Question

I have a simple collection that gets updated in real-time. The data is displayed in a DataGrid in WPF. When a user sorts the DataGrid and the data changes the grid is updated with the new data but does not resort the data.

Anyone find a good way to resort the data when the underlying collection changes? I can easily determine when a collection change occured but so far I haven’t had much success in resorting.

Found I can do this:

SortDescription description = grdData.Items.SortDescriptions[0];
grdData.ItemsSource = null;
grdData.ItemsSource = Data;
grdData.Items.SortDescriptions.Add(description);

if(description.PropertyName=="Value")
{
    grdData.Columns[1].SortDirection = description.Direction;
}
else
{
    grdData.Columns[0].SortDirection = description.Direction;
}

But it’s quite the hack. Anything come up with something better?

Answer 1

This is a bit tricky and largely depends on the underlying data source, but here’s what I do:

First, and foremost, you need a data type that is sortable. For this, I’ve created a “SortableObservableCollection” since my underlying data type is an ObservableCollection:

public class SortableObservableCollection<T> : ObservableCollection<T>
{        
    public event EventHandler Sorted;       

    public void ApplySort(IEnumerable<T> sortedItems)
    {
        var sortedItemsList = sortedItems.ToList();

        foreach (var item in sortedItemsList)
            Move(IndexOf(item), sortedItemsList.IndexOf(item));       

        if (Sorted != null)
            Sorted(this, EventArgs.Empty);
    }
}

Now, with that as the data source, I can detect sorts on my DataGrid and resort the actual data. To do this, I’ve added the following event handler to my DataGrid’s Items’ CollectionChanged event:

... In the constructor or initialization somewhere

ItemCollection view = myDataGrid.Items as ItemCollection;
((INotifyCollectionChanged)view.SortDescriptions).CollectionChanged += MyDataGrid_ItemsCollectionChanged;

...

private void MyDataGrid_ItemsCollectionChanged(object sender, NotifyCollectionChangedEventArgs e)
{
    // This is how we detect if a sorting event has happend on the grid.
    if (e.NewItems != null &&
        e.NewItems.Count == 1 &&
        (e.NewItems[0] is SortDescription))
    {
        MyItem[] myItems = new MyItem[MyDataGrid.Items.Count]; // MyItem would by type T of whatever is in the SortableObservableCollection
        myDataGrid.Items.CopyTo(myItems, 0);
        myDataSource.ApplySort(myItems);  // MyDataSource would be the instance of SortableObservableCollection
    }
} 

One of the reasons this works a little better than using the SortDirection is in the instance of doing combined sorted (hold shift down when doing sorts on your columns and you’ll see what I mean).