c# - 如何使用 TreeView 转换器从列表创建树结构？

Question

某些函数提供了一个简单的字符串列表，如下所示:

var list = new List<int>{1,2,3,4};

TreeView 现在想要将此列表转换为具有两个根节点的树(一个用于奇数，一个用于偶数)。这只是一个示例，因为真实场景必须创建更加层次化的结构。 重点是后端提供了一个平面列表， View 想要将其转换为树。

我们尝试使用转换器作为 ItemsSource，但当它创建一个新结构时，它基本上破坏了与原始列表的绑定(bind)(使得异步填充变得不可能)。 Explaining why

这是一个小的复制代码:

隐藏代码:

public partial class MainWindow : Window
    {
        public ObservableCollection<int> TreeViewSource { get; set; }
        public MainWindow()
        {
            InitializeComponent();
            DataContext = this;
            TreeViewSource = new ObservableCollection<int>();

            Action filler =() => { Enumerable.Range(0, 100).ToList().ForEach((i) => { Thread.Sleep(20); TreeViewSource.Add(i); }); };

            Task.Run(() => filler()); // ASYNC CALL DOES NOT WORK
            //filler(); // SYNC CALL DOES WORK
        }
    }
    public class TreeConverter : IValueConverter
    {

        public object Convert(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
        {
            var sourceCollection = value as ObservableCollection<int>;          
            var outputCollection = new List<Item>();

            var odd = new Item { Text = "not divisible by 2" };
            var even = new Item { Text = "divisible by 2" };
            even.Children.AddRange(sourceCollection.Where(x => x % 2 == 0).Select(x => new Item { Text = x.ToString() }));
            odd.Children.AddRange(sourceCollection.Where(x => x % 2 != 0).Select(x => new Item { Text = x.ToString() }));

            outputCollection.Add(odd);
            outputCollection.Add(even);
            return outputCollection;
        }

        public object ConvertBack(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
        {
            throw new NotImplementedException();
        }
    }
    public class Item
    {
        public string Text { get; set; }
        public List<Item> Children { get; set; }
        public Item()
        {
            Children = new List<Item>();
        }
    }

Xaml:

<Window x:Class="TreeViewAsync.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:local="clr-namespace:TreeViewAsync"
        Title="MainWindow" Height="350" Width="525">
    <Window.Resources>
        <local:TreeConverter x:Key="treeConverter"/>
    </Window.Resources>
    <Grid>
        <TreeView Name="treeView" 
                  ItemsSource="{Binding TreeViewSource, Converter={StaticResource treeConverter}}">
            <TreeView.ItemTemplate>
                <HierarchicalDataTemplate DataType="{x:Type local:Item}" ItemsSource="{Binding Children}">
                    <TextBlock Text="{Binding Text}" />
                </HierarchicalDataTemplate>
            </TreeView.ItemTemplate>
        </TreeView>
    </Grid>
</Window>

这个link表示应该在 ItemsSource 上使用 DataTemplate 而不是 Converter。但由于模板是单独应用于每个项目的，它如何创建完整的树呢？

再次强调:ViewModel 不需要数据是树，因此不提供树结构，而仅提供平面列表。为了方便起见， View 希望将其显示为树。 由于 MVVM，我们希望避免 View 中的代码隐藏。

Answer 1

A TreeView now wants to transform this list into a tree with two root nodes The ViewModel does not need the data to be a tree and therefore does not provide a treestructure but a flat list only

您误解了 View 模型概念。如果 View 需要分层数据，那么 View 模型的职责就是使数据分层并准备好与 View 绑定(bind)。

But since the template is applied to each item individually

通常，模板适用于任何类型的项目。

public sealed class NestedItemsViewModel
{
    public string Name { get; set; }
    public int[] Items { get; set; }
}

public sealed class ViewModel
{
    private readonly List<int> list = new List<int> { 1, 2, 3, 4 };
    private NestedItemsViewModel[] items;

    public NestedItemsViewModel[] Items
    {
        get
        {
            if (items == null)
            {
                items = new[]
                {
                    new NestedItemsViewModel
                    { 
                        Name = "Even",
                        Items = list.Where(x => x % 2 == 0).ToArray() 
                    },
                    new NestedItemsViewModel
                    { 
                        Name = "Odd",
                        Items = list.Where(x => x % 2 != 0).ToArray() 
                    },                        
                };
            }
            return items;
        }
    }
}

XAML:

<!-- Assuming, that TreeView.DataContext = new ViewModel()  -->
<TreeView ItemsSource="{Binding Items, IsAsync=True}">
    <TreeView.Resources>
        <HierarchicalDataTemplate DataType="{x:Type local:NestedItemsViewModel}" ItemsSource="{Binding Items}">
            <TextBlock Text="{Binding Name}"/>
        </HierarchicalDataTemplate>
        <!-- If nested items will be more complex, then here will be their data template(s) -->
    </TreeView.Resources>
</TreeView>