WinUI中DataGrid选中项数量绑定至可见性失效问题求助

WinUI里DataGrid.SelectedItems.Count的绑定不会在选中项变化时自动更新，这是因为SelectedItems的Count属性不会触发属性更改通知，导致转换器只在初始化时执行一次，后续选中项变化时绑定目标不会更新。下面是几种可行的解决方法：

方法一：后台代码监听SelectionChanged事件 #

直接在后台处理选中变化事件，手动更新控件可见性：

1. XAML中给DataGrid添加事件：

<DataGrid x:Name="MyDataGrid" SelectionChanged="MyDataGrid_SelectionChanged" />

2. 后台实现事件处理：

private void MyDataGrid_SelectionChanged(object sender, SelectionChangedEventArgs e)
{
    // TargetControl是你要控制可见性的控件名称
    TargetControl.Visibility = MyDataGrid.SelectedItems.Count == 1 ? Visibility.Visible : Visibility.Collapsed;
}

方法二：用附加属性封装选中计数（适配MVVM） #

创建一个附加属性，自动监听DataGrid的选中变化并更新计数，这样就能正常绑定：

1. 编写附加属性类：

public static class DataGridExtensions
{
    public static readonly DependencyProperty SelectedCountProperty =
        DependencyProperty.RegisterAttached(
            "SelectedCount",
            typeof(int),
            typeof(DataGridExtensions),
            new PropertyMetadata(0));

    public static int GetSelectedCount(DependencyObject obj) => (int)obj.GetValue(SelectedCountProperty);
    public static void SetSelectedCount(DependencyObject obj, int value) => obj.SetValue(SelectedCountProperty, value);

    public static readonly DependencyProperty TrackSelectedCountProperty =
        DependencyProperty.RegisterAttached(
            "TrackSelectedCount",
            typeof(bool),
            typeof(DataGridExtensions),
            new PropertyMetadata(false, OnTrackSelectedCountChanged));

    public static bool GetTrackSelectedCount(DependencyObject obj) => (bool)obj.GetValue(TrackSelectedCountProperty);
    public static void SetTrackSelectedCount(DependencyObject obj, bool value) => obj.SetValue(TrackSelectedCountProperty, value);

    private static void OnTrackSelectedCountChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
    {
        if (d is DataGrid dataGrid)
        {
            if ((bool)e.NewValue)
            {
                dataGrid.SelectionChanged += DataGrid_SelectionChanged;
                UpdateSelectedCount(dataGrid);
            }
            else
            {
                dataGrid.SelectionChanged -= DataGrid_SelectionChanged;
            }
        }
    }

    private static void DataGrid_SelectionChanged(object sender, SelectionChangedEventArgs e)
    {
        if (sender is DataGrid dataGrid) UpdateSelectedCount(dataGrid);
    }

    private static void UpdateSelectedCount(DataGrid dataGrid)
    {
        SetSelectedCount(dataGrid, dataGrid.SelectedItems.Count);
    }
}

2. XAML中使用：

<!-- 先引入命名空间，local是你的项目命名空间 -->
<DataGrid x:Name="MyDataGrid" local:DataGridExtensions.TrackSelectedCount="True" />
<YourControl Visibility="{Binding ElementName=MyDataGrid, Path=(local:DataGridExtensions.SelectedCount), Converter={StaticResource CountToVisibilityConverter}}" />

方法三：结合ViewModel的MVVM方案 #

如果用MVVM模式，可以在ViewModel中维护选中计数，通过事件更新：

1. ViewModel实现：

public class MainViewModel : INotifyPropertyChanged
{
    private int _selectedItemsCount;
    public int SelectedItemsCount
    {
        get => _selectedItemsCount;
        set
        {
            if (_selectedItemsCount != value)
            {
                _selectedItemsCount = value;
                OnPropertyChanged();
            }
        }
    }

    public event PropertyChangedEventHandler PropertyChanged;
    protected void OnPropertyChanged([CallerMemberName] string propertyName = null)
    {
        PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
    }
}

2. 后台页面绑定ViewModel并监听事件：

public MainPage()
{
    InitializeComponent();
    ViewModel = new MainViewModel();
    MyDataGrid.SelectionChanged += (s, e) => ViewModel.SelectedItemsCount = MyDataGrid.SelectedItems.Count;
}

public MainViewModel ViewModel { get; set; }

3. XAML绑定：

<YourControl Visibility="{x:Bind ViewModel.SelectedItemsCount, Converter={StaticResource CountToVisibilityConverter}, Mode=OneWay}" />

内容的提问来源于stack exchange，提问作者miniHessel