheObservableCollection")
        End Set
    End Property
End Class

结果是 Silverlight 呈现数据网格没有结果，因为 TheObservableCollection 尚未由事件处理程序填充。这是可以预料的。我所希望的是，在异步调用 Web 服务之后，TheObservableCollection 的 Setter 被调用，随后 RaisePropertyChanged 也被调用。

我的期望是 Silverlight 会通过将 dataGrid 重新绑定到属性来处理这个问题...使用 INotifyPropertyChanged 的​​目的不是为了让我不必以编程方式重新绑定网格，还是我对此感到困惑？

Answer 1

在 Silverlight 中（与 WPF 不同），您无需在服务响应处理程序中分配 ObservableCollection。这将打破绑定。

相反，请清除已绑定的集合并使用服务响应中的项目重新填充它。更新集合将正确引发通知事件。如果重新分配集合的引用，与 UI 元素的绑定将被破坏。

INotifyPropertyChanged 背后的目的是通知所有绑定成员属性的值已更改。对于集合，重新分配引用不算作值更改。更新集合成员的数量即可。同样，只要您调用 PropertyChanged 事件，更改单个属性就会引发通知事件。该接口的目的是为 View 提供一个监听 ViewModel 的契约。