绑定到集合的最后N个元素

Question

I use wpf toolkit charts to display some data stored in a ObservableCollection . When there are more than N items stored in that collection only the last N items should be displayed (I cannot delete any items).

XAML

<chartingToolkit:LineSeries DependentValueBinding="{Binding DoubleValue,Converter={StaticResource DoubleValueConverter}}" IndependentValueBinding="{Binding Count}" ItemsSource="{Binding Converter={StaticResource DataSourceConverter}}"/>

DataSourceConverter

public object Convert(object value, System.Type targetType, object parameter, System.Globalization.CultureInfo culture)
    {
        ICollection<object> items = value as ICollection<object>;

        int N = 300;

        if (items != null)
        {
            return items.Skip(Math.Max(0, items.Count - N)).Take(N);
        }
        else
        {
            return value;
        }  
    }

ItemSource is bound to a ObservableCollection containing both "DoubleValue" and "Count". It seems like DataSourceConverter is called only once and not when my ObservableCollection is updated.

Answer 1

忘记转换器，在viewmodel类中创建一个新属性，该属性将返回最后300个项目（就像您现在在转换器中声明的一样），并绑定到该属性。

Answer 2

You could use an ICollectionView and set a Filter on it.

From your ObservableCollection, create a new CollectionView:

CollectionView topNItems = (CollectionView) CollectionViewSource.GetDefaultView(myObservableCollection);

Next, create the filter on your CollectionView:

topNItems.Filter += new FilterEventHandler(ShowOnlyTopNItems);

And finally the filter event:

private void ShowOnlyBargainsFilter(object sender, FilterEventArgs e)
{
    int n = 300;
    int listCount = myObservableCollection.Count;
    int indexOfItem = myObservableCollection.IndexOf(e.Item);
    e.Accepted = (listCount - indexOfItem) < n;
}

Now, bind your chart to the new topNItems instead of to the ObservableCollection.