绑定到ObservableCollection的ItemsControl在属性更改时未更新UI

Question

绑定到ObservableCollection的ItemsControl在属性更改时未更新UI

4

在经过长时间的头疼和加班后，我放弃了尝试自己解决这个问题。虽然有很多与此类似的问题的文献可以找到，但我还没有找到一个确切的解决方案来解决我的特定问题。

我遇到的问题是，在使用画布作为ItemsPanel的ItemsControl中，当其ItemsSource中的项的属性被修改后，无法更新UI。

我创建了一个非常干净的示例应用程序来演示发生了什么。

在我的示例应用程序中，有一个视图'MainWindow.xaml'，一个继承'ViewModelBase.vb'的视图模型'MainWindowViewModel.vb'，以及最后一个命令代理'DelegateCommand.vb'，用于创建RelayCommands以更新我的ItemsControl的ItemSource。

首先，是MainWindow.xaml：

<Window
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:local="clr-namespace:SampleApp"
    x:Class="MainWindow" Title="MainWindow" Height="347" Width="525" Background="Black">
    <Window.DataContext>
        <local:MainWindowViewModel/>
    </Window.DataContext>
    <Grid>
        <!-- LINE SEGMENTS -->
        <ItemsControl x:Name="ic1" ItemsSource="{Binding LineData, Mode=OneWay, NotifyOnTargetUpdated=True}" HorizontalAlignment="Left" Height="246" VerticalAlignment="Top" Width="517" Background="#FF191919" BorderBrush="#FF444444">
            <ItemsControl.ItemsPanel>
                <ItemsPanelTemplate>
                    <Canvas IsItemsHost="True"/>
                </ItemsPanelTemplate>
            </ItemsControl.ItemsPanel>
            <ItemsControl.ItemTemplate>
                <DataTemplate>
                    <Line X1="{Binding X1}" Y1="{Binding Y1}" X2="{Binding X2}" Y2="{Binding Y2}" Stroke="White" StrokeThickness="6"/>
                </DataTemplate>
            </ItemsControl.ItemTemplate>
        </ItemsControl>

        <Button Content="Refresh Canvas" HorizontalAlignment="Left" Margin="350,261,0,0" VerticalAlignment="Top" Width="124" Height="40" FontFamily="Verdana" FontWeight="Bold" Click="Button_Click"/>
        <Button Content="Command 1" Command="{Binding Command1}" HorizontalAlignment="Left" Margin="45,261,0,0" VerticalAlignment="Top" Width="124" Height="40" FontFamily="Verdana" FontWeight="Bold"/>
        <Button Content="Command 2" Command="{Binding Command2}" HorizontalAlignment="Left" Margin="198,261,0,0" VerticalAlignment="Top" Width="124" Height="40" FontWeight="Bold" FontFamily="Verdana"/>
    </Grid>
</Window>

如您所见，我的窗口的DataContext是MainWindowViewModel，而ItemSource的绑定是LineData（位于该VM中）。

此外，我有三个按钮。前两个按钮执行ICommands，而第三个按钮执行后台代码刷新ItemsControl（这是为了调试目的，以证明在UI不更新的情况下ItemSource中的绑定属性正在更新）。稍后会详细介绍。

第一个按钮绑定到VM中的Command1，而第二个按钮绑定到VM中的Command2。

接下来是MainWindowViewModel.vb:

Imports System.Collections.ObjectModel

Public Class MainWindowViewModel
    Inherits ViewModelBase

    ' Sample line data variable
    Private _LineData As ObservableCollection(Of LineStructure) = GetLineData()
    Public Property LineData As ObservableCollection(Of LineStructure)
        Get
            Return _LineData
        End Get
        Set(value As ObservableCollection(Of LineStructure))
            _LineData = value
            OnPropertyChanged("LineData")
        End Set
    End Property

    ' ICommands
    Private _Command1 As ICommand
    Public ReadOnly Property Command1 As ICommand
        Get
            If _Command1 Is Nothing Then
                _Command1 = New MVVM.RelayCommand(AddressOf ExecuteCommand1)
            End If
            Return _Command1
        End Get
    End Property

    Private _Command2 As ICommand
    Public ReadOnly Property Command2 As ICommand
        Get
            If _Command2 Is Nothing Then
                _Command2 = New MVVM.RelayCommand(AddressOf ExecuteCommand2)
            End If
            Return _Command2
        End Get
    End Property

    ' ICommand Methods
    Private Sub ExecuteCommand1()
        ' Re-arrange LineData(0) to make a plus sign on the canvas
        ' This works - Assigning a new value to an item of the collection updates the canvas
        LineData(0) = New LineStructure With {.X1 = "175", .Y1 = "50", .X2 = "175", .Y2 = "150"}
    End Sub

    Private Sub ExecuteCommand2()
        ' Put LineData(0) back into its original position
        ' This doesn't work - Modifying the PROPERTY of an item in the collection does not update the canvas.. even with INotifyPropertyChange being called
        LineData(0).X1 = "50"
        LineData(0).Y1 = "50"
        LineData(0).X2 = "300"
        LineData(0).Y2 = "50"

        OnPropertyChanged("LineData")
    End Sub

    ' Misc methods
    Private Function GetLineData() As ObservableCollection(Of LineStructure)
        Dim tmpList As New ObservableCollection(Of LineStructure)

        ' Create two horizontal parallel lines
        tmpList.Add(New LineStructure With {.X1 = "50", .Y1 = "50", .X2 = "300", .Y2 = "50"})
        tmpList.Add(New LineStructure With {.X1 = "50", .Y1 = "100", .X2 = "300", .Y2 = "100"})

        Return tmpList
    End Function
End Class

Public Class LineStructure
    Public Property X1
    Public Property Y1
    Public Property X2
    Public Property Y2
End Class

在我的ViewModel中，我立即定义了LineData（这就是我的ItemsSource绑定的内容），因此我们在执行时有一些准备好在画布中显示的数据。它由GetLineData()函数定义，该函数简单地返回填充的两条线的ObservableCollection。

应用程序首次启动时，会显示两条水平、平行的线。

LineData变量是我定义的LineStructure类的ObservableObject，该类仅包含X1、Y1、X2、Y2字符串，用于绑定和显示画布中的对象。

Command1（再次说明，这与第一个按钮绑定）将新的LineStructure分配给LineData的第一个索引。当执行此操作时，UI会按预期更新，一切都很顺利。这将使线段在画布上呈现出十字形。

问题从这里开始：

Command2不会像Command1那样将新的LineStructure分配给第一个LineData索引，而是会逐个重新定义第一个LineData索引内的属性。如果这能够起作用，它将重新排列第一条线，画布上的两条线将再次水平平行。

然而，这并没有更新画布/UI——我无法弄清楚原因。我已经阅读了许多文章，并尝试了许多不同的解决方案，但都没有成功。

如果有人能解释为什么修改属性而不是重新声明整个LineStructure索引时，绑定不会更新，请告诉我，我将不胜感激。

最后要注意的一件事是，我已经找到了一个解决方案，可以完成我需要做的事情，但我不认为我应该使用它。我认为绑定应该能够检测到任何属性更改。

对于任何感兴趣的人，请参见以下片段，以获取关于在属性更改时更新画布的临时解决方法。

我在xaml中的ItemsControl声明中添加了NotifyOnTargetUpdated=True和TargetUpdated="RefreshCanvas"。

这将调用一个名为RefreshCanvas()的方法，该方法从MainWindow的代码后台执行ic1.Items.Refresh()（您可以在本文末尾找到代码后台）。这将刷新ItemsControl项，因此画布被刷新并显示绑定集合的更新。

<ItemsControl x:Name="ic1" TargetUpdated="RefreshCanvas" ItemsSource="{Binding LineData, Mode=OneWay, UpdateSourceTrigger=PropertyChanged, NotifyOnTargetUpdated=True}" HorizontalAlignment="Left" Height="246" VerticalAlignment="Top" Width="517" Background="#FF191919" BorderBrush="#FF444444">

为了参考，我会包含其他文件，因为它可能相关:

ViewModelBase.vb:

Imports System.ComponentModel

Public MustInherit Class ViewModelBase
    Implements INotifyPropertyChanged, IDisposable

#Region "Constructor"
    Protected Sub New()
    End Sub
#End Region ' Constructor

#Region "DisplayName"

    ' Returns the user-friendly name of this object.
    ' Child classes can set this property to a new value, or override it to determine the value on-demand.

    Private privateDisplayName As String

    Public Overridable Property DisplayName() As String
        Get
            Return privateDisplayName
        End Get
        Protected Set(ByVal value As String)
            privateDisplayName = value
        End Set
    End Property
#End Region ' DisplayName

#Region "Debugging Aids"
    ' Warns the developer if this object does not have a public property with the specified name. 
    ' This method does not exist in a Release build.
    <Conditional("DEBUG"), DebuggerStepThrough()> _
    Public Sub VerifyPropertyName(ByVal propertyName As String)
        ' Verify that the property name matches a real, public, instance property on this object.
        If TypeDescriptor.GetProperties(Me)(propertyName) Is Nothing Then
            Dim msg As String = "Invalid property name: " & propertyName

            If Me.ThrowOnInvalidPropertyName Then
                Throw New Exception(msg)
            Else
                Debug.Fail(msg)
            End If
        End If
    End Sub

    ' Returns whether an exception is thrown, or if a Debug.Fail() is used when an invalid property name is passed to the VerifyPropertyName method.
    ' The default value is false, but subclasses used by unit tests might override this property's getter to return true.
    Private privateThrowOnInvalidPropertyName As Boolean
    Protected Overridable Property ThrowOnInvalidPropertyName() As Boolean
        Get
            Return privateThrowOnInvalidPropertyName
        End Get
        Set(ByVal value As Boolean)
            privateThrowOnInvalidPropertyName = value
        End Set
    End Property
#End Region ' Debugging Aides

#Region "INotifyPropertyChanged Members"
    ' Raised when a property on this object has a new value.
    Public Event PropertyChanged As PropertyChangedEventHandler Implements INotifyPropertyChanged.PropertyChanged

    ' Raises this object's PropertyChanged event.
    ' <param name="propertyName">The property that has a new value.</param>
    Protected Overridable Sub OnPropertyChanged(ByVal propertyName As String)
        Me.VerifyPropertyName(propertyName)

        Dim handler As PropertyChangedEventHandler = Me.PropertyChangedEvent
        If handler IsNot Nothing Then
            Dim e = New PropertyChangedEventArgs(propertyName)
            handler(Me, e)
        End If
    End Sub
#End Region ' INotifyPropertyChanged Members

#Region "IDisposable Support"
    Private disposedValue As Boolean ' To detect redundant calls

    ' IDisposable
    Protected Overridable Sub Dispose(disposing As Boolean)
        If Not Me.disposedValue Then
            If disposing Then
                ' TODO: dispose managed state (managed objects).
            End If

            ' TODO: free unmanaged resources (unmanaged objects) and override Finalize() below.
            ' TODO: set large fields to null.
        End If
        Me.disposedValue = True
    End Sub

    ' Invoked when this object is being removed from the application and will be subject to garbage collection.
    Public Sub Dispose() Implements IDisposable.Dispose
        Me.OnDispose()
    End Sub

    ' Child classes can override this method to perform clean-up logic, such as removing event handlers.
    Protected Overridable Sub OnDispose()
    End Sub

    ' Controla el tancament del ViewModel.
    ' <returns></returns>
    ' <remarks></remarks>
    Public Overridable Function CanClose() As Boolean
        Return Nothing
    End Function

#If DEBUG Then
    ' Useful for ensuring that ViewModel objects are properly garbage collected.
    Protected Overrides Sub Finalize()
        Dim msg As String = String.Format("{0} ({1}) ({2}) Finalized", Me.GetType().Name, Me.DisplayName, Me.GetHashCode())
        System.Diagnostics.Debug.WriteLine(msg)
    End Sub
#End If
#End Region

End Class

DelegateCommand.vb:

Imports System.Windows.Input

Namespace MVVM
    Public NotInheritable Class RelayCommand
        Implements ICommand

#Region " Declarations "
        Private ReadOnly _objCanExecuteMethod As Predicate(Of Object) = Nothing
        Private ReadOnly _objExecuteMethod As Action(Of Object) = Nothing
#End Region

#Region " Events "
        Public Custom Event CanExecuteChanged As EventHandler Implements System.Windows.Input.ICommand.CanExecuteChanged
            AddHandler(ByVal value As EventHandler)
                If _objCanExecuteMethod IsNot Nothing Then
                    AddHandler CommandManager.RequerySuggested, value
                End If
            End AddHandler

            RemoveHandler(ByVal value As EventHandler)
                If _objCanExecuteMethod IsNot Nothing Then
                    RemoveHandler CommandManager.RequerySuggested, value
                End If
            End RemoveHandler

            RaiseEvent(ByVal sender As Object, ByVal e As System.EventArgs)
                If _objCanExecuteMethod IsNot Nothing Then
                    CommandManager.InvalidateRequerySuggested()
                End If
            End RaiseEvent
        End Event
#End Region

#Region " Constructor "
        Public Sub New(ByVal objExecuteMethod As Action(Of Object))
            Me.New(objExecuteMethod, Nothing)
        End Sub

        Public Sub New(ByVal objExecuteMethod As Action(Of Object), ByVal objCanExecuteMethod As Predicate(Of Object))
            If objExecuteMethod Is Nothing Then
                Throw New ArgumentNullException("objExecuteMethod", "Delegate comamnds can not be null")
            End If

            _objExecuteMethod = objExecuteMethod
            _objCanExecuteMethod = objCanExecuteMethod
        End Sub
#End Region

#Region " Methods "
        Public Function CanExecute(ByVal parameter As Object) As Boolean Implements System.Windows.Input.ICommand.CanExecute
            If _objCanExecuteMethod Is Nothing Then
                Return True
            Else
                Return _objCanExecuteMethod(parameter)
            End If
        End Function

        Public Sub Execute(ByVal parameter As Object) Implements System.Windows.Input.ICommand.Execute
            If _objExecuteMethod Is Nothing Then
                Return
            Else
                _objExecuteMethod(parameter)
            End If
        End Sub
#End Region
    End Class
End Namespace


Namespace MVVM
    Public NotInheritable Class RelayCommand(Of T)
        Implements ICommand

#Region " Declarations "
        Private ReadOnly _objCanExecuteMethod As Predicate(Of T) = Nothing
        Private ReadOnly _objExecuteMethod As Action(Of T) = Nothing
#End Region

#Region " Events "
        Public Custom Event CanExecuteChanged As EventHandler Implements System.Windows.Input.ICommand.CanExecuteChanged
            AddHandler(ByVal value As EventHandler)
                If _objCanExecuteMethod IsNot Nothing Then
                    AddHandler CommandManager.RequerySuggested, value
                End If
            End AddHandler

            RemoveHandler(ByVal value As EventHandler)
                If _objCanExecuteMethod IsNot Nothing Then
                    RemoveHandler CommandManager.RequerySuggested, value
                End If
            End RemoveHandler

            RaiseEvent(ByVal sender As Object, ByVal e As System.EventArgs)
                If _objCanExecuteMethod IsNot Nothing Then
                    CommandManager.InvalidateRequerySuggested()
                End If
            End RaiseEvent
        End Event
#End Region

#Region " Constructors "
        Public Sub New(ByVal objExecuteMethod As Action(Of T))
            Me.New(objExecuteMethod, Nothing)
        End Sub

        Public Sub New(ByVal objExecuteMethod As Action(Of T), ByVal objCanExecuteMethod As Predicate(Of T))
            If objExecuteMethod Is Nothing Then
                Throw New ArgumentNullException("objExecuteMethod", "Delegate comamnds can not be null")
            End If

            _objExecuteMethod = objExecuteMethod
            _objCanExecuteMethod = objCanExecuteMethod
        End Sub
#End Region

#Region " Methods "
        Public Function CanExecute(ByVal parameter As Object) As Boolean Implements ICommand.CanExecute
            If _objCanExecuteMethod Is Nothing Then
                Return True
            Else
                Return _objCanExecuteMethod(DirectCast(parameter, T))
            End If
        End Function

        Public Sub Execute(ByVal parameter As Object) Implements ICommand.Execute
            _objExecuteMethod(DirectCast(parameter, T))
        End Sub
#End Region
    End Class
End Namespace

MainWindow.xaml.vb（MainWindow的代码后台）：

Class MainWindow 
    Private Sub Button_Click(sender As Object, e As RoutedEventArgs)
        ic1.Items.Refresh()
    End Sub

    Private Sub RefreshCanvas(sender As Object, e As DataTransferEventArgs)
        sender.Items.Refresh()
    End Sub
End Class

感谢提供任何帮助指引我正确的方向，希望这也能帮助到其他人。

***** 更新，问题已解决 *****

E-Bat 友情提示，LineData 结构的属性本身需要实现 INotifyPropertyChanged 接口。我已经实现了这个更改，并添加了更新后可用的“MainWindowViewModel.xaml”代码如下：

Imports System.ComponentModel
Imports System.Collections.ObjectModel

Public Class MainWindowViewModel
    Inherits ViewModelBase

    ' Sample line data variable
    Private _LineData As ObservableCollection(Of LineData) = GetLineData()
    Public Property LineData As ObservableCollection(Of LineData)
        Get
            Return _LineData
        End Get
        Set(value As ObservableCollection(Of LineData))
            _LineData = value
            OnPropertyChanged("LineData")
        End Set
    End Property

    ' ICommands
    Private _Command1 As ICommand
    Public ReadOnly Property Command1 As ICommand
        Get
            If _Command1 Is Nothing Then
                _Command1 = New MVVM.RelayCommand(AddressOf ExecuteCommand1)
            End If
            Return _Command1
        End Get
    End Property

    Private _Command2 As ICommand
    Public ReadOnly Property Command2 As ICommand
        Get
            If _Command2 Is Nothing Then
                _Command2 = New MVVM.RelayCommand(AddressOf ExecuteCommand2)
            End If
            Return _Command2
        End Get
    End Property

    ' ICommand Methods
    Private Sub ExecuteCommand1()
        ' Re-arrange LineData(0) to make a plus sign on the canvas
        ' This works - Assigning a new value to an item of the collection updates the canvas
        LineData(0) = New LineData With {.X1 = "175", .Y1 = "50", .X2 = "175", .Y2 = "150"}
    End Sub

    Private Sub ExecuteCommand2()
        ' Put LineData(0) back into its original position
        ' Now it works, it's voodoo!
        LineData(0).X1 = "50"
        LineData(0).Y1 = "50"
        LineData(0).X2 = "300"
        LineData(0).Y2 = "50"
    End Sub

    ' Misc methods
    Private Function GetLineData() As ObservableCollection(Of LineData)
        Dim tmpList As New ObservableCollection(Of LineData)

        ' Create two horizontal parallel lines
        tmpList.Add(New LineData With {.X1 = "50", .Y1 = "50", .X2 = "300", .Y2 = "50"})
        tmpList.Add(New LineData With {.X1 = "50", .Y1 = "100", .X2 = "300", .Y2 = "100"})

        OnPropertyChanged("LineData")

        Return tmpList
    End Function
End Class

Public Class LineData
    Implements INotifyPropertyChanged

    Private _X1 As String
    Public Property X1 As String
        Get
            Return _X1
        End Get
        Set(value As String)
            _X1 = value
            OnPropertyChanged("X1")
        End Set
    End Property

    Private _Y1 As String
    Public Property Y1 As String
        Get
            Return _Y1
        End Get
        Set(value As String)
            _Y1 = value
            OnPropertyChanged("Y1")
        End Set
    End Property

    Private _X2 As String
    Public Property X2 As String
        Get
            Return _X2
        End Get
        Set(value As String)
            _X2 = value
            OnPropertyChanged("X2")
        End Set
    End Property

    Private _Y2 As String
    Public Property Y2 As String
        Get
            Return _Y2
        End Get
        Set(value As String)
            _Y2 = value
            OnPropertyChanged("Y2")
        End Set
    End Property

    Public Event PropertyChanged As PropertyChangedEventHandler Implements INotifyPropertyChanged.PropertyChanged

    Protected Sub OnPropertyChanged(ByVal name As String)
        RaiseEvent PropertyChanged(Me, New PropertyChangedEventArgs(name))
    End Sub
End Class

- mt1985

很高兴看到你找到了解决方案，但我想作为旁注评论一下，如果你想让ObservableCollection的更改事件在集合中的项的属性更改时触发，你可以订阅ObservableCollection的CollectionChanged事件，将PropertyChanged处理程序添加到集合中的每个项，并手动引发ObservableCollection的PropertyChanged事件。这里有一个例子 :) - Rachel

1个回答

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原文链接

- E-Bat · Accepted Answer

当您替换ObservableCollection中的项目时，旧引用将首先被移除，然后添加新引用，因此ObservableCollection将提高其事件，这就是为什么第一个命令会像魔术一样起作用。

现在，对于第二个命令要刷新UI，您必须使项本身(LineStructure)实现INotifyPropertyChanged，以便通过绑定刷新其属性的任何更改。因此，对于此类别，自动属性不适用，需手动实现。

Public Class LineStructure
    Implements INotifyPropertyChanged

    Private _x1 As String
    Public Property X1 As String
        Get
            Return _x1
        End Get
        Set(value As String)
            If _x1 = value Then Return
            _x1 = value
            OnPropertyChanged("X1")
        End Set
    End Property
End Class