ObservableCollection FileSystemWatcher ListBox 更新问题

Question

我有一个 ObservableCollection，它使用 FileSystemWatcher 自动添加已添加到目录中的其他 PNG 图像。 ListBox 使用以下 XAML 将 ItemsSource 数据绑定到 Photos 对象。

<ListBox ItemsSource="{Binding Source={StaticResource Photos}}" IsSynchronizedWithCurrentItem="True"/>

但是，当 PNG 文件添加到受监视目录时，将调用 OnPhotoCreated 事件（断点确认这一点），但未更新 ListBox UI。有什么想法吗？

Public Class Photos
Inherits Collections.ObjectModel.ObservableCollection(Of BitmapImage)

' Events
Public Event ItemsUpdated As EventHandler

' Fields
Private FileSystemWatchers As Dictionary(Of String, FileSystemWatcher) = New Dictionary(Of String, FileSystemWatcher)

' Methods
Protected Overrides Sub ClearItems()
    MyBase.ClearItems()
    Me.FileSystemWatchers.Clear()
End Sub

Protected Overrides Sub InsertItem(ByVal index As Integer, ByVal item As BitmapImage)
    MyBase.InsertItem(index, item)
    Dim ImagePath As String = IO.Path.GetDirectoryName(item.UriSource.LocalPath)
    If Not Me.FileSystemWatchers.ContainsKey(ImagePath) Then
        Dim FileWatcher As New FileSystemWatcher(ImagePath, "*.png")
        FileWatcher.EnableRaisingEvents = True
        AddHandler FileWatcher.Created, New FileSystemEventHandler(AddressOf Me.OnPhotoCreated)
        AddHandler FileWatcher.Deleted, New FileSystemEventHandler(AddressOf Me.OnPhotoDeleted)
        AddHandler FileWatcher.Renamed, New RenamedEventHandler(AddressOf Me.OnPhotoRenamed)
        Me.FileSystemWatchers.Add(ImagePath, FileWatcher)
    End If
End Sub

Private Sub OnPhotoCreated(ByVal sender As Object, ByVal e As FileSystemEventArgs)
    MyBase.Items.Add(New BitmapImage(New Uri(e.FullPath)))
    RaiseEvent ItemsUpdated(Me, New EventArgs)
End Sub

Private Sub OnPhotoDeleted(ByVal sender As Object, ByVal e As FileSystemEventArgs)
    Dim index As Integer = -1
    Dim i As Integer
    For i = 0 To MyBase.Items.Count - 1
        If (MyBase.Items.Item(i).UriSource.AbsolutePath = e.FullPath) Then
            index = i
            Exit For
        End If
    Next i
    If (index >= 0) Then
        MyBase.Items.RemoveAt(index)
    End If
    RaiseEvent ItemsUpdated(Me, New EventArgs)
End Sub

Private Sub OnPhotoRenamed(ByVal sender As Object, ByVal e As RenamedEventArgs)
    Dim index As Integer = -1
    Dim i As Integer
    For i = 0 To MyBase.Items.Count - 1
        If (MyBase.Items.Item(i).UriSource.AbsolutePath = e.OldFullPath) Then
            index = i
            Exit For
        End If
    Next i
    If (index >= 0) Then
        MyBase.Items.Item(index) = New BitmapImage(New Uri(e.FullPath))
    End If
    RaiseEvent ItemsUpdated(Me, New EventArgs)
End Sub


End Class

更新#1：我尝试了如下所示的事件。这会导致 InvalidOperationException 崩溃，“调用线程无法访问此对象，因为另一个线程拥有它”当新图像试图滚动到视图中时。我希望不需要 Refresh 方法。

Dim Photos As Photos = CType(Me.FindResource("Photos"), Photos)
AddHandler Photos.ItemsUpdated, AddressOf Me.Photos_ItemsUpdated

Private Sub RefreshPhotos()
    '
    If Me.ImageListBox.Dispatcher.CheckAccess = True Then
        Me.ImageListBox.Items.Refresh()
    Else
        Dispatcher.BeginInvoke(DispatcherPriority.Normal, New DispatcherMethodCallback(AddressOf Me.RefreshPhotos))
    End If
    '
End Sub

Private Sub Photos_ItemsUpdated(ByVal sender As Object, ByVal e As EventArgs)
    '
    Debug.WriteLine("PhotosUpdated")
    Me.RefreshPhotos()
    '
End Sub

Answer 1

看看 ObservableCollection 类中的 InsertItem 代码（来自 Reflector）：

Protected Overrides Sub InsertItem(ByVal index As Integer, ByVal item As T)
    Me.CheckReentrancy
    MyBase.InsertItem(index, item)
    Me.OnPropertyChanged("Count")
    Me.OnPropertyChanged("Item[]")
    Me.OnCollectionChanged(NotifyCollectionChangedAction.Add, item, index)
End Sub

现在使用它作为指导 ObservableCollection 应该执行哪些通知。 OnCollectionChanged 方法是 wpf 通知系统的主要连接，如果它被适当地调用，那么这个 calss 工作正常，而其他地方就是问题所在。

另外，在你的 OnPhotoCreated 方法中，你调用了Items.Add 方法，它不做通知，它不属于 ObservableCollection 类，而是属于 Collection<T> 类ObservableCollection 继承。

至于 InvalidOperationException 错误，对我来说，这听起来像是从错误线程更新 UI 的情况。