WPF ListView: Changing ItemsSource does not change ListView

Question

I am using a ListView control to display some lines of data. There is a background task which receives external updates to the content of the list. The newly received data may contain less, more or the same number of items and also the items itself may have changed.

The ListView.ItemsSource is bound to an OberservableCollection (_itemList) so that changes to _itemList should be visible also in the ListView.

_itemList = new ObservableCollection<PmemCombItem>();
_itemList.CollectionChanged += new NotifyCollectionChangedEventHandler(OnCollectionChanged);
L_PmemCombList.ItemsSource = _itemList;

In order to avoid refreshing the complete ListView I do a simple comparison of the newly retrieved list with the current _itemList, change items which are not the same and add/remove items if necessary. The collection "newList" contains newly created objects, so replacing an item in _itemList is correctly sending a "Refresh" notification (which I can log by using the event handler OnCollectionChanged of the ObservableCollection`)

Action action = () =>
{
    for (int i = 0; i < newList.Count; i++)
    {
        // item exists in old list -> replace if changed
        if (i < _itemList.Count)
        {
            if (!_itemList[i].SameDataAs(newList[i]))
                _itemList[i] = newList[i];
        }
        // new list contains more items -> add items
        else
            _itemList.Add(newList[i]);
     }
     // new list contains less items -> remove items
     for (int i = _itemList.Count - 1; i >= newList.Count; i--)
         _itemList.RemoveAt(i);
 };
 Dispatcher.BeginInvoke(DispatcherPriority.Background, action);

My problem is that if many items are changed in this loop, the ListView is NOT refreshing and the data on screen stay as they are...and this I don't understand.

Even a simpler version like this (exchanging ALL elements)

List<PmemCombItem> newList = new List<PmemCombItem>();
foreach (PmemViewItem comb in combList)
    newList.Add(new PmemCombItem(comb));

if (_itemList.Count == newList.Count)
    for (int i = 0; i < newList.Count; i++)
        _itemList[i] = newList[i];
else
{
    _itemList.Clear();
    foreach (PmemCombItem item in newList)
        _itemList.Add(item);
}

is not working properly

Any clue on this?

UPDATE

If I call the following code manually after updating all elements, everything works fine

OnCollectionChanged(new NotifyCollectionChangedEventArgs(NotifyCollectionChangedAction.Reset));

But of course this causes the UI to update everything which I still want to avoid.

Answer 1

After a change, you can use the following to refresh the Listview, it's more easy

listView.Items.Refresh();

Answer 2

This is what I had to do to get it to work.

MyListView.ItemsSource = null;
MyListView.ItemsSource = MyDataSource;

Answer 3

I know that's an old question, but I just stumbled upon this issue. I didn't really want to use the null assignation trick or the refresh for just a field that was updated.

So, after looking at MSDN, I found this article: https://learn.microsoft.com/en-us/dotnet/api/system.componentmodel.inotifypropertychanged?redirectedfrom=MSDN&view=netframework-4.7.2

To summarize, you just need the item to implement this interface and it will automatically detect that this object can be observed.

public class MyItem : INotifyPropertyChanged
{
    private string status;

    public string Status
    {
        get => status;
        set
        {
            OnPropertyChanged(nameof(Status));
            status = value;
        }
    }

    public event PropertyChangedEventHandler PropertyChanged;

    [NotifyPropertyChangedInvocator]
    protected virtual void OnPropertyChanged([CallerMemberName] string propertyName = null)
    {
        PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
    }
}

So, the event will be called everytime someone changes the Status. And, in your case, the listview will add a handler automatically on the PropertyChanged event.

This doesn't really handle the issue in your case (add/remove). But for that, I would suggest that you have a look at BindingList<T> https://learn.microsoft.com/en-us/dotnet/api/system.componentmodel.bindinglist-1?view=netframework-4.7.2

Using the same pattern, your listview will be updated properly without using any tricks.

Answer 4

You should not reset ItemsSource of ListView each time observable collection changed. Just set proper binding that will do your trick. In xaml:

<ListView ItemsSource='{Binding ItemsCollection}'
 ...
</ListView>

And in code-behind (suggest to use MVVM) property that will be responsible for holding _itemList:

public ObservableCollection<PmemCombItem> ItemsCollection
{
   get 
   {
      if (_itemList == null)
      {
          _itemList = new ObservableCollection<PmemCombItem>();
      }
      return _itemList;
   }
}

---

UPDATE: There is similar post which most probably will Answer your question: How do I update an ObservableCollection via a worker thread?

Answer 5

I found a way to do it. It is not really that great but it works.

YourList.ItemsSource = null;
// Update the List containing your elements (lets call it x)
YourList.ItemsSource = x;

this should refresh your ListView (it works for my UAP :) )

Answer 6

An alternative on Xopher's answer.

MyListView.ItemsSource = MyDataSource.ToList();

This refreshes the Listview because it's a other list.

Answer 7

Please check this answer: Passing ListView Items to Commands using Prism Library

List view Items needs to notify about changes (done is setter)

public ObservableCollection<Model.Step> Steps 
{
    get { return _steps; }
    set { SetProperty(ref _steps, value); }
} 

and UpdateSourceTrigger need to be set in xaml

<Image Source="{Binding ImageData, UpdateSourceTrigger=PropertyChanged}" />