[Bug] No Data Virtualization in CollectionView

[jefffhaynes]

Description

CollectionView is being groomed as a replacement for ListView. However, due to a lack of real data virtualization it is not a viable option for many scenarios. While CollectionView does support "incremental data load", it is up to the consumer to swap new data in and is cumbersome to say the least. At a minimum, the eager loading of the entire data source is not expected behavior.

Steps to Reproduce

1. Define a data source:

   public class Source : IReadOnlyList<string>
   {
   public IEnumerator<string> GetEnumerator()
   {
       return Enumerable.Range(0, 100).Select(i =>
       {
           Debug.WriteLine($"Enumerator: {i}");
           return i.ToString();
       }).GetEnumerator();
   }
   
   IEnumerator IEnumerable.GetEnumerator()
   {
       return GetEnumerator();
   }
   
   public int Count => 100;
   
   public string this[int index]
   {
       get
       {
           Debug.WriteLine($"Indexer: {index}");
           return index.ToString();
       }
   }
   }

2. Assign the source to a CollectionView

Expected Behavior

Only visible data items should be retrieved. ListView does this when defining an IReadOnlyList data source, although even that is a seemingly arbitrary distinction.

Actual Behavior

All items are retrieved.

Basic Information

• Version with issue: 4.5.0.617
• Last known good version: N/A
• IDE: VS 2019 16.5.4
• Platform Target Frameworks:
  • iOS: 13.4

[BlueRaja]

Any news on this? CollectionView is basically unusable at this point