Folder Tree WPF Control: Part 1 - Lazy Loading

FolderTree.xaml

<TreeView ItemsSource="{Binding Path=Drives}" TreeViewItem.Expanded="FolderTree_Expanded">
    <TreeView.ItemContainerStyle>
                <Style TargetType="TreeViewItem">
                    <Setter Property="IsExpanded" Value="{Binding Path=IsExpanded, Mode=TwoWay}" />
                </Style>
            </TreeView.ItemContainerStyle>
    <TreeView.ItemTemplate>
        <HierarchicalDataTemplate ItemsSource="{Binding Path=Folders}">
            <TextBlock Text="{Binding Path=Label}" />
        </HierarchicalDataTemplate>
    </TreeView.ItemTemplate>
</TreeView>

FolderTree.xaml.cs

public partial class FolderTree : UserControl
{
    ...

    private void FolderTree_Expanded(object sender, RoutedEventArgs e)
    {
        var treeViewItem = (TreeViewItem)e.OriginalSource;
        var node = (FolderTreeItem)treeViewItem.Header;
        node.LoadChildren();
    }
}

Node.cs

namespace GeekJ.FolderTreeControl.Model
{
    public abstract class FolderTreeItem : ViewModelBase
    {
        private bool _isExpanded;
        public bool IsExpanded
        {
            get
            {
                return _isExpanded;
            }
            set
            {
                _isExpanded = value;
                OnPropertyChanged("IsExpanded");
            }
        }

        public abstract void LoadChildren();
    }
}