如何在启用ChangeTrackEnabled时避免ListView.SelectedItem被设为null

首先得搞清楚问题根源：当你给ReactiveList<Attachment>开启ChangeTrackEnabled后，ReactiveUI会为集合里的每个Attachment对象创建ChangeTrackingItem包装器。这会导致ListView在判断选中项是否存在于集合中时，因为引用或相等性判断的变化，找不到原来的选中项，最终把SelectedItem设为null。

下面是几个针对性的解决方案，按推荐程度排序：

1. 为Attachment类实现稳定的Equals和GetHashCode #

这是最根本的解决办法。ChangeTrackEnabled会监控对象的属性变化，如果你的Equals方法依赖可变属性，当属性变更时，对象的相等性会改变，ListView就会认为原来的选中项不在集合里了。

你应该基于对象的唯一标识（比如Id字段）来实现相等判断，示例代码如下：

public class Attachment : IEquatable<Attachment>
{
    public Guid Id { get; set; } // 假设这是对象的唯一标识
    // 其他属性...

    public bool Equals(Attachment other)
    {
        if (other is null) return false;
        return Id.Equals(other.Id);
    }

    public override bool Equals(object obj) => Equals(obj as Attachment);

    public override int GetHashCode() => Id.GetHashCode();
}

这样即使对象的其他属性变化，只要Id不变，ListView就能正确识别选中项。

2. 在ViewModel中手动维护选中项的引用 #

如果没法修改Attachment类的相等性实现，你可以在AttachmentsViewModel里监听集合的变更事件，手动保持选中项的引用有效。

假设你有一个SelectedAttachmentViewModel属性，代码示例：

public class AttachmentsViewModel : ReactiveObject, ISupportsActivation
{
    private ReactiveList<Attachment> _attachments;
    public ReactiveList<Attachment> Attachments 
    { 
        get => _attachments; 
        set => this.RaiseAndSetIfChanged(ref _attachments, value); 
    }

    private IReactiveDerivedList<AttachmentViewModel> _attachmentViewModels;
    public IReactiveDerivedList<AttachmentViewModel> AttachmentViewModels 
    { 
        get => _attachmentViewModels; 
        set => this.RaiseAndSetIfChanged(ref _attachmentViewModels, value); 
    }

    private AttachmentViewModel _selectedAttachmentViewModel;
    public AttachmentViewModel SelectedAttachmentViewModel
    {
        get => _selectedAttachmentViewModel;
        set => this.RaiseAndSetIfChanged(ref _selectedAttachmentViewModel, value);
    }

    public ViewModelActivator Activator { get; } = new ViewModelActivator();

    public AttachmentsViewModel()
    {
        this.WhenActivated(disposables =>
        {
            // 初始化集合并开启ChangeTrackEnabled
            Attachments = new ReactiveList<Attachment> { ChangeTrackingEnabled = true };

            // 监听集合项的属性变更，维护选中项引用
            Attachments.ItemChanged.Subscribe(args =>
            {
                if (SelectedAttachmentViewModel?.Attachment == args.Sender)
                {
                    var matchingVm = AttachmentViewModels.FirstOrDefault(vm => vm.Attachment.Id == args.Sender.Id);
                    if (matchingVm != null)
                    {
                        SelectedAttachmentViewModel = matchingVm;
                    }
                }
            }).DisposeWith(disposables);

            // 监听集合变更，确保选中项存在于派生列表中
            Attachments.CollectionChanged.Subscribe(args =>
            {
                if (SelectedAttachmentViewModel != null && !AttachmentViewModels.Contains(SelectedAttachmentViewModel))
                {
                    var matchingVm = AttachmentViewModels.FirstOrDefault(vm => vm.Attachment.Id == SelectedAttachmentViewModel.Attachment.Id);
                    SelectedAttachmentViewModel = matchingVm ?? null;
                }
            }).DisposeWith(disposables);
        });
    }
}

3. 绑定SelectedItem时使用标识匹配 #

在View层绑定SelectedItem的时候，不要直接绑定引用，而是通过唯一标识来匹配对应的ViewModel。比如在WPF中，可以用SelectedValuePath和SelectedValue：

<ListView ItemsSource="{Binding AttachmentViewModels}"
          SelectedValuePath="Attachment.Id"
          SelectedValue="{Binding SelectedAttachmentId}"/>

然后在ViewModel中维护SelectedAttachmentId属性，并通过它来获取对应的SelectedAttachmentViewModel：

private Guid? _selectedAttachmentId;
public Guid? SelectedAttachmentId
{
    get => _selectedAttachmentId;
    set => this.RaiseAndSetIfChanged(ref _selectedAttachmentId, value);
}

// 在构造函数中绑定关联关系
this.WhenAnyValue(x => x.SelectedAttachmentId)
    .Select(id => id.HasValue ? AttachmentViewModels.FirstOrDefault(vm => vm.Attachment.Id == id.Value) : null)
    .BindTo(this, x => x.SelectedAttachmentViewModel)
    .DisposeWith(disposables);

这种方式完全避开了引用相等性的问题，非常可靠。

内容的提问来源于stack exchange，提问作者Mitkins