更改 ContentControl 綁定會導致空 DataContext

Question

也許你們中的一位可以就以下問題啟發我。 我使用列表框作為在 2 個視圖模型之間導航的方式。

<Window x:Class="MyWPFApp.ShellView"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
                 xmlns:cal="http://www.caliburnproject.org">

  <Grid Background="White">
     <Grid.ColumnDefinitions>
        <ColumnDefinition Width="2*"/>
        <ColumnDefinition Width="8*"/>
     </Grid.ColumnDefinitions>
     <ListBox Grid.Column="0" cal:Message.Attach="[Event SelectionChanged] 
= [Action onViewSelectionChange($this.SelectedItem.Text)]">
        <TextBlock Text="FirstView"/>
        <TextBlock Text="SecondView"/>
     </ListBox>
        <ContentControl Grid.Column="1" Content="{Binding MyViewModel}"/>
  </Grid>

namespace MyWPFApp
{
    public class ShellViewModel : Caliburn.Micro.PropertyChangedBase, IShell
   {
      private object _vm;

      public object MyViewModel
      {
         get { return _vm; }
         set { _vm = value; NotifyOfPropertyChange(() => MyViewModel); }
      }

      public FirstViewModel vm1;
      public SecondViewModel vm2;



      public ShellViewModel()
      {
         vm1 = new FirstViewModel();
         vm2 = new SecondViewModel();
         MyViewModel = vm1;

      }

      public void onViewSelectionChange(string str)
      {

         switch (str)
         {
            case "FirstView":
               MyViewModel = vm1;
               break;
            case "SecondView":
               MyViewModel = vm2;
               break;
            default:
               break;

         }
      }
   }
}

我創建了 2 個數據模板來將視圖模型與其視圖相關聯

<Application x:Class="MyWPFApp.App"
         xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
         xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
         xmlns:local="clr-MyWPFApp">
  <Application.Resources>
  <ResourceDictionary>
     <ResourceDictionary.MergedDictionaries>
        <ResourceDictionary>
           <local:AppBootstrapper x:Key="bootstrapper" />
        </ResourceDictionary>
     </ResourceDictionary.MergedDictionaries>
     <DataTemplate DataType="{x:Type local:FirstViewModel}">
        <Grid>
           <local:FirstView></local:FirstView>
        </Grid>
     </DataTemplate>
     <DataTemplate DataType="{x:Type local:SecondViewModel}">
        <Grid>
           <local:SecondView></local:SecondView>
        </Grid>
     </DataTemplate>
  </ResourceDictionary>
  </Application.Resources>
  </Application>

FirstView 實際上具有與 FirstViewModel 中的屬性綁定的控件

   <UserControl x:Class="MyWPFApp.FirstView"
            xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
            xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
            xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" 
            xmlns:d="http://schemas.microsoft.com/expression/blend/2008" 
            xmlns:local="clr-namespace:MyWPFApp"
            xmlns:cal="http://www.caliburnproject.org"
            mc:Ignorable="d" 
            d:DesignHeight="300" d:DesignWidth="300">
  <Grid Background="Aquamarine">
     <ListBox SelectedIndex="{Binding SelectedIndex}" 
              SelectedItem="{Binding SelectedItem}"
              cal:Message.Attach="[Event SelectionChanged]=[Action OnLBSelectionChanged($source)]">
        <ListBoxItem>apple</ListBoxItem>
        <ListBoxItem>orange</ListBoxItem>
        <ListBoxItem>pear</ListBoxItem>
     </ListBox>
  </Grid>

using Caliburn.Micro;
using System;
using System.Collections.Generic;
using System.Collections.ObjectModel;
using System.Diagnostics;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace MyWPFApp
{
   public class FirstViewModel : PropertyChangedBase
   {

      private int _selectedindex;

      public int SelectedIndex
      {
         get { return _selectedindex; }
         set { _selectedindex = value; NotifyOfPropertyChange(() => SelectedIndex); }
      }


      private string _selecteditem;

      public string SelectedItem
      {
         get { return _selecteditem; }
         set { _selecteditem = value; NotifyOfPropertyChange(() => SelectedItem); }
      }



      public FirstViewModel()
      {
      }


      public void OnLBSelectionChanged(System.Windows.Controls.ListBox lb)
      {
         if(lb.DataContext == null)
         {
            Debug.WriteLine("The DataContext is null");
         }

      }

   }
}

SecondView 不包含任何控件。

當 ShellView 列表框選擇從 FirstView 更改為 SecondView 時，我注意到 FirstView ListBox DataContext 變為 null。

  public void OnLBSelectionChanged(System.Windows.Controls.ListBox lb)
  {
     if(lb.DataContext == null)
     {
        Debug.WriteLine("The DataContext is null");
     }

  }

我寧願不必處理尋找空 DataContext 的選擇更改。 相反，我想知道實際發生了什么。 謝謝。

Answer 1

發生這種情況的原因是，當ContentControl的Content屬性通過數據綁定設置為FirstViewModel時， FirstView的DataContext由 WPF 隱式連接。

一旦綁定更改為SecondViewModel ， FirstViewModel就會超出范圍，因此 WPF 會自行清理，刪除FirstViewModel 、 FirstView及其隱式DataContext綁定。

我希望這有幫助。