我想这个可以：

using CommunityToolkit.Mvvm.ComponentModel;
using Microsoft.UI.Xaml;
using Microsoft.UI.Xaml.Controls;
using System;
using System.Collections.Generic;
using System.Collections.ObjectModel;
using System.Linq;

namespace TreeViewExample;

public class Visit
{
    public int Id { get; set; }
    public DateTime Date { get; set; }
    public string VisitNo { get; set; } = string.Empty;
    public string Details { get; set; } = string.Empty;
}

public partial class YearItem : ObservableObject
{
    public YearItem(int year)
    {
        Year = year;
    }

    [ObservableProperty]
    private int year;

    [ObservableProperty]
    private ObservableCollection<Visit> visits = new();
}

public class TreeViewTemplateSelector : DataTemplateSelector
{
    public DataTemplate? YearTemplate { get; set; }

    public DataTemplate? VisitTemplate { get; set; }

    protected override DataTemplate? SelectTemplateCore(object item)
    {
        return item switch
        {
            YearItem => YearTemplate,
            Visit => VisitTemplate,
            _ => throw new NotSupportedException(),
        };
    }
}

public partial class MainPageViewModel : ObservableObject
{
    [ObservableProperty]
    private ObservableCollection<YearItem> yearItems = new();

    public MainPageViewModel()
    {
        List<Visit> visits = new()
        {
            new Visit { Id = 1, Date = DateTime.Now.AddYears(-1).AddMonths(-3), VisitNo = "001", Details = "Visit 1" },
            new Visit { Id = 2, Date = DateTime.Now.AddYears(-1), VisitNo = "002", Details = "Visit 2" },
            new Visit { Id = 3, Date = DateTime.Now.AddMonths(-2), VisitNo = "003", Details = "Visit 3" },
            new Visit { Id = 4, Date = DateTime.Now.AddMonths(-1), VisitNo = "004", Details = "Visit 4" },
            new Visit { Id = 5, Date = DateTime.Now, VisitNo = "005", Details = "Visit 5" }
        };

        foreach (Visit visit in visits)
        {
            if (YearItems
                .Where(x => x.Year == visit.Date.Year)
                .FirstOrDefault() is not YearItem visitsViewModel)
            {
                visitsViewModel = new(visit.Date.Year);
                YearItems.Add(visitsViewModel);
            }

            visitsViewModel.Visits.Add(visit);
        }
    }
}

<Page
    x:Class="TreeViewExample.MainPage"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
    xmlns:local="using:TreeViewExample"
    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
    Background="{ThemeResource ApplicationPageBackgroundThemeBrush}"
    mc:Ignorable="d">
    <Page.DataContext>
        <local:MainPageViewModel x:Name="ViewModel" />
    </Page.DataContext>

    <Page.Resources>
        <local:TreeViewTemplateSelector x:Key="TreeViewTemplateSelector">
            <local:TreeViewTemplateSelector.YearTemplate>
                <DataTemplate x:DataType="local:YearItem">
                    <TreeViewItem
                        Content="{x:Bind Year}"
                        ItemsSource="{x:Bind Visits}" />
                </DataTemplate>
            </local:TreeViewTemplateSelector.YearTemplate>
            <local:TreeViewTemplateSelector.VisitTemplate>
                <DataTemplate x:DataType="local:Visit">
                    <TextBlock Text="{x:Bind VisitNo}" />
                </DataTemplate>
            </local:TreeViewTemplateSelector.VisitTemplate>
        </local:TreeViewTemplateSelector>
    </Page.Resources>

    <Grid>
        <TreeView
            ItemTemplateSelector="{StaticResource TreeViewTemplateSelector}"
            ItemsSource="{x:Bind ViewModel.YearItems, Mode=OneWay}" />
    </Grid>
</Page>