Making multiple selection in a DataGrid bindable
A customer recently asked how to change the selection of a row in the Silverlight 3 DataGrid based on the values of the columns. The brief answer is to add an event handler for LoadingRow and call dataGrid.SelectedItems.Add(e.Row.DataContext), but the customer did mention that he was using M-V-VM and that got me thinking. The selection state of a DataGrid should be pulled from the model, not calculated as part of the UI processing when the row gets loaded. The DataGrid supports this concept for single-item selection because DataGrid.SelectedItem is data-bindable. The problem is that DataGrid.SelectedItems is pre-populated with an internal collection which implements IList (the return type of DataGrid.SelectedItems). I turned to using attached properties - the trusty duct-tape of Silverlight - to fill the gap.
Below is the C# code for a class containing an attached property called SelectedItemsSource which can be applied to a DataGrid to keep its SelectedItems internal collection in-sync with a source list you specify. The synchronization is two-way and works with multiple DataGrid instances synced to the same source list. The only limitation is that the source list must implement IList and INotifyCollectionChanged. Disclaimer: This plays nice for the most part if the DataGrid.ItemsSource is bound to a collection implementing ICollectionView (which handles single-item currency). In my limited testing this workaround falls down when you have multiple DataGrids with the SelectedItemsSource attached property set to the same list and ItemsSource bound to the same ICollectionView instance.
The full source for the class is dowloadable from a link at the end of the post.
using System;
using System.Collections;
using System.Collections.Generic;
using System.Collections.Specialized;
using System.Diagnostics;
using System.Windows;
using System.Windows.Controls;
namespace Samples
{
/// <summary>
/// Class providing support to faux-bind the SelectedItems for a DataGrid to an IList implementing INotifyCollectionChanged
/// </summary>
public static class DataGridMultipleSelection
{
/// <summary>
/// Used in the selectedItemsSources private registry to hold state
/// </summary>
private class DataGridsAndInitiatedSelectionChange
{
public readonly List<WeakReference> BoundDataGridReferences = new List<WeakReference>();
public bool InitiatedSelectionChange { get; set; }
}
// Use each source list's hashcode as the key so that we don't hold on
// to any references in case the DataGrid gets disposed without telling
// to remove the source list from our registry.
private static Dictionary<int, DataGridsAndInitiatedSelectionChange> selectedItemsSources = new Dictionary<int, DataGridsAndInitiatedSelectionChange>();
#region InitiatedSelectionChangeProperty
/// <summary>
/// For use only by this implementation to track if a DataGrid is the one changing the selection
/// </summary>
private static readonly DependencyProperty InitiatedSelectionChangeProperty = DependencyProperty.RegisterAttached("InitiatedSelectionChange", typeof(bool), typeof(DataGridMultipleSelection), new PropertyMetadata(false));
/// <summary>
/// Accessor to get the InitiatedSelectionChange DependencyProperty
/// </summary>
private static bool GetInitiatedSelectionChange(DataGrid element)
{
return (bool)element.GetValue(DataGridMultipleSelection.InitiatedSelectionChangeProperty);
}
/// <summary>
/// Accessor to set the InitiatedSelectionChange DependencyProperty
/// </summary>
private static void SetInitiatedSelectionChange(DataGrid element, bool value)
{
element.SetValue(DataGridMultipleSelection.InitiatedSelectionChangeProperty, value);
}
#endregion
#region SelectedItemsSourceProperty
/// <summary>
/// Holds an IList implementing INotifyCollectionChanged to use as an items source for DataGrid.SelectedItems
/// </summary>
public static readonly DependencyProperty SelectedItemsSourceProperty = DependencyProperty.RegisterAttached("SelectedItemsSource", typeof(INotifyCollectionChanged), typeof(DataGridMultipleSelection), new PropertyMetadata(null, SelectedItemsSourceChanged));
/// <summary>
/// Accessor to get the SelectedItemsSource DependencyProperty
/// </summary>
public static INotifyCollectionChanged GetSelectedItemsSource(DataGrid element)
{
return element.GetValue(DataGridMultipleSelection.SelectedItemsSourceProperty) as INotifyCollectionChanged;
}
/// <summary>
/// Accessor to set the SelectedItemsSource DependencyProperty
/// </summary>
public static void SetSelectedItemsSource(DataGrid element, INotifyCollectionChanged value)
{
element.SetValue(DataGridMultipleSelection.SelectedItemsSourceProperty, value);
}
/// <summary>
/// Updates the items source registry when the SelectedItemsSource for a DataGrid changes
/// </summary>
private static void SelectedItemsSourceChanged(DependencyObject sender, DependencyPropertyChangedEventArgs args)
{
DataGrid dataGrid = sender as DataGrid;
IList selectedItemsSource = null;
// Check if the app is setting the source to a new or different list, or if it is removing the binding
if (args.NewValue != null)
{
selectedItemsSource = args.NewValue as IList;
if (selectedItemsSource == null)
{
throw new ArgumentException("The value for SelectedItemsSource must implement IList.");
}
INotifyCollectionChanged collection = args.NewValue as INotifyCollectionChanged;
if (collection == null)
{
throw new ArgumentException("The value for SelectedItemsSource must implement INotifyCollectionChanged.");
}
// Don't add the event handler if the DataGrid is not setting its SelectedItemsSource for the first time
if (args.OldValue == null)
{
// Sign up for changes to the DataGrid's selected items to enable a two-way binding effect
dataGrid.SelectionChanged += UpdateSourceListOnDataGridSelectionChanged;
}
// Track this DataGrid instance for the specified source list
DataGridsAndInitiatedSelectionChange sourceListInfo = null;
if (DataGridMultipleSelection.selectedItemsSources.TryGetValue(selectedItemsSource.GetHashCode(), out sourceListInfo))
{
sourceListInfo.BoundDataGridReferences.Add(new WeakReference(dataGrid));
}
else
{
// This is a new source collection
sourceListInfo = new DataGridsAndInitiatedSelectionChange() { InitiatedSelectionChange = false };
sourceListInfo.BoundDataGridReferences.Add(new WeakReference(dataGrid));
DataGridMultipleSelection.selectedItemsSources.Add(selectedItemsSource.GetHashCode(), sourceListInfo);
// Sign up for changes to the source only on the first time the source is added
collection.CollectionChanged += UpdateDataGridsOnSourceCollectionChanged;
}
// Now force the DataGrid to update its SelectedItems to match the current
// contents of the source list
sourceListInfo.InitiatedSelectionChange = true;
UpdateDataGrid(dataGrid, selectedItemsSource, new NotifyCollectionChangedEventArgs(NotifyCollectionChangedAction.Reset));
sourceListInfo.InitiatedSelectionChange = false;
}
else
{
// This DataGrid is removing its SelectedItems binding to any list
dataGrid.SelectionChanged -= UpdateSourceListOnDataGridSelectionChanged;
dataGrid.SelectedItems.Clear();
}
if (args.OldValue != null)
{
// Clean up the items source that was the old value
// Remove the DataGrid from the source list's registry and remove the source list
// if there are no more DataGrids bound to it.
DataGridsAndInitiatedSelectionChange sourceListInfo = DataGridMultipleSelection.selectedItemsSources[args.OldValue.GetHashCode()];
WeakReference dataGridReferenceNeedingRemoval = null;
foreach (WeakReference dataGridReference in sourceListInfo.BoundDataGridReferences)
{
if (dataGridReference.IsAlive && (dataGridReference.Target == dataGrid))
{
dataGridReferenceNeedingRemoval = dataGridReference;
break;
}
}
sourceListInfo.BoundDataGridReferences.Remove(dataGridReferenceNeedingRemoval);
if (sourceListInfo.BoundDataGridReferences.Count == 0)
{
DataGridMultipleSelection.selectedItemsSources.Remove(args.OldValue.GetHashCode());
// Detach the event handlers and clear DataGrid.SelectedItems since the source is now null
INotifyCollectionChanged collection = args.OldValue as INotifyCollectionChanged;
if (collection != null)
{
collection.CollectionChanged -= UpdateDataGridsOnSourceCollectionChanged;
}
}
}
}
/// <summary>
/// INotifyCollectionChanged.CollectionChanged handler for updating DataGrid.SelectedItems when the source list changes
/// </summary>
private static void UpdateDataGridsOnSourceCollectionChanged(object source, NotifyCollectionChangedEventArgs collectionChangedArgs)
{
DataGridsAndInitiatedSelectionChange sourceListInfo = DataGridMultipleSelection.selectedItemsSources[source.GetHashCode()];
// For each DataGrid that is bound to this list, is alive, and did not initate selection changes, update its selection
sourceListInfo.InitiatedSelectionChange = true;
IList sourceList = source as IList;
Debug.Assert(sourceList != null, "SelectedItemsSource must be of type IList");
DataGrid dataGrid = null;
foreach (WeakReference dataGridReference in sourceListInfo.BoundDataGridReferences)
{
if (dataGridReference.IsAlive && !DataGridMultipleSelection.GetInitiatedSelectionChange(dataGridReference.Target as DataGrid))
{
dataGrid = dataGridReference.Target as DataGrid;
UpdateDataGrid(dataGrid, sourceList, collectionChangedArgs);
}
}
sourceListInfo.InitiatedSelectionChange = false;
}
/// <summary>
/// Helper method to update the items in DataGrid.SelectedItems based on the changes defined in the given NotifyCollectionChangedEventArgs
/// </summary>
/// <param name="dataGrid">DataGrid which owns the SelectedItems collection to update</param>
/// <param name="sourceList">IList which is the SelectedItemsSource</param>
/// <param name="collectionChangedArgs">The NotifyCollectionChangedEventArgs that was passed into the CollectionChanged event handler</param>
private static void UpdateDataGrid(DataGrid dataGrid, IList sourceList, NotifyCollectionChangedEventArgs collectionChangedArgs)
{
switch (collectionChangedArgs.Action)
{
case NotifyCollectionChangedAction.Add:
foreach (object newItem in collectionChangedArgs.NewItems)
{
dataGrid.SelectedItems.Add(newItem);
}
break;
case NotifyCollectionChangedAction.Remove:
foreach (object oldItem in collectionChangedArgs.OldItems)
{
dataGrid.SelectedItems.Remove(oldItem);
}
break;
case NotifyCollectionChangedAction.Reset:
// Unfortunately can not do the following two steps as an atomic change
// so the target list could raise multiple notifications as it gets updated
dataGrid.SelectedItems.Clear();
foreach (object item in sourceList)
{
dataGrid.SelectedItems.Add(item);
}
break;
default:
throw new NotImplementedException("Only Add, Remove, and Reset actions are implemented.");
}
}
/// <summary>
/// DataGrid.SelectionChanged handler to update the source list given the SelectionChangedEventArgs
/// </summary>
private static void UpdateSourceListOnDataGridSelectionChanged(object sender, SelectionChangedEventArgs selectionChangedArgs)
{
DataGrid dataGrid = sender as DataGrid;
IList selectedItemsSource = DataGridMultipleSelection.GetSelectedItemsSource(dataGrid) as IList;
Debug.Assert(selectedItemsSource != null, "SelectedItemsSource must be of type IList");
// If the source list initiated the changes then don't pass the DataGrid's changes back down to the source list
if (!DataGridMultipleSelection.selectedItemsSources[selectedItemsSource.GetHashCode()].InitiatedSelectionChange)
{
DataGridMultipleSelection.SetInitiatedSelectionChange(dataGrid, true);
foreach (object removedItem in selectionChangedArgs.RemovedItems)
{
selectedItemsSource.Remove(removedItem);
}
foreach (object addedItem in selectionChangedArgs.AddedItems)
{
selectedItemsSource.Add(addedItem);
}
DataGridMultipleSelection.SetInitiatedSelectionChange(dataGrid, false);
}
}
#endregion
}
}
Usage of the attached property is extremely simple. In the first example, the view model is assumed to be already set as the DataGrid's DataContext. The second example assumes that the view model has been instantiated as a static resource either for the App or some other container. Remember to set SelectionMode="Extended" to enable multiple selection in the DataGrid.
<data:DataGrid ... SelectionMode="Extended" samples:DataGridMultipleSelection.SelectedItemsSource="{Binding SelectedProducts}" />
<data:DataGrid ... SelectionMode="Extended" samples:DataGridMultipleSelection.SelectedItemsSource="{Binding SelectedProducts, Source={StaticResource CatalogModel}}" />