ListView将演员表项目绑定到ListViewItem

Question

I have a ListView bound to an instance of data

ObservalbeCollection<ActivityItem> ActivityItems

            <ListView
                x:Name="ActivityItemsList"
                ItemsSource="{Binding ActivityItems}"
                ItemTemplate="{StaticResource Herke80ItemTemplate}"
                Header="{Binding DateFilterListBox.SelectedItem}" />

When I run a filter, I want to select the ListViewItem bound to the ActivityItem within the ListView, and change its visibility depending on the filter selected.

I was doing this by keeping another ObservableCollection instance, meaning the data instance was duplicated. I then removed or added items accordingly. This took up a lot of loading time. So I figured I'd try keep to one binding, and disable or enable the UI elements.

        foreach (ActivityItem activityItem in ActivityItemsList.Items)
        {
            if (activityItem == null) continue;

            var index = ActivityItemsList.Items.IndexOf(activityItem);

            (ActivityItemsList.Items[index] as ListViewItem).Visibility = Visibility.Collapsed;

            int startComparer = DateTime.Compare(activityItem.Start, selectedStartDate);
            int endComparer = DateTime.Compare(selectedEndDate, activityItem.End);

            if (OverdueToggleSwitch.IsOn)
            {
                (ActivityItemsList.Items[index] as ListViewItem).Visibility = Visibility.Visible;
            }
            else
            {
                if (startComparer >= 0 && endComparer >= 0)
                {
                    (ActivityItemsList.Items[index] as ListViewItem).Visibility = Visibility.Visible;
                }
            }
        }

ex is a NullReferenceException, due to the fact that the ListViewItem is not actually a ListViewItem but an ActivityItem.

What is an alternative or the correct way of doing this?

Answer 1

I think you have over complicated it. If you use Snoop you can see that when you bind a ListView to a collection via ItemSource it ends up populated with a collection of ListViewItems where each ListViewItem has two things.

1. a DataContext set to the data (eg ActivityItem)
2. a ContentPresenter filled with the controls defined in your DataTemplate

This means that in theory you can browse both collections

from x in listView.Items select x as ListViewItem

or

from x in listView.Items select x.DataContext as ActivityItem

However if you simply want a filtered list then can I suggest changing your binding,

 <ListView
            x:Name="ActivityItemsList"
            ItemsSource="{Binding FilteredItems}"

and

public class MyViewModel :INotifyPropertyChanged    
{
   public IEnumerable<ActivityItem> AllItems {get;set;} //needs to NotifyPropertyChanged(FilteredItems)
   public Func<ActivityItem, bool> Filter { get;set;} //needs to NotifyPropertyChanged(FilteredItems)
   public IEnumerable<ActivityItem> FilteredItems { get { return from x in AllItems where Filter(x) select x; }}
}

If you are still struggling with performance issues then have a quick read through Bea Costa's series on TreeView performance part one , part two and part three . It might help you to get WPF doing all the hard work for you.