Prevent flickering of visible UICollectionView cells

Question

I am using an infinite scrolling UICollectionView which is bound to a custom ObservableCollection which is in the ViewModel. The cell has only a UITextView which is databound to a string attribute in the model object.

When I start scrolling down I am displaying more items by calling the AddRange on the OservableCollection.

public class MyCollection<T> : ObservableCollection<T>
{
    public MyCollection()
        : base()
    {
    }

    public MyCollection(IEnumerable<T> collection)
        : base(collection)
    {
    }

    public MyCollection(List<T> list)
        : base(list)
    {
    }

    public void AddRange(IEnumerable<T> range)
    {
        foreach (var item in range)
        {             
            Items.Add(item);
        }

        var dispatcher = Mvx.Resolve<IMvxMainThreadDispatcher>();
        dispatcher.RequestMainThreadAction(() =>
        {
            this.OnPropertyChanged(new PropertyChangedEventArgs("Count"));
            this.OnPropertyChanged(new PropertyChangedEventArgs("Item[]"));
            this.OnCollectionChanged(new NotifyCollectionChangedEventArgs(NotifyCollectionChangedAction.Add, range));
        });
    }
}

The new items are added successfully, however once the collectionview get´s reloaded from the notification change event the visible cell items may flicker and show the wrong data for a split second. Is it possible to prevent the flickering of the visible cells when loading more data into the UICollectionView?

Are you reloading the whole collectionview? Don't do that. Just add the news cells — Paulw11
@Paulw11 thanks for the suggestions. Yes, I asume the propertyChangeEvent triggers reloading of the whole collectionview. Do you know how I can only trigger the reload of the new cells which are not visible? I could call collectionView.ReloadItems(indexPaths[]) but I don't know how I can retrieve the indexpaths for the new items. — doorman

pnavk pnavk · Accepted Answer · 2016-07-29T23:09:29

You can create the IndexPath[] array to pass into collectionView.ReloadItems(indexPaths[]) by doing this:

public void AddRange(IEnumerable<T> range)
{
    ...
    ...
    ...

    var originalCount = Count;

    foreach (var item in range)
    {             
        Items.Add(item);
    }

    List<NSIndexPath> reloadPaths = new List<NSIndexPath>();
    for (int i = originalCount; i < Count-1; i++) {
        reloadPaths.Add(NSIndexPath.FromRowSection(i, 0));
    }

    ...
    ...
    ...
}

Then you can execute collectionView.ReloadItems(reloadPaths.ToArray())

Prevent flickering of visible UICollectionView cells

1 Answers