在更新绑定属性更改(如果已更改)之前，请与用户确认

Question

我有一个下拉框，可以更改屏幕上显示的内容。如果IsDirty变量为true，如果用户试图更改下拉菜单，我希望弹出一个通知用户存在未保存的更改。

下拉框与属性绑定，我试图将消息框放入属性集中，但效果不佳。似乎每次我试图拦截机会并修改它时，结果都是属性上的集合再次启动。

我尝试了一些我在网上找到的解决方案，但没有成功，有人有什么建议吗？

我最近的尝试是附加一个行为，但它有同样的问题，即多次激发，而且值无论如何都发生了变化。

我正在使用.net 4.5

 public class CancellableSelectionBehavior : Behavior<ComboBox>
    {
    protected override void OnAttached()
    {
        base.OnAttached();
        AssociatedObject.SelectionChanged += OnSelectionChanged;
    }
    protected override void OnDetaching()
    {
        base.OnDetaching();
        AssociatedObject.SelectionChanged -= OnSelectionChanged;
    }
    #region Properties
    #region IsDirtyCheck
    public static readonly DependencyProperty IsDirtyCheckProperty =
        DependencyProperty.Register(
            Reflection.GetPropertyName<CancellableSelectionBehavior>(m => m.IsDirtyCheck),
            typeof(bool),
            typeof(CancellableSelectionBehavior));
    public bool IsDirtyCheck
    {
        get { return (bool)GetValue(IsDirtyCheckProperty); }
        set { SetValue(IsDirtyCheckProperty, value); }
    }
    #endregion
    #region Selected Value
    public static readonly DependencyProperty SelectedValueProperty =
        DependencyProperty.Register(Reflection.GetPropertyName<CancellableSelectionBehavior>(m => m.SelectedValue),
                                    typeof(object), typeof(CancellableSelectionBehavior),
                                    new FrameworkPropertyMetadata(null,
                                                                  FrameworkPropertyMetadataOptions
                                                                      .BindsTwoWayByDefault, OnSelectedItemChanged));
    public object SelectedValue
    {
        get { return GetValue(SelectedValueProperty); }
        set { SetValue(SelectedValueProperty, value); }
    }
    #endregion
    #endregion
    private static void OnSelectedItemChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
    {
        var behavior = (CancellableSelectionBehavior)d;
        var selector = behavior.AssociatedObject;
        if (e.OldValue != null)
        {
            if (e.OldValue != e.NewValue)
            {
                if (behavior.IsDirtyCheck)
                {
                    string message = string.Format(UserMessages.UnSavedFaultPlygon, Environment.NewLine);
                    if (ServiceLocationContext.ServiceFinder.GetService<IMessageBox>()
                                              .Show(new MessageDialogOptions(message,
                                                                             UserMessages
                                                                                 .FaultPlygons,
                                                                             MessageDialogIcon
                                                                                 .Exclamation,
                                                                             MessageDialogButtons
                                                                                 .YesNo)) == MessageDialogResult.Yes)
                    {
                        selector.SelectedValue = e.NewValue;
                    }
                    else
                    {
                        selector.SelectedValue = e.OldValue;
                    }
                }
                else
                {
                    selector.SelectedValue = e.NewValue;
                }
            }
        }
    }
    /// <summary>
    /// Called when the associated selector's selection is changed.
    /// Tries to assign it to the <see cref="SelectedItem"/> property.
    /// If it fails, updates the selector's with  <see cref="SelectedItem"/> property's current value.
    /// </summary>
    private void OnSelectionChanged(object sender, SelectionChangedEventArgs e)
    {
        if ((e.AddedItems == null || e.AddedItems.Count == 0)) return;
        SelectedValue = AssociatedObject.SelectedValue;
    }
}


<ribbonDropDown:RibbonDropDownListView
                            Label="{Binding Path=MapModel.SelectedPostingItemName}"
                            Image="{Binding Path=LargeImageSource, ElementName=_postingTypeMenuButton}" 
                            Width="100"
                            ItemsSource="{Binding Path=MapModel.PostingHorizonItems}"                            
                            SelectedValuePath="Key"
                            Visibility="{Binding Path=MapModel.SelectedTypeKey, Converter={StaticResource EnumMatchToVisibilityConverter}, ConverterParameter={x:Static map2:MapPostingType.Horizon}}"
                            ToolTip="{Binding Path=MapModel.SelectedPostingItemDescription}">
                            <i:Interaction.Behaviors>
                                <common:CancellableSelectionBehavior IsDirtyCheck="{Binding Path=MapControlViewModel.IsFaultPolygonDirty}"
                                                                     SelectedValue="{Binding Path=MapModel.SelectedHorizonPostingItem}"/>
                            </i:Interaction.Behaviors>

Answer 1

我可能过于简化了答案，但我认为问题是您正在捕获onselectionchanged事件，然后更改该代码块内的选择。

这种行为看起来很巧妙，但我认为你必须把它联系起来，而不是基于这种事件。

我会这样解决问题的。（如果你认为可能会抛出异常，也许可以尝试使用finally重新附加事件。）

private void OnSelectionChanged(object sender, SelectionChangedEventArgs e)
{
    if ((e.AddedItems == null || e.AddedItems.Count == 0)) return;
    AssociatedObject.SelectionChanged -= OnSelectionChanged;
    SelectedValue = AssociatedObject.SelectedValue;
    AssociatedObject.SelectionChanged += OnSelectionChanged;
}