删除集合中的项目时,ObservableCollection依赖项属性不会更新

Question

我在控件上有一个ObservableCollection类型的附加属性.如果我在集合中添加或删除项目,则ui不会更新.但是,如果我用新的替换集合,则ui会更新ViewModel.

有人能给我一个我在Dependency对象中需要做什么的例子,以便它可以处理集合中的变化吗？

下面列出了依赖项对象的一部分:

public class RadCalendarBehavior : DependencyObject
{
private static void OnSpecialDaysChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
{
  var calendar = d as RadCalendar;
  if (e.NewValue != null)
  {
    calendar.DayTemplateSelector = new SpecialDaySelector((ObservableCollection<DateTime>)e.NewValue, GetSpecialDayTemplate(d));
  }
}

public static ObservableCollection<DateTime> GetSpecialDays(DependencyObject obj)
{
  return (ObservableCollection<DateTime>)obj.GetValue(SpecialDaysProperty);
}

public static void SetSpecialDays(DependencyObject obj, ObservableCollection<DateTime> value)
{
  obj.SetValue(SpecialDaysProperty, value);
}

public static readonly DependencyProperty SpecialDaysProperty =
    DependencyProperty.RegisterAttached("SpecialDays", typeof(ObservableCollection<DateTime>), typeof(RadCalendarBehavior), new UIPropertyMetadata(null, OnSpecialDaysChanged));
}
}

我知道我需要注册该集合已经更改,但我不确定如何在依赖项属性中执行此操作

Answer 1

集合中的更改不会触发OnSpecialDaysChanged回调,因为依赖项属性的值未更改.如果您需要做出反应以检测集合的更改,则需要CollectionChanged手动处理事件事件:

private static void OnSpecialDaysChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
{
  var calendar = d as RadCalendar;

  if (e.OldValue != null)
  {
    var coll = (INotifyCollectionChanged)e.OldValue;
    // Unsubscribe from CollectionChanged on the old collection
    coll.CollectionChanged -= SpecialDays_CollectionChanged;
  }

  if (e.NewValue != null)
  {
    var coll = (ObservableCollection<DateTime>)e.NewValue;
    calendar.DayTemplateSelector = new SpecialDaySelector(coll, GetSpecialDayTemplate(d));
    // Subscribe to CollectionChanged on the new collection
    coll.CollectionChanged += SpecialDays_CollectionChanged;
  }
}

private static void SpecialDays_CollectionChanged(object sender, NotifyCollectionChangedEventArgs e)
{
    // handle CollectionChanged
}

Answer 2

如果您有集合类型依赖属性，请记住以下几点：

如果您的属性是引用类型，则依赖属性元数据中指定的默认值不是每个实例的默认值；相反，它是适用于该类型的所有实例的默认值。[...]
要纠正此问题，您必须将集合依赖项属性值重置为唯一实例，作为类构造函数调用的一部分。

（请参阅 MSDN集合类型依赖属性）

回答 Sam 的问题（我刚刚遇到了同样的问题）：

使您的 CollectionChanged 处理程序成为非静态的，并在实例级别取消订阅/重新订阅。

private static void OnSpecialDaysChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
{
  var calendar = (RadCalendar)d;

  if (e.OldValue != null)
  {
    var coll = (INotifyCollectionChanged)e.OldValue;
    // Unsubscribe from CollectionChanged on the old collection of the DP-instance (!)
    coll.CollectionChanged -= calendar.SpecialDays_CollectionChanged;
  }

  if (e.NewValue != null)
  {
    var coll = (ObservableCollection<DateTime>)e.NewValue;
    calendar.DayTemplateSelector = new SpecialDaySelector(coll, GetSpecialDayTemplate(d));
    // Subscribe to CollectionChanged on the new collection of the DP-instance (!)
    coll.CollectionChanged += calendar.SpecialDays_CollectionChanged;
  }
}

private void SpecialDays_CollectionChanged(object sender, NotifyCollectionChangedEventArgs e)
{
    // handle CollectionChanged on instance-level
}

Answer 3

这只是为了增加托马斯的答案.在我的代码中,我通过创建一个类似于下面的处理程序对象来与DependencyObject的属性进行交互:

private static void OnSpecialDaysChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
{
    var action = new NotifyCollectionChangedEventHandler(
            (o, args) =>
                {
                    var calendar = d as RadCalendar;

                    if (calendar!= null)
                    {
                        // play with calendar's properties/methods
                    }
                });

    if (e.OldValue != null)
    {
       var coll = (INotifyCollectionChanged)e.OldValue;
       // Unsubscribe from CollectionChanged on the old collection
       coll.CollectionChanged -= action;
    }

    if (e.NewValue != null)
    {
       var coll = (ObservableCollection<DateTime>)e.NewValue;
       // Subscribe to CollectionChanged on the new collection
       coll.CollectionChanged += action;
    }
}

希望这对某人有帮助.