如何在ViewModel中获取CollectionView的SelectedItems？

问题根源 #

CollectionView 的 SelectedItems 属性类型为 IList<object>，双向绑定时：

1. 直接绑定 ObservableCollection<User> 会因类型不匹配报错
2. 原转换器的 ConvertBackTo 逻辑错误：CollectionView 返回的选中项集合并非 ObservableCollection<User>，而是内部私有集合，value as ObservableCollection<User> 会返回 null，导致 ViewModel 的 SelectedMembers 始终为空

方案一：修正类型转换器（推荐程度：★★★） #

重写转换器，正确处理 IList<object> 和 ObservableCollection<User> 之间的双向转换：

using CommunityToolkit.Maui.Converters;
using System.Collections.ObjectModel;
using System.Globalization;
using System.Linq;

namespace Portal.ValueConverters
{
    internal class SelectedItemsConverter : BaseConverter<ObservableCollection<User>, IList<object>>
    {
        public override IList<object> DefaultConvertReturnValue => [];
        public override ObservableCollection<User> DefaultConvertBackReturnValue => [];

        // ViewModel → View：将User集合转为object列表
        public override IList<object> ConvertFrom(ObservableCollection<User> value, CultureInfo culture)
        {
            return value?.Cast<object>().ToList() ?? [];
        }

        // View → ViewModel：将选中的object列表转为User集合
        public override ObservableCollection<User> ConvertBackTo(IList<object> value, CultureInfo culture)
        {
            var selectedUsers = new ObservableCollection<User>();
            if (value != null)
            {
                foreach (var item in value.OfType<User>())
                {
                    selectedUsers.Add(item);
                }
            }
            return selectedUsers;
        }
    }
}

XAML 中保持原绑定逻辑，确保转换器已在资源字典注册：

<ResourceDictionary>
    <valueConverters:SelectedItemsConverter x:Key="UserObservableCollectionToIListOfObjectsConverter" />
</ResourceDictionary>

方案二：直接使用 IList<object> 并封装转换属性（推荐程度：★★★★★） #

无需转换器，直接绑定 IList<object>，在 ViewModel 中封装一个只读属性获取强类型的选中用户集合：

public partial class SelectGroupMembersViewModel:ObservableObject
{
    private readonly IUserFriendshipService _userFriendshipService;

    public Command GetUserFriendsCommand { get; }
    public Command GetSelectedMembersCommand { get; }
    
    // 直接绑定CollectionView的SelectedItems
    public IList<object> SelectedMembers { get; set; } = [];
    
    // 封装强类型的选中用户集合
    public IEnumerable<User> SelectedUsers => SelectedMembers.OfType<User>();
    
    public ObservableCollection<User> Friends { get; }

    public SelectGroupMembersViewModel(IUserFriendshipService userFriendshipService)
    {
        _userFriendshipService = userFriendshipService;
        GetUserFriendsCommand = new(async () => await GetUserFriendsAsync());
        GetSelectedMembersCommand = new(async () => await GetSelectedMembersAsync());
        Friends = [];
    }

    public async Task GetUserFriendsAsync()
    {
        IList<User> userFriends = await _userFriendshipService.GetCurrentUserFriendsAsync();
        Friends.Clear();
        foreach (User friend in userFriends)
            Friends.Add(friend);
    }

    public async Task GetSelectedMembersAsync()
    {
        // 使用强类型集合处理选中项
        var selectedUsers = SelectedUsers.ToList();
        // 后续业务逻辑...
    }
}

XAML 简化绑定（移除转换器）：

<CollectionView SelectedItems="{Binding SelectedMembers, Mode=TwoWay}" 
                ItemsSource="{Binding Friends}" 
                SelectionMode="Multiple" 
                BackgroundColor="#222">
    <CollectionView.ItemTemplate>
        <DataTemplate x:DataType="models:User">
            <HorizontalStackLayout Padding="20">
                <ImageButton Source="{Binding ImageSource}" CornerRadius="25" WidthRequest="50" HeightRequest="50" HorizontalOptions="Center"/>
                <Label Text="{Binding Name}" FontSize="Title" Margin="5" VerticalOptions="Center"/>
            </HorizontalStackLayout>
        </DataTemplate>
    </CollectionView.ItemTemplate>
</CollectionView>

方案三：使用 Behavior 监听选中事件（推荐程度：★★★） #

通过自定义 Behavior 监听 SelectionChanged 事件，直接更新 ViewModel 的 ObservableCollection<User>：

using Microsoft.Maui.Controls;
using System.Collections.ObjectModel;

namespace Portal.Behaviors
{
    public class CollectionViewSelectionBehavior : Behavior<CollectionView>
    {
        public static readonly BindableProperty SelectedItemsProperty =
            BindableProperty.Create(nameof(SelectedItems), typeof(ObservableCollection<User>), typeof(CollectionViewSelectionBehavior), new ObservableCollection<User>());

        public ObservableCollection<User> SelectedItems
        {
            get => (ObservableCollection<User>)GetValue(SelectedItemsProperty);
            set => SetValue(SelectedItemsProperty, value);
        }

        protected override void OnAttachedTo(CollectionView bindable)
        {
            base.OnAttachedTo(bindable);
            bindable.SelectionChanged += OnSelectionChanged;
        }

        protected override void OnDetachingFrom(CollectionView bindable)
        {
            base.OnDetachingFrom(bindable);
            bindable.SelectionChanged -= OnSelectionChanged;
        }

        private void OnSelectionChanged(object sender, SelectionChangedEventArgs e)
        {
            SelectedItems.Clear();
            foreach (var item in e.CurrentSelection.OfType<User>())
            {
                SelectedItems.Add(item);
            }
        }
    }
}

XAML 中绑定 Behavior：

<CollectionView ItemsSource="{Binding Friends}" 
                SelectionMode="Multiple" 
                BackgroundColor="#222">
    <CollectionView.Behaviors>
        <behaviors:CollectionViewSelectionBehavior SelectedItems="{Binding SelectedMembers, Mode=TwoWay}" />
    </CollectionView.Behaviors>
    <CollectionView.ItemTemplate>
        <!-- 模板内容不变 -->
    </CollectionView.ItemTemplate>
</CollectionView>

ViewModel 保持原 ObservableCollection<User> SelectedMembers 定义即可。

内容的提问来源于stack exchange，提问作者Hossam Fares