c# - 在 WPF 的 ContentControl 中托管 ViewModel

Question

我有一个传统的表单布局，顶部有一个菜单栏，底部有一个状态栏。当用户选择一个菜单项时，中间的空间(表单的整个剩余客户区)将被用户控件替换 - 想象一个可以托管多种类型文档的 SDI 应用程序。

如果您知道解决此问题的更好方法，请插话。目前，我正在尝试让它在带有 ContentControl 的非常简化的版本中工作，但我无法让它在以下情况下更新屏幕它的 DataContext 已设置。

下面是 ViewModelA 的非常简单的代码。 ViewModelB 除了 B 之外是相同的。

namespace Dynamic_ContentControl
{
    public class ViewModelA: ViewModelBase
    {
        public ViewModelA()
        {
            DisplayName = "This is A";
        }
    }
}

主窗口非常简单。它基本上声明了一个属性来保存托管控件的 View 模型，并公开了两个命令来分配 View 模型 A 或 B。

namespace Dynamic_ContentControl
{
    public class MainViewModel: ViewModelBase
    {
        private ViewModelBase clientContent = null;

        public ICommand ShowA { get; private set; }
        public ICommand ShowB { get; private set; }
        public ViewModelBase ClientContent {
            get
            {
                return clientContent;
            }
            private set
            {
                clientContent = value;
                OnPropertyChanged("ClientContent");
            }
        }
        public MainViewModel()
        {
            ShowA = new RelayCommand((obj) =>
            {
                ClientContent = new ViewModelA();
            });
            ShowB = new RelayCommand((obj) =>
            {
                ClientContent = new ViewModelB();
            });
        }
    }
}

最后，XAML 声明一个 ContentControl 并将其 ContentTemplate 设置为一个名为 ClientAreaTemplate 的 DataTemplate，其 ContentPresenter 指向另一个名为 TextBlockLayout 的 DataTemplate:

<Window x:Class="Dynamic_ContentControl.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:vm="clr-namespace:Dynamic_ContentControl"
        Title="MainWindow" Height="350" Width="525">
    <Window.DataContext>
        <vm:MainViewModel />
    </Window.DataContext>
    <Window.Resources>
        <DataTemplate x:Key="TextBlockLayout">
            <TextBlock Text="{Binding Path=DisplayName}" />
        </DataTemplate>
        <DataTemplate x:Key="ButtonLayout">
            <Button Content="{Binding Path=DisplayName}" />
        </DataTemplate>
        <DataTemplate x:Key="CheckBoxLayout">
            <CheckBox Content="{Binding Path=DisplayName}" />
        </DataTemplate>
        <DataTemplate x:Key="ClientAreaTemplate">
            <ContentPresenter x:Name="ContentArea" ContentTemplate="{StaticResource ResourceKey=TextBlockLayout}"/>
            <DataTemplate.Triggers>
                <DataTrigger Binding="{Binding Path=DataContext}"
                             Value="{x:Type vm:ViewModelB}">
                    <Setter TargetName="ContentArea"
                            Property="ContentTemplate"
                            Value="{StaticResource ResourceKey=ButtonLayout}" />
                </DataTrigger>
                <DataTrigger Binding="{Binding Path=DataContext}"
                             Value="{x:Type vm:ViewModelB}">
                    <Setter TargetName="ContentArea"
                            Property="ContentTemplate"
                            Value="{StaticResource ResourceKey=CheckBoxLayout}" />
                </DataTrigger>
            </DataTemplate.Triggers>
        </DataTemplate>
    </Window.Resources>
    <Grid>
        <Button Content="Show A"
                Command="{Binding Path=ShowA}"
                HorizontalAlignment="Left"
                Margin="10,10,0,0"
                VerticalAlignment="Top"
                Width="75" />
        <Button Content="Show B"
                Command="{Binding ShowB}"
                HorizontalAlignment="Left"
                Margin="90,10,0,0"
                VerticalAlignment="Top"
                Width="75" />
        <Label Content="{Binding Path=ClientContent.DisplayName}"
               HorizontalAlignment="Left"
               Margin="170,8,0,0"
               VerticalAlignment="Top" />
        <ContentControl DataContext="{Binding Path=ClientContent}"
                        Content="{Binding}"
                        ContentTemplate="{StaticResource ResourceKey=ClientAreaTemplate}"
                        HorizontalAlignment="Left"
                        Margin="10,37,0,0"
                        VerticalAlignment="Top"
                        Height="198"
                        Width="211" />

    </Grid>
</Window>

预期行为

当屏幕打开时，我希望显示 TextBoxLayout。如果用户随后单击两个按钮之一，它应该加载 ButtonLayout 或 CheckBoxLayout，具体取决于分配的 View 模型的实际运行时类型。

实际行为

屏幕打开时加载了 TextBoxLayout，但当我单击按钮时它永远不会更改为另一种类型。

我认为问题在于 DataTrigger 尝试与类型进行比较的方式，但在所有输出窗口中都没有绑定(bind)消息。

Answer 1

在这种情况下，你需要使用DataTemplateSelector :

Provides a way to choose a DataTemplate based on the data object and the data-bound element.

Here是动态 DataTemplateSelector 的一个版本，它根据类型返回所需的 DataTemplate:

/// <summary>
/// Provides a means to specify DataTemplates to be selected from within WPF code
/// </summary>
public class DynamicTemplateSelector : DataTemplateSelector
{
    /// <summary>
    /// Generic attached property specifying <see cref="Template"/>s
    /// used by the <see cref="DynamicTemplateSelector"/>
    /// </summary>
    /// <remarks>
    /// This attached property will allow you to set the templates you wish to be available whenever
    /// a control's TemplateSelector is set to an instance of <see cref="DynamicTemplateSelector"/>
    /// </remarks>
    public static readonly DependencyProperty TemplatesProperty =
        DependencyProperty.RegisterAttached("Templates", typeof(TemplateCollection), typeof(DataTemplateSelector),
        new FrameworkPropertyMetadata(new TemplateCollection(), FrameworkPropertyMetadataOptions.Inherits));


    /// <summary>
    /// Gets the value of the <paramref name="element"/>'s attached <see cref="TemplatesProperty"/>
    /// </summary>
    /// <param name="element">The <see cref="UIElement"/> who's attached template's property you wish to retrieve</param>
    /// <returns>The templates used by the givem <paramref name="element"/>
    /// when using the <see cref="DynamicTemplateSelector"/></returns>
    public static TemplateCollection GetTemplates(UIElement element)
    {
        return (TemplateCollection)element.GetValue(TemplatesProperty);
    }

    /// <summary>
    /// Sets the value of the <paramref name="element"/>'s attached <see cref="TemplatesProperty"/>
    /// </summary>
    /// <param name="element">The element to set the property on</param>
    /// <param name="collection">The collection of <see cref="Template"/>s to apply to this element</param>
    public static void SetTemplates(UIElement element, TemplateCollection collection)
    {
        element.SetValue(TemplatesProperty, collection);
    }

    /// <summary>
    /// Overriden base method to allow the selection of the correct DataTemplate
    /// </summary>
    /// <param name="item">The item for which the template should be retrieved</param>
    /// <param name="container">The object containing the current item</param>
    /// <returns>The <see cref="DataTemplate"/> to use when rendering the <paramref name="item"/></returns>
    public override System.Windows.DataTemplate SelectTemplate(object item, System.Windows.DependencyObject container)
    {
        //This should ensure that the item we are getting is in fact capable of holding our property
        //before we attempt to retrieve it.
        if (!(container is UIElement))
            return base.SelectTemplate(item, container);

        //First, we gather all the templates associated with the current control through our dependency property
        TemplateCollection templates = GetTemplates(container as UIElement);
        if (templates == null || templates.Count == 0)
            base.SelectTemplate(item, container);

        //Then we go through them checking if any of them match our criteria
        foreach (var template in templates)
            //In this case, we are checking whether the type of the item
            //is the same as the type supported by our DataTemplate
            if (template.Value.IsInstanceOfType(item))
                //And if it is, then we return that DataTemplate
                return template.DataTemplate;

        //If all else fails, then we go back to using the default DataTemplate
        return base.SelectTemplate(item, container);
    }
}

/// <summary>
/// Holds a collection of <see cref="Template"/> items
/// for application as a control's DataTemplate.
/// </summary>
public class TemplateCollection : List<Template>
{

}

/// <summary>
/// Provides a link between a value and a <see cref="DataTemplate"/>
/// for the <see cref="DynamicTemplateSelector"/>
/// </summary>
/// <remarks>
/// In this case, our value is a <see cref="System.Type"/> which we are attempting to match
/// to a <see cref="DataTemplate"/>
/// </remarks>
public class Template : DependencyObject
{
    /// <summary>
    /// Provides the value used to match this <see cref="DataTemplate"/> to an item
    /// </summary>
    public static readonly DependencyProperty ValueProperty = DependencyProperty.Register("Value", typeof(Type), typeof(Template));

    /// <summary>
    /// Provides the <see cref="DataTemplate"/> used to render items matching the <see cref="Value"/>
    /// </summary>
    public static readonly DependencyProperty DataTemplateProperty =
       DependencyProperty.Register("DataTemplate", typeof(DataTemplate), typeof(Template));

    /// <summary>
    /// Gets or Sets the value used to match this <see cref="DataTemplate"/> to an item
    /// </summary>
    public Type Value
    { get { return (Type)GetValue(ValueProperty); } set { SetValue(ValueProperty, value); } }

    /// <summary>
    /// Gets or Sets the <see cref="DataTemplate"/> used to render items matching the <see cref="Value"/>
    /// </summary>
    public DataTemplate DataTemplate
    { get { return (DataTemplate)GetValue(DataTemplateProperty); } set { SetValue(DataTemplateProperty, value); } }
}

使用示例

<local:DynamicTemplateSelector x:Key="MyTemplateSelector" />

<DataTemplate x:Key="StringTemplate">
    <TextBlock>
        <Run Text="String: " />
        <Run Text="{Binding}" />
    </TextBlock>
</DataTemplate>

<DataTemplate x:Key="Int32Template">
    <TextBlock>
        <Run Text="Int32: " />
        <Run Text="{Binding}" />
    </TextBlock>
</DataTemplate>

<Style x:Key="MyListStyle" TargetType="ListView">
    <Setter Property="ItemTemplateSelector" Value="{StaticResource MyTemplateSelector}"/>
    <Setter Property="local:DynamicTemplateSelector.Templates">
        <Setter.Value>
            <local:Templates>
                <local:Template Value={x:Type String} DataTemplate={StaticResource StringTemplate}/>
                <local:Template Value={x:Type Int32} DataTemplate={StaticResource Int32Template}/>
            </local:Templates>
        </Setter.Value>
    </Setter>
</Style>