How to search in a lazy-loading WPF MVVM TreeView? - wpf

Question

I need a TreeView to represent some hierarchical data from multiple tables stored in a SQL Server CE database. Before, the data was stored in xml and was simple deserialized on startup and everything was good. Now I was asked to move data to a database and I've faced a several problems.
My first problem was that it takes quite a long time to retrieve many items from DB and build a TreeView ViewModel from this items (still not sure what is longer - to get items or to construct this tree). So I implemented lazy loading and now I'm getting items only when a TreeViewItem is expanding.
Now, I need to perform a text search over all the nodes, but to make it work, all nodes must be loaded.
I tried to load all of them but the UI freezes while the tree is loading. Doing this inside a BackgroundWorker is also impossible for me because the items are stored in an ObservableCollection and I'm getting "InvalidOperationException". Using Dispatcher helps with this but it is also freezes UI...
The excerpt from my TreeViewItem VM is below, if more code is needed please let me know. Maybe I am totally wrong with my design, so any comments are very appreciated!
public class TreeViewItemViewModel: DisplayableItem, IItemsHost
{
internal static DummyTreeViewItemViewModel _dummy = new DummyTreeViewItemViewModel();
public TreeViewItemViewModel(){}
public TreeViewItemViewModel(IDisplayableItem displayableItem)
{
Data = displayableItem;
}
public TreeViewItemViewModel(IDisplayableItem displayableItem, IDisplayableItem parent)
:this(displayableItem)
{
Parent = parent as TreeViewItemViewModel;
}
private TreeViewItemViewModel _parent;
public TreeViewItemViewModel Parent
{
get { return _parent; }
set { _parent = value; InvokePropertyChanged(new PropertyChangedEventArgs("Parent")); }
}
private IDisplayableItem _data;
public new IDisplayableItem Data
{
get { return _data; }
set { _data = value; InvokePropertyChanged(new PropertyChangedEventArgs("Data")); }
}
private bool _isSelected;
public new bool IsSelected
{
get { return _isSelected; }
set { _isSelected = value; InvokePropertyChanged(new PropertyChangedEventArgs("IsSelected")); }
}
private bool _isEnabled=true;
public new bool IsEnabled
{
get { return _isEnabled; }
set { _isEnabled = value; InvokePropertyChanged(new PropertyChangedEventArgs("IsEnabled")); }
}
private bool _isVisible = true;
public new bool IsVisible
{
get { return _isVisible; }
set { _isVisible = value; InvokePropertyChanged(new PropertyChangedEventArgs("IsVisible")); }
}
private void FillItems()
{
if (Items.Contains(_dummy))
{
Items.Remove(_dummy);
var itemshost = Data as IItemsHost;
if (itemshost != null)
{
_items = new ObservableCollection<IDisplayableItem>();
foreach (var item in itemshost.Items)//getting 'Items' actually requesting them from a database
{
var treeItem = new TreeViewItemViewModel(item, this);
_items.Add(treeItem);
}
InvokePropertyChanged(new PropertyChangedEventArgs("Items"));
}
}
}
protected bool _isExpanded;
public bool IsExpanded
{
get { return _isExpanded; }
set
{
if(value)
{
FillItems();
}
_isExpanded = value;
InvokePropertyChanged(new PropertyChangedEventArgs("IsExpanded"));
}
}
protected SObservableCollection<IDisplayableItem> _items = new SObservableCollection<IDisplayableItem>();
public SObservableCollection<IDisplayableItem> Items
{
get
{
var itemshost = Data as IItemsHost;
if (itemshost != null)
{
if (_items.Count == 0 && itemshost.Items.Count > 0)
_items.Add(_dummy);
}
return _items;
}
set { _items = value; InvokePropertyChanged(new PropertyChangedEventArgs("Items")); }
}
UPDATE: for those who would search for a similar solution - my problem was in my query method. I shouldn't open a new SQL Server CE connection each time I need to perform a query...

Answer 1

What about a new DB table that holds a flattened representation of the entire hierarchy, and have your search logic query this table? You'll obviously need to keep this table updated as you insert/update/delete records in the other tables.
Each record in the new table would need to include some information about where the item sits in the hierarchy, so that when you get the search results back you can load and populate just those tree nodes containing the "hits".

Answer 2

Since reading from database is being done asynchronously so the performance bottle-neck should be constructing View from ViewModel. I suggest the following method:
Read all essential Model data from database in one async call and store them in an object called SearchHelper.
Add a a simple property (Model.Id or Model's hash code) to every ViewModel that you create in order to find the equivalent view model of an specific model.
Create only visible ViewModels. (lazy loading for ViewModel only)
Use the SearchHelper to find matches for the search query, then using the Id or hash code of the results, you can easily locate their equivalent view models.
Please Consider:
Once loaded, SearchHelper does not update itself, so you might want to manually update it.
For this method to have optimal performance, try avoiding iteration of all nodes. instead, store the sequence of traced items (their index or Id) in order to find them in view model. if each Model item knows its parent, then the back-tracking should be easy.