ContentPresenter 丢失 DataContext答案

【问题标题】：ContentPresenter losing DataContextContentPresenter 丢失 DataContext
【发布时间】：2010-10-03 14:11:36
【问题描述】：

我想创建一个“FlipPanel”，它提供同一对象的两个不同视图。这是我正在采取的方法。

这是主页面，它由一个 ItemsControl 组成，它的 ItemTemplate 是一个 FlipPanel。 FlipPanel 公开了两个属性，它们定义了用于 Front 和 Back 的 DataTemplate。

<UserControl.Resources>

    <ControlTemplate x:Key="MyFlipTemplate">
        <StackPanel>
            <Button Content="Flip" x:Name="PART_FlipButton"/>
            <ContentPresenter Content="{TemplateBinding Content}" x:Name="PART_FlipContent"/>
        </StackPanel>
    </ControlTemplate>

    <DataTemplate x:Key="Front">
        <StackPanel Orientation="Horizontal">
            <TextBlock Text="Front"/>
            <TextBlock Text="{Binding Name}"/>
        </StackPanel>
    </DataTemplate>

    <DataTemplate x:Key="Back">
        <StackPanel>
            <TextBlock Text="Back"/>
            <TextBlock Text="{Binding Description}"/>
        </StackPanel>
    </DataTemplate>
</UserControl.Resources>

<StackPanel>
    <ItemsControl x:Name="_items">
        <ItemsControl.ItemsPanel>
            <ItemsPanelTemplate>
                <StackPanel></StackPanel>
            </ItemsPanelTemplate>
        </ItemsControl.ItemsPanel>
        <ItemsControl.ItemTemplate>
            <DataTemplate>
                <SLTest:FlipPanel Template="{StaticResource MyFlipTemplate}" FrontDataTemplate="{StaticResource Front}" BackDataTemplate="{StaticResource Back}" Side="Front"/>
            </DataTemplate>
        </ItemsControl.ItemTemplate>
    </ItemsControl>
</StackPanel>

主页背后的代码非常简单，因为它只是将 ItemsControl 的 DataContext 设置为测试数据列表。

使用 System.Collections.Generic；使用 System.Windows.Controls；命名空间 SLTest { 公共部分类 NewPage : UserControl { 公共新页面（） { 初始化组件（）； _items.ItemsSource = 物品； } 公共 IList 项 { 得到 { 返回新列表 { 新的NewClass {名称=“名称1”，描述=“描述1”}，新的NewClass {名称=“名称2”，描述=“描述2”}，新的NewClass {名称=“名称3”，描述=“描述3”}，新的NewClass {名称=“名称4”，描述=“描述4”} }; } } } 公共类 NewClass { 公共字符串名称；公共字符串描述； } }

FlipPanel 代码也相对简单，因为它尝试根据 Side DependencyProperty 更改 DataTemplate。问题似乎是 ContentPresenter 的 DataContext 在某些时候丢失了。在代码中，我有两个 cmets 指示 ContentPresenter 的 DataContext 的有效性。

使用系统；使用 System.Windows；使用 System.Windows.Controls；命名空间 SLTest { [模板部分（名称 = FlipPanel.ButtonPart，类型 = typeof（按钮））] [模板部分（名称= FlipPanel.ContentPart，类型= typeof（ContentPresenter））] 公共部分类 FlipPanel：ContentControl { 私有常量字符串 ButtonPart = "PART_FlipButton"; 私有常量字符串 ContentPart = "PART_FlipContent"; 公共枚举 FlipSide { 正面，后退 } 私人翻转 _flipSide; 公共静态只读 DependencyProperty SideProperty = DependencyProperty.RegisterAttached("FlipSide", typeof(FlipSide), typeof(FlipPanel), new PropertyMetadata(FlipSide.Front, FlipSidePropertyChanged)); public static readonly DependencyProperty FrontDataTemplateProperty = DependencyProperty.Register("FrontDataTemplate", typeof (DataTemplate), typeof (FlipPanel), null); 公共静态只读 DependencyProperty BackDataTemplateProperty = DependencyProperty.Register("BackDataTemplate", typeof(DataTemplate), typeof(FlipPanel), null); 私人按钮_flipButton；私有 ContentPresenter _content；公共翻转面板（） { 初始化组件（）； } 公共数据模板 FrontDataTemplate { 得到 { 返回 (DataTemplate) GetValue(FrontDataTemplateProperty); } 放 { SetValue(FrontDataTemplateProperty, 值); } } 公共数据模板 BackDataTemplate { 得到 { 返回 (DataTemplate)GetValue(BackDataTemplateProperty); } 放 { SetValue(BackDataTemplateProperty, value); } } 私有静态无效 FlipSidePropertyChanged（DependencyObject d，DependencyPropertyChangedEventArgs e） { var flipSide = (FlipSide)e.NewValue; var 翻转面板 = d 作为翻转面板； flipPanel._content.ContentTemplate = flipSide == FlipSide.Front ？ flipPanel.FrontDataTemplate : FlipPanel.BackDataTemplate; } 公共覆盖无效 OnApplyTemplate() { _flipButton = GetTemplateChild(ButtonPart) 作为按钮； _flipButton.Click += OnFlipClicked; _content = GetTemplateChild(ContentPart) 作为 ContentPresenter； // _content.DataContext 在这里有效... _content.ContentTemplate = Side == FlipSide.Front ？前数据模板：后数据模板； base.OnApplyTemplate(); } 私人无效 OnFlipClicked（对象发送者，RoutedEventArgs e） { // _content.DataContext 现在为 NULL！！！！边 =（边 == FlipSide.Front）？ FlipSide.Back : FlipSide.Front; } 公共 FlipSide 侧 { 得到 { 返回（FlipSide）GetValue（SideProperty）； } 放 { SetValue(SideProperty, 值); } } } }

有什么想法吗？

我不确定这是否是解决我的要求的正确方法，如果有更好的方法我会欢迎任何进一步的建议。

谢谢

【问题讨论】：

标签： silverlight silverlight-2.0

【解决方案1】：

克里斯，

我不知道你是否还在寻找一种方法来做到这一点。我知道有很多类似问题的指南，但不完全是您正在寻找的。根据我对您想要什么的理解，您需要一个具有两个面（实际上是 x 个面）的控件，用户可以在其中按下按钮并导致面板“翻转”并显示不同的数据。但是，您希望此数据显示为足够通用，以便此翻转面板可以在其他位置使用，只需不同的实现而不是完全不同的代码。我是否正确理解您的需求？如果没有，请澄清我误入歧途的地方，我可能会为你得到更好的答案。话虽如此，这就是我所做的（谷歌代码演示项目在底部）：

我创建了一个控件库来容纳我的 FlipPanel（因为这是我做事的方式；这样我就可以在以后的其他项目中使用这些控件。）
我对控件库中的控件进行了样式设置，以包含您在方案中需要的上述属性。
我创建了一个 Silverlight 2.0 应用程序来创建控件的实例。
我创建了一个用于绑定的基本对象，它具有一些属性，以便我可以展示控件的潜力。

以下是在 Silverlight 2.0 页面中使用的可能定义：

    <Grid x:Name="LayoutRoot" Background="White">
  <controls:FlipPanel x:Name="TestingFlipPanel" Side="Front" >
   <controls:FlipPanel.BackDataTemplate>
    <DataTemplate>
     <StackPanel Orientation="Horizontal">
      <TextBlock Text="Back: "/>
      <TextBlock Text="{Binding BackText}" />
     </StackPanel>
    </DataTemplate>
   </controls:FlipPanel.BackDataTemplate>
   <controls:FlipPanel.FrontDataTemplate>
    <DataTemplate>
     <StackPanel Orientation="Horizontal">
      <TextBlock Text="Front: "/>
      <TextBlock Text="{Binding FrontText}" />
     </StackPanel>
    </DataTemplate>
   </controls:FlipPanel.FrontDataTemplate>
  </controls:FlipPanel>
    </Grid>

另一种方法是在用户控件（页面级别，甚至应用级别）中定义数据模板，如下所示：

这样您就可以知道我的绑定对象在这里的定义（是的，它是 VB……对不起！）：

Public Class BindingObject
 Private _FrontText As String
 Private _BackText As String

 Public Sub New(ByVal frontText As String, ByVal backText As String)
  MyBase.New()
  _FrontText = frontText
  _BackText = backText
 End Sub

 Public Property FrontText() As String
  Get
   Return _FrontText
  End Get
  Set(ByVal value As String)
   _FrontText = value
  End Set
 End Property
 Public Property BackText() As String
  Get
   Return _BackText
  End Get
  Set(ByVal value As String)
   _BackText = value
  End Set
 End Property

End Class

在我后面的代码中，这是我的页面的定义和翻转面板的数据上下文设置：

Partial Public Class Page
 Inherits UserControl
 Dim _BindingObject As New BindingObject("This is the front", "This is the back")
 Public Sub New()
  InitializeComponent()
 End Sub

 Private Sub Page_Loaded(ByVal sender As Object, ByVal e As System.Windows.RoutedEventArgs) Handles Me.Loaded
  TestingFlipPanel.DataContext = _BindingObject
 End Sub
End Class

因此，所有这些都摆在您面前，我们希望控件会显示一个按钮（在控件样式中）和一个文本块（实际上是两个），上面写着“前面：这是前面”，当按下按钮它被“翻转”以显示“后退：这是后退”。

说了这么多，下面是我用于控件的样式：

    <Style TargetType="controls:FlipPanel">
  <Setter Property="Template">
   <Setter.Value>
    <ControlTemplate TargetType="controls:FlipPanel">
     <Grid x:Name="LayoutRoot">
      <Grid>
       <Grid.RowDefinitions>
        <RowDefinition Height="Auto" />
        <RowDefinition Height="Auto" />
       </Grid.RowDefinitions>
       <Button x:Name="FlipButton" Content="Flip" Grid.Row="0" />
       <ContentPresenter Grid.Row="1" x:Name="FrontContentPresenter" Content="{TemplateBinding DataContext}" ContentTemplate="{TemplateBinding FrontDataTemplate}" />
       <ContentPresenter Grid.Row="1" x:Name="BackContentPresenter"  Content="{TemplateBinding DataContext}" ContentTemplate="{TemplateBinding BackDataTemplate}" />
      </Grid>

     </Grid>
    </ControlTemplate>
   </Setter.Value>
  </Setter>
 </Style>

最后，控件的定义（注意 - 它很长）：

<TemplatePart(Name:=FlipPanel.LayoutRoot_ElementName, Type:=GetType(FrameworkElement))> _
<TemplatePart(Name:=FlipPanel.FrontContentPresenter_ElementName, Type:=GetType(FrameworkElement))> _
<TemplatePart(Name:=FlipPanel.BackContentPresenter_ElementName, Type:=GetType(FrameworkElement))> _
<TemplatePart(Name:=FlipPanel.FlipButton_ElementName, Type:=GetType(FrameworkElement))> _
Public Class FlipPanel
 Inherits Control

 Public Const LayoutRoot_ElementName As String = "LayoutRoot"
 Public Const FlipButton_ElementName As String = "FlipButton"
 Public Const FrontContentPresenter_ElementName As String = "FrontContentPresenter"
 Public Const BackContentPresenter_ElementName As String = "BackContentPresenter"

 Public Enum Sides
  Front
  Back
 End Enum

 Private _LayoutRoot As FrameworkElement = Nothing
 Private _FlipButton As FrameworkElement = Nothing
 Private _FrontContentPresenter As FrameworkElement = Nothing
 Private _BackContentPresenter As FrameworkElement = Nothing

 Private _ControlUpdating As Boolean = False

 Public Sub New()
  MyBase.New()
  MyBase.DefaultStyleKey = GetType(FlipPanel)
 End Sub

 Public Overrides Sub OnApplyTemplate()
  MyBase.OnApplyTemplate()
  UpdateControl()
 End Sub

 Private Sub UpdateControl()
  If _ControlUpdating Then Exit Sub
  _ControlUpdating = True

  If _LayoutRoot Is Nothing Then _LayoutRoot = TryCast(GetTemplateChild(LayoutRoot_ElementName), FrameworkElement)
  If _LayoutRoot IsNot Nothing Then
   Dim element As Grid = TryCast(_LayoutRoot, Grid)
   If element IsNot Nothing Then
    ' Update LayoutGrid here.  
   End If
  End If
  If _FlipButton Is Nothing Then _FlipButton = TryCast(GetTemplateChild(FlipButton_ElementName), FrameworkElement)
  If _FlipButton IsNot Nothing Then
   Dim element As Button = TryCast(_FlipButton, Button)
   If element IsNot Nothing Then
    ' Update Button
    RemoveHandler element.Click, AddressOf _FlipButton_Click
    AddHandler element.Click, AddressOf _FlipButton_Click
   End If
  End If
  If _FrontContentPresenter Is Nothing Then _FrontContentPresenter = TryCast(GetTemplateChild(FrontContentPresenter_ElementName), FrameworkElement)
  If _FrontContentPresenter IsNot Nothing Then
   Dim element As ContentPresenter = TryCast(_FrontContentPresenter, ContentPresenter)
   If element IsNot Nothing Then
    ' Update FrontContentPresenter here.
    If Side = Sides.Front Then
     element.Visibility = Windows.Visibility.Visible
    Else
     element.Visibility = Windows.Visibility.Collapsed
    End If
   End If
  End If
  If _BackContentPresenter Is Nothing Then _BackContentPresenter = TryCast(GetTemplateChild(BackContentPresenter_ElementName), FrameworkElement)
  If _BackContentPresenter IsNot Nothing Then
   Dim element As ContentPresenter = TryCast(_BackContentPresenter, ContentPresenter)
   If element IsNot Nothing Then
    ' Update BackContentPresenter here.  
    If Side = Sides.Front Then
     element.Visibility = Windows.Visibility.Collapsed
    Else
     element.Visibility = Windows.Visibility.Visible
    End If
   End If
  End If


  _ControlUpdating = False
 End Sub

 Private Sub _FlipButton_Click(ByVal sender As Object, ByVal e As RoutedEventArgs)
  Select Case Side
   Case Sides.Front
    Side = Sides.Back
   Case Sides.Back
    Side = Sides.Front
   Case Else
    Throw New ArgumentOutOfRangeException("Side")
  End Select
  UpdateControl()
 End Sub

 Private Sub FlipPanel_LayoutUpdated(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.LayoutUpdated
  UpdateControl()
 End Sub

#Region " FrontDataTemplateProperty Dependency Property "

#Region " FrontDataTemplate Property "

 Public Property FrontDataTemplate() As DataTemplate
  Get
   Return DirectCast(GetValue(FrontDataTemplateProperty), DataTemplate)
  End Get
  Set(ByVal value As DataTemplate)
   SetValue(FrontDataTemplateProperty, value)
  End Set
 End Property

#End Region

#Region " FrontDataTemplate Dependency Property "

 Public Shared ReadOnly FrontDataTemplateProperty As DependencyProperty = DependencyProperty.Register("FrontDataTemplate", GetType(DataTemplate), GetType(FlipPanel), New PropertyMetadata(Nothing, AddressOf OnFrontDataTemplatePropertyChanged))

#End Region

#Region " FrontDataTemplate Property Changed CallBack "

 Private Shared Sub OnFrontDataTemplatePropertyChanged(ByVal d As DependencyObject, ByVal e As DependencyPropertyChangedEventArgs)
  If e.OldValue Is Nothing AndAlso e.NewValue Is Nothing Then Exit Sub
  If e.OldValue IsNot Nothing AndAlso e.OldValue.Equals(e.NewValue) Then Exit Sub

  Dim source As FlipPanel = TryCast(d, FlipPanel)
  If source Is Nothing Then Throw New ArgumentException("source is not an instance of FlipPanel!")

  ' Provide any other validation here.

  ' Apply any other changes here.

 End Sub

#End Region

#End Region

#Region " BackDataTemplateProperty Dependency Property "

#Region " BackDataTemplate Property "

 Public Property BackDataTemplate() As DataTemplate
  Get
   Return DirectCast(GetValue(BackDataTemplateProperty), DataTemplate)
  End Get
  Set(ByVal value As DataTemplate)
   SetValue(BackDataTemplateProperty, value)
  End Set
 End Property

#End Region

#Region " BackDataTemplate Dependency Property "

 Public Shared ReadOnly BackDataTemplateProperty As DependencyProperty = DependencyProperty.Register("BackDataTemplate", GetType(DataTemplate), GetType(FlipPanel), New PropertyMetadata(Nothing, AddressOf OnBackDataTemplatePropertyChanged))

#End Region

#Region " BackDataTemplate Property Changed CallBack "

 Private Shared Sub OnBackDataTemplatePropertyChanged(ByVal d As DependencyObject, ByVal e As DependencyPropertyChangedEventArgs)
  If e.OldValue Is Nothing AndAlso e.NewValue Is Nothing Then Exit Sub
  If e.OldValue IsNot Nothing AndAlso e.OldValue.Equals(e.NewValue) Then Exit Sub

  Dim source As FlipPanel = TryCast(d, FlipPanel)
  If source Is Nothing Then Throw New ArgumentException("source is not an instance of FlipPanel!")

  ' Provide any other validation here.

  ' Apply any other changes here.

 End Sub

#End Region

#End Region

#Region " SideProperty Dependency Property "

#Region " Side Property "

 Public Property Side() As Sides
  Get
   Return DirectCast(GetValue(SideProperty), Sides)
  End Get
  Set(ByVal value As Sides)
   SetValue(SideProperty, value)
  End Set
 End Property

#End Region

#Region " Side Dependency Property "

 Public Shared ReadOnly SideProperty As DependencyProperty = DependencyProperty.Register("Side", GetType(Sides), GetType(FlipPanel), New PropertyMetadata(Sides.Front, AddressOf OnSidePropertyChanged))

#End Region

#Region " Side Property Changed CallBack "

 Private Shared Sub OnSidePropertyChanged(ByVal d As DependencyObject, ByVal e As DependencyPropertyChangedEventArgs)
  If e.OldValue Is Nothing AndAlso e.NewValue Is Nothing Then Exit Sub
  If e.OldValue IsNot Nothing AndAlso e.OldValue.Equals(e.NewValue) Then Exit Sub

  Dim source As FlipPanel = TryCast(d, FlipPanel)
  If source Is Nothing Then Throw New ArgumentException("source is not an instance of FlipPanel!")

  ' Provide any other validation here.

  ' Apply any other changes here.

 End Sub

#End Region

#End Region

End Class

现在，您一直在等待什么，代码！

享受吧！

Google Code - http://code.google.com/p/stackoverflow-answers-by-scott/

Google Code - Source Code (Zip)

谢谢！

【讨论】：

Scott，刚刚注意到您的回复（我知道，有点晚了:)）。虽然我不再需要这种类型的控制，但我非常感谢您的彻底回应，如果需要，我一定会将此作为参考。再次感谢，克里斯