How to paginate an ObservableCollection?

Question

I have a ListBox with way too many items in it and the UI is getting slower and slower (virtualization is on, etc). So I was thinking about displaying only the first 20 item

Answer 1

This facility is not directly available in the base ObservableCollecton class. You can extend the ObservableCollection and create a custom Collection which does this. You need to hide the original Collection inside this new class and based on a FromIndex and ToIndex give dynamically add the range of items to the class. Override InsertItem and RemoveItem. I am giving a not-tested version bellow. But please take this as just pseudo code.

 //This class represents a single Page collection, but have the entire items available in the originalCollection
public class PaginatedObservableCollection : ObservableCollection<object>
{
    private ObservableCollection<object> originalCollection;

    public int CurrentPage { get; set; }
    public int CountPerPage { get; set; }

    protected override void InsertItem(int index, object item)
    {
        //Check if the Index is with in the current Page then add to the collection as bellow. And add to the originalCollection also
        base.InsertItem(index, item);
    }

    protected override void RemoveItem(int index)
    {
        //Check if the Index is with in the current Page range then remove from the collection as bellow. And remove from the originalCollection also
        base.RemoveItem(index);
    }
}

UPDATE: I have a blog post about this topic on here - http://jobijoy.blogspot.com/2008/12/paginated-observablecollection.html and the source code is uploaded to Codeplex.