WPF: switching UserControls depending on correspon

问题:

I'll try to simplify the task I'm working on by imagining this example:

Let's suppose that we have the following hierarchy of model classes:

Animal
   Lion
   Snake
   Bird

...corresponding ViewModels:

AnimalCollectionViewModel
   AnimalViewModel
      LionViewModel
      SnakeViewModel
      BirdViewModel

... and corresponding views:

AnimalCollectionView
   LionView
   SnakeView
   BirdView

It's supposed that AnimalCollection contains a list filled with objects of different types of animals and below the list it has a property grid for setting the properties of a selected animal. Obviously the property grids will have different properties and should change when the type of a selected item changes.

The question is: How to implement switching of the property grids in WPF according to the MVVM pattern? Using what mechanism?

Currently I have an abstract enum property in the base ViewModel (AnimalViewModel.PropertyGridType = {Lion, Snake, Bird}) which the derived classes implement by returning corresponding values. And the AnimalCollectionView changes the property grid user controls depending on the value of this property. Something like this:

...

<UserControl.Resources>
    <Style x:Key="PropertyGridStyle" TargetType="ContentControl">
        <Style.Triggers>
            <DataTrigger Binding="{Binding PropertyGridType}" Value="Lion">
                <Setter Property="Content">
                    <Setter.Value>
                        <view:LionPropertyGridView />
                    </Setter.Value>
                </Setter>
            </DataTrigger>
            <DataTrigger Binding="{Binding PropertyGridType}" Value="Snake">
                <Setter Property="Content">
                    <Setter.Value>
                        <view:SnakePropertyGridView />
                    </Setter.Value>
                </Setter>
            </DataTrigger>
        </Style.Triggers>
    </Style>
</UserControl.Resources>

<ContentControl Style="{StaticResource PropertyGridStyle}" />

...

But I'm not sure whether it is the right approach. (At least I don't like introducing the auxiliary enum property. Is it possible to deduce the necessary user control based on a ViewModel type?) Can anybody advise other options? Thanks in advance!

回答1:

Is it possible to deduce the necessary user control based on a ViewModel type?

You mean, like this?

<Window.Resources>
   <DataTemplate DataType="{x:Type vm:LionViewModel}">
      <v:LionView />
   </DataTemplate>
   <DataTemplate DataType="{x:Type vm:SnakeViewModel}">
      <v:SnakeView />
   </DataTemplate>
   <DataTemplate DataType="{x:Type vm:BirdViewModel}">
      <v:BirdView"/>
   </DataTemplate>
</Window.Resources>

See "Applying a View to a View Model" in Josh Smith's article on MVVM.

Edit:

Here's a trivial example of type-based template selection that you can paste into Kaxaml to prove to yourself that it really works:

<Page
  xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
  xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
  xmlns:sys="clr-namespace:System;assembly=mscorlib">
  <Page.Resources>
    <sys:String x:Key="string">this is a string</sys:String>
    <sys:Int32 x:Key="int32">1234</sys:Int32>
    <DataTemplate DataType="{x:Type sys:String}">
      <TextBlock Foreground="Red" Text="{Binding}"/>
    </DataTemplate>
    <DataTemplate DataType="{x:Type sys:Int32}">
      <TextBlock Foreground="Blue" Text="{Binding}"/>
    </DataTemplate>
  </Page.Resources>
  <StackPanel>  
    <ContentControl Content="{Binding Source={StaticResource string}}"/>
    <ContentControl Content="{Binding Source={StaticResource int32}}"/>
  </StackPanel>
</Page>

回答2:

You can use DataTemplateSelector for that. The method of choosing the right template is up to you. You can use enums or test for class type if you wish.