DataContext not binding in Style.Trigger

问题:

So I have some code similar to the following: (Forgive any typos-- I tried to simplify in the SO editor for the post)

<my:CustomContentControl>
        <my:CustomContentControl.Style>
            <Style TargetType="{x:Type my:CustomContentControl}">
                <Style.Triggers>                        
                    <DataTrigger Binding="{Binding Path=CurrentView}" Value="MyCustomView">
                        <Setter Property="Content">
                            <Setter.Value>
                                <my:CustomView DataContext="{Binding DataContextForMyCustomView"/>
                            </Setter.Value>
                        </Setter>
                    </DataTrigger>
                </Style.Triggers>
            </Style>
        </m:CustomContentControl.Style>
</my:CustomContentControl>

The problem is that whenever the DataTrigger occurs, the setter does set the Content property to my:CustomView, but it does not bind DataContext. If I move the same code outside of the trigger the DataContext binding works just fine.

Any ideas? If this is a limitation of some sorts, is there any work around?

Update:

I received the following error in the output window:

System.Windows.Data Error: 3 : Cannot find element that provides DataContext. BindingExpression:Path=DataContextForMyCustomView; DataItem=null; target element is 'CustomView' (Name='customView'); target property is 'DataContext' (type 'Object')

回答1:

The error you posted makes it sound like your custom control is in an object that doesn't have a DataContext, such as a DataGridColumn.Header.

To get around that, you can create a Freezeable object in your .Resources containing the binding you're looking for, then bind your my:CustomView.DataContext to that object

<my:CustomContentControl.Resources>
    <local:BindingProxy x:Key="proxy" 
        Data="{Binding DataContextForMyCustomView, ElementName=MyControl}" />
</my:CustomContentControl.Resources>

...

<my:CustomView DataContext="{Binding Source={StaticResource proxy}}"/>

Here's the code for a sample Freezable object copied from here:

public class BindingProxy : Freezable
{
    #region Overrides of Freezable

    protected override Freezable CreateInstanceCore()
    {
        return new BindingProxy();
    }

    #endregion

    public object Data
    {
        get { return (object)GetValue(DataProperty); }
        set { SetValue(DataProperty, value); }
    }

    // Using a DependencyProperty as the backing store for Data.  
    // This enables animation, styling, binding, etc...
    public static readonly DependencyProperty DataProperty =
        DependencyProperty.Register("Data", typeof(object), 
            typeof(BindingProxy), new UIPropertyMetadata(null));
}

Also, you really should use ContentTemplate instead of Content to avoid an exception if more than one object applies that style :)

回答2:

I solved a similar problem by putting the UserControl into the resources and then changing the Content with that.

e.g. from my own code (different names, same concept)

<ContentControl Grid.Column="1"
                Margin="7,0,7,0">
    <ContentControl.Resources>
        <mapping:Slide11x4MappingView x:Key="Slide11X4MappingView" DataContext="{Binding MappingViewModel}"/>
        <mapping:MicrotubeMappingView x:Key="MicrotubeMappingView"  DataContext="{Binding MappingViewModel}"/>
    </ContentControl.Resources>
    <ContentControl.Style>
        <Style TargetType="{x:Type ContentControl}">
            <Style.Triggers>
                <DataTrigger Binding="{Binding Acquirer.Sorter.TrayType}" Value="{x:Static mapping:TrayType.SLIDES11X4}">
                    <Setter Property="Content" Value="{StaticResource Slide11X4MappingView}"/>
                </DataTrigger>
                <DataTrigger Binding="{Binding Acquirer.Sorter.TrayType}" Value="{x:Static mapping:TrayType.VIALS}">
                    <Setter Property="Content" Value="{StaticResource MicrotubeMappingView}"/>
                </DataTrigger>
            </Style.Triggers>
        </Style>
    </ContentControl.Style>
</ContentControl>