如何在 XAML 中添加 TreeView.Resources 的条件模板？

Question

我想要一个有条件的 XAML 来选择是 TreeView.Resources 还是其他。

我正在使用MaterialDesignInXAML toolkit 的Card，里面是TreeView。

假设我有一个 Fruits 集合，其中包含另一个名为 Trees 的集合。

<TreeView.Resources>
    <HierarchicalDataTemplate DataType="{x:Type domain:Tree}" ItemsSource="{Binding Fruits}">
        // tree information here ...
    </HierarchicalDataTemplate>
    <DataTemplate DataType="{x:Type domain:Fruit}">
        // fruit information here ...
    </DataTemplate>
</TreeView.Resources>

这里的问题是，有些水果不是从树上长出来的。所以我想要的是拥有另一个资源，但仍然会继续使用模板。

<TreeView.Resources>
    <HierarchicalDataTemplate DataType="{x:Type domain:Fruit}">
        // fruit information here
    </HierarchicalDataTemplate>
</TreeView.Resources>

我希望我的输出如下所示：

> Apple Tree
    - Apples
> Mango Tree
    - Mangoes
> Watermelon

编辑：

我使用了 @BionicCode 的第二个建议，即使用DataTemplateSelector。我将此添加到我的 XAML：

<UserControl DataContext="{Binding ViewModel}">
    <UserControl.Resources>
        <domain:DtmSelector x:Key="DtmSelector">
            <domain:DtmSelector.WithTree>
                <HierarchicalDataTemplate DataType="{x:Type domain:Tree}" ItemsSource="{Binding Fruits}">
                    <HierarchicalDataTemplate.ItemTemplate>
                        <DataTemplate DataType="{x:Type domain:Fruit}">
                            <TextBlock Text="{Binding Name}"/>
                        </DataTemplate>
                    </HierarchicalDataTemplate.ItemTemplate>
                    <TextBlock Text="{Binding Name}"/>
                </HierarchicalDataTemplate>
            </domain:DtmSelector.WithTree>
            <domain:DtmSelector.WithoutTree>
                <DataTemplate DataType="{x:Type domain:Tree}">
                    <TextBlock Text="{Binding Fruits[0].Name}"/>
                </DataTemplate>
            </domain:DtmSelector.WithoutTree>
        </domain:DtmSelector>
    </UserControl.Resources>

    <TreeView
        ItemsSource="{Binding Trees}"
        ItemTemplateSelector="{StaticResource DtmSelector}"                
    />
</UserControl>

在我的DtmSelector.cs中：

public class DtmSelector: DataTemplateSelector
{
    public DataTemplate WithTrees { get; set; }
    public DataTemplate WithoutTrees { get; set; }

    public override DataTemplate SelectTemplate(object item, DependencyObject container)
    {
        Tree key = item as Tree;
        if (key.Fruits.Count() == 0)
        {
            return WithTrees;
        }
        else
        {
            if (key.Fruits.Select(c=>c.Name).Contains(key.Name))
            {
                return WithTrees;
            }
            else
            {
                return WithoutTrees;
            }
        }
    }
}

我ViewModel的概念是这样的：

foreach (var item in fruits)
{
    if ( item.Tree == null)
    {
        if (!Trees.Select(c => c.Name).Contains(item.Name))
        {
            Trees.Add(new Tree(item.Name));
        }

        foreach (var tree in Trees.ToList())
        {
            if (tree.Name == item.Name)
            {
                tree.Fruits.Add(item);
            }
        }
    }
    else
    {
        if (!Trees.Select(c => c.Name).Contains(item.Tree.Name))
        {
            Trees.Add(item.Tree);
        }

        foreach (var tree in Trees.ToList())
        {
            if (tree.Name == item.Tree.Name)
            {
                tree.Fruits.Add(item);
            }
        }
    }
}

问题

这组代码将首先检查Trees，而不添加Fruits。所以这条线if (key.Fruits.Count() == 0) 将永远返回true。有什么我可以做的吗？我错过了什么吗？

我发现 an answer from another thread 可能会有所帮助，但是，我不知道这将如何在 ViewModel 上工作，我不知道如何通过绑定的 Property 选择某个设置器。

Answer 1

这里有一些选择。我将根据数据结构（模型）设计向您展示两个建议。

第一个建议

对完整的树结构使用单一数据类型：

ViewModel.cs

class ViewModel : INotifyPropertyChanged
{
  ViewModel()
  {
    this.Fruits = new ObservableCollection<Fruit>()
    {
      new Fruit("Apple Tree", new ObservableCollection<Fruit>() {new Fruit("Apples")}),
      new Fruit("Mango Tree", new ObservableCollection<Fruit>() {new Fruit("Mangos")}),
      new Fruit("Watermelon")
    }

  ObservableCollection<Fruit> fruits;
  ObservableCollection<Fruit> Fruits
  {
    get => this.fruits;
    set
    {
      this.fruits = value;
      OnPropertyChanged();
    }
  }

  // INotifyPropertyChanged implementation here...
}

Fruit.cs（数据模型）

class Fruit : INotifyPropertyChanged
{
  Fruit(string name) : this(name, new List<Fruit>())
  {
  }

  Fruit(string name, IEnumerable<Fruit> children)
  {
    this.Name = name;
    this.Children = new ObservableCollection<Fruit>(children ?? new List<Fruit>());
  }

  string name;
  string Name 
  {
    get => this.name;
    set
    {
      this.name = value;
      OnPropertyChanged();
    }
  }

  ObservableCollection<Fruit> children;
  ObservableCollection<Fruit> Children 
  {
    get => this.children;
    set
    {
      this.children = value;
      OnPropertyChanged();
    }
  }

  // INotifyPropertyChanged implementation here...
}

TreeViewXAML

<Window>
  <Window.DataContext>
    <ViewModel />
  </Window.DataContext>

  <TreeView ItemsSource={Binding Fruits}>
    <TreeView.ItemTemplate>
      <HierarchicalDataTemplate DataType="{x:Type domain:Fruit}" ItemsSource="{Binding Children}">
        <HierarchicalDataTemplate.ItemTemplate>
          <DataTemplate DataType="{x:Type domain:Fruit}">
            <TextBlock Text="{Binding Name}" />
          </DataTemplate>
        </HierarchicalDataTemplate.ItemTemplate>

        <TextBlock Text="{Binding Name}" />
      </HierarchicalDataTemplate>
    </TreeView.ItemTemplate>
  </TreeView>
</Window>

---

第二个建议

如果您需要不同类型的不同角色（节点），您必须使用DataTemplateSelector：

ViewModel.cs

class ViewModel : INotifyPropertyChanged
{
  ViewModel()
  {
    this.Fruits = new ObservableCollection<INode>()
    {
      new Category("Apple Tree", new ObservableCollection<INode>() {new Fruit("Apples")}),
      new Category("Mango Tree", new ObservableCollection<INode>() {new Fruit("Mangos")}),
      new Fruit("Watermelon")
    }

  ObservableCollection<INode> fruits;
  ObservableCollection<INode> Fruits
  {
    get => this.fruits;
    set
    {
      this.fruits = value;
      OnPropertyChanged();
    }
  }

  // INotifyPropertyChanged implementation here...
}

INode.cs（数据模型）

// Common base type for the node collection and all tree nodes
interface INode: INotifyPropertyChanged
{
  string Name {get; set; }

  ObservableCollection<INode> Children { get; set; }
}

Category.cs（数据模型）

class Category : INode
{
  Category(string name) : this(name, new List<INode>())
  {
  }

  Category(string name, IEnumerable<INode> children) 
  {
    this.Name = name;
    this.Children = new ObservableCollection<INode>(children ?? new List<INode>());
  }

  string name;
  string Name 
  {
    get => this.name;
    set
    {
      this.name = value;
      OnPropertyChanged();
    }
  }

  ObservableCollection<INode> children;
  ObservableCollection<INode> Children 
  {
    get => this.children;
    set
    {
      this.children = value;
      OnPropertyChanged();
    }
  }

  // INotifyPropertyChanged implementation here...
}

Fruit.cs（数据模型）

class Fruit : INode
{
  Fruit(string name) : this(name, new List<INode>())
  {
  }

  Fruit(string name, IEnumerable<INode> children)
  {
    this.Name = name;
    this.Children = new ObservableCollection<INode>(children ?? new List<INode>());
  }

  string name;
  string Name 
  {
    get => this.name;
    set
    {
      this.name = value;
      OnPropertyChanged();
    }
  }

  ObservableCollection<INode> children;
  ObservableCollection<INode> Children 
  {
    get => this.children;
    set
    {
      this.children = value;
      OnPropertyChanged();
    }
  }

  // INotifyPropertyChanged implementation here...
}

NodeTemplateSelector.cs

class NodeTemplateSelector : DataTemplateSelector
{
  public DataTemlplate CategoryNodeTemplate { get; set; }
  public DataTemlplate FruitNodeTemplate { get; set; }

  public override DataTemplate SelectTemplate(object item, DependencyObject container)
  {
    switch (item)
    {
        case Category _:
           return this.CategoryNodeTemplate;
        case Fruit _:
           return this.FruitNodeTemplate;
        default:
           return this.FruitNodeTemplate;
    }
  }
}

TreeViewXAML

<Window>
  <Window.DataContext>
    <ViewModel />
  </Window.DataContext>
  <Window.Resources>
    <NodeTemplateSelector x:Key="NodeTemplateSelector">
      <NodeTemplateSelector.CategoryNodeTemplate>
        <HierarchicalDataTemplate DataType="{x:Type Category}" ItemsSource="{Binding Children}">
          <TextBlock Text="{Binding Name}" />
        </HierarchicalDataTemplate>
      </NodeTemplateSelector.CategoryNodeTemplate>
      <NodeTemplateSelector.FruitNodeTemplate>
        <DataTemplate DataType="{x:Type Fruit}">
          <TextBlock Text="{Binding Name}" />
        </DataTemplate>
      </NodeTemplateSelector.FruitNodeTemplate>
    </NodeTemplateSelector>
  </Window.Resources>

  <TreeView ItemsSource="{Binding Fruits}" 
            ItemTemplateSelector="{StaticResource NodeTemplateSelector}"  />
</Window>