为什么我的 ListView 在使用分组时不显示任何内容？

Question

我正在尝试使用启用分组的 ListView 创建一个简单的过滤器菜单。目前我正在通过从父页面生成一个模式页面来做到这一点，该页面传递了一个我需要填充菜单的对象，其中包含一些按钮和列表页面。然而，我什至无法让最简单的分组 ListView 出现。我已经按照教程here 到达了我所在的位置。

我将包含 XAML、代码隐藏和过滤器页面的 ViewModel，以及教程中描述的过滤器组和过滤器选项的模型。请注意，虽然页面在我的代码中传递了一个对象，但我还没有以任何方式使用它来发布我在这里发布的内容。

XAML

    <ContentPage xmlns="http://xamarin.com/schemas/2014/forms"
             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
             xmlns:d="http://xamarin.com/schemas/2014/forms/design"
             xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
             mc:Ignorable="d"
             x:Class="MyProject.Views.FilterMenu">
        <ContentPage.Content>

            <Grid>
                <Grid.RowDefinitions>
                    <RowDefinition Height="Auto"/>
                    <RowDefinition Height="Auto"/>
                    <RowDefinition Height="*"/>
                    <RowDefinition Height="Auto"/>
                </Grid.RowDefinitions>
                <Grid.ColumnDefinitions>
                    <ColumnDefinition />
                    <ColumnDefinition />
                </Grid.ColumnDefinitions>

                <Button Text="Close"
                        Grid.Column="0"
                        Grid.Row="0"
                        HorizontalOptions="FillAndExpand"
                        Clicked="CloseButton_OnClicked"/>

                <Button Text="Clear Filter"
                        Grid.Column="1"
                        Grid.Row="0"
                        HorizontalOptions="FillAndExpand"
                        Clicked="ClearButton_OnClicked"/>

                <Label Text="Filter By"
                       Grid.Column="0"
                       Grid.ColumnSpan="2"
                       Grid.Row="1"
                       VerticalOptions="FillAndExpand" 
                       HorizontalOptions="StartAndExpand"
                       HorizontalTextAlignment="Start"/>


                <StackLayout Grid.Column="0"
                             Grid.ColumnSpan="2"
                             Grid.Row="2">
                    <ListView ItemsSource="{Binding FilterOptionList}"
                              IsGroupingEnabled="True"
                              GroupDisplayBinding="{Binding Name}">
                        <ListView.ItemTemplate>
                            <DataTemplate>
                                <TextCell Text="{Binding Name}"></TextCell>
                            </DataTemplate>
                        </ListView.ItemTemplate>
                    </ListView>
                </StackLayout>
                <Button
                    Text="Apply Filter"
                    Grid.Column="0"
                    Grid.ColumnSpan="2"
                    Grid.Row="3"
                    Clicked="ApplyFilterButton_OnClicked"/>
            </Grid>

        </ContentPage.Content>
    </ContentPage>

代码背后

using System;
    using MyProject.Models;
    using MyProject.ViewModels;
    using MvvmHelpers;
    using Xamarin.Forms;
    using Xamarin.Forms.Xaml;
    
    namespace MyProject.Views
    {
        [XamlCompilation(XamlCompilationOptions.Compile)]
        public partial class FilterMenu : ContentPage
        {
            public FilterMenu(ObservableRangeCollection<Unit> units)
            {
                InitializeComponent();
                BindingContext = new FilterMenuViewModel(Navigation, units);
            }
    
            private void CloseButton_OnClicked(object sender, EventArgs e)
            {
                (BindingContext as FilterMenuViewModel)?.Close();
            }
    
            private void ClearButton_OnClicked(object sender, EventArgs e)
            {
                (BindingContext as FilterMenuViewModel)?.ClearFilter();
            }
    
            private void ApplyFilterButton_OnClicked(object sender, EventArgs e)
            {
                (BindingContext as FilterMenuViewModel)?.ApplyFilter();
            }
        }
    }

视图模型

using System;
using System.Collections.Generic;
using System.Collections.ObjectModel;
using System.ComponentModel;
using System.Diagnostics;
using System.Linq;
using System.Runtime.CompilerServices;
using System.Text;
using MyProject.Annotations;
using MyProject.Models;
using Microsoft.Extensions.Options;
using Xamarin.Forms;

namespace MyProject.ViewModels
{
    public class FilterMenuViewModel : INotifyPropertyChanged
    {
        public ObservableCollection<Unit> Units { get; private set; }
        public ObservableCollection<FilterOptionGroup> FilterOptionList = new ObservableCollection<FilterOptionGroup>();
        private readonly INavigation _navigation;
        public event PropertyChangedEventHandler PropertyChanged;       

        public FilterMenuViewModel(INavigation navigation, ObservableCollection<Unit> units )
        {
            _navigation = navigation;
            this.Units = units;

            PopulateCategories();
            FilterOptionList.Add(new FilterOptionGroup("Group One", new []
            {
                new FilterOption
                {
                    Name = "Option One", IsFiltered = false
                },
                new FilterOption
                {
                    Name = "Option One", IsFiltered = false
                }
            }));

            FilterOptionList.Add(new FilterOptionGroup("Group Two", new[]
            {
                new FilterOption
                {
                    Name = "Option One", IsFiltered = false
                },
                new FilterOption
                {
                    Name = "Option Two", IsFiltered = false
                },
                new FilterOption
                {
                    Name = "Option Three", IsFiltered = false
                }
            }));
        }

        [NotifyPropertyChangedInvocator]
        protected virtual void OnPropertyChanged([CallerMemberName] string propertyName = null)
        {
            PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));

        }

        public async void Close()
        {
            await _navigation.PopModalAsync();
        }

        public void ClearFilter()
        {
            Debug.WriteLine("Clear Filter Clicked.");
        }

        public void ApplyFilter()
        {
            Debug.WriteLine("Apply Filter Clicked.");
        }
    }
}

过滤选项 GROUP 集合

using System.Collections.ObjectModel;
using System.Text;

namespace MyProject.Models
{
    public class FilterOptionGroup : ObservableCollection<FilterOption>
    {
        public string Name { get; private set; }

        public FilterOptionGroup(string name)
            : base()
        {
            Name = name;
        }

        public FilterOptionGroup(string name, IEnumerable<FilterOption> source)
            : base(source)
        {
            Name = name;
        }
    }
}

过滤选项类

using System;
using System.Collections.Generic;
using System.Text;

namespace DormRemote.Models
{
    public class FilterOption
    {
        private string _name;
        private bool _isFiltered;

        public string Name
        {
            get { return _name; }
            set { _name = value; }
    }

        public bool IsFiltered
        {
            get => _isFiltered;
            set => _isFiltered = value;
        }
    }
}

据我所知，我的绑定设置正确，因为我的按钮打印了相应的调试文本，如果我用一个简单的 ListView 替换当前的 ListView 而没有启用分组，并将它绑定到一个简单的 List 它会正确显示。

此外，我已经测试了 FilterOptionList 的填充是否正确，因为我已使用调试代码打印出每个组的内容。

我不确定我是否只是遗漏了一些东西，或者本教程中的实现是否不起作用。任何帮助将不胜感激。

Answer 1

您正在绑定到FilterOptionList，这不是您的 VM 的公共属性。您只能绑定到公共属性。

C# 属性有 get; 和/或 set; 否则它们只是字段。