c# - 选择项目后，我的 c# ComboBox 不调用 getter。为什么？

Question

我有一个带有 SelectedItem 的组合框。如果我选择一个项目，我的 setter 会进行一些计算，也许我想将值重置为旧值。不幸的是，我的 View 没有刷新。

我有以下组合框:

<ComboBox BorderThickness="0" VerticalAlignment="Center" Margin="2,0"
  DisplayMemberPath="Name" 
  ItemsSource="{Binding ItemsVS.View}" 
  SelectedItem="{Binding SelectedItem, Mode=TwoWay}" >

</ComboBox>

这是我的 ViewModel 的属性:

private CollectionViewSource _itemsVS;
public CollectionViewSource ItemsVS
{
    get { return _itemsVS; }
    set
    {
        _itemsVS = value;
        if(PropertyChanged!=null)
            PropertyChanged(this, new PropertyChangedEventArgs("ItemsVS"));
    }
}


private ItemViewModel _selectedItem;
public ItemViewModel SelectedItem
{
    get
    { 
        // after setting the old value in the setter the getter is not called
        // and the view is not refreshed
        return _selectedItem; 
    }
    set
    {
        var old = _selectedItem;
        _selectedItem = value;
        // After some logic I want to have the old value!!!!!!!!!!!!!!
        _selectedItem = old;
        if (PropertyChanged != null)
            PropertyChanged(this, new PropertyChangedEventArgs("SelectedItem"));


    }
}

Answer 1

如果你想让 ComboBox 在设置新值后读回当前值，你需要添加一个“no op” Converter 到你的 有效地什么都不做的绑定(bind)。这是一个有用的小技巧，因为绑定(bind)通常不会检查实际应用的源值是否与绑定(bind)提供的新值相匹配。添加转换器会强制它进行检查。

public sealed class NoOpConverter : IValueConverter
{
    public static readonly NoOpConverter Instance = new NoOpConverter();

    public object Convert(
        object value,
        Type targetType,
        object parameter,
        CultureInfo culture)
    {
        return value;
    }

    public object ConvertBack(
        object value,
        Type targetType,
        object parameter,
        CultureInfo culture)
    {
        return value;
    }
}

<ComboBox SelectedItem="{Binding Path=SelectedItem, 
                                 Mode=TwoWay,
                                 Converter={x:Static NoOpConverter.Instance}}"
          ... />