Expand and Collapse in WinUI TreeView

27 Feb 20259 minutes to read

The TreeView allows you to expand and collapse the nodes either by user interaction on the nodes or by programmatically.

Expand Action Trigger

Expanding and collapsing of nodes can be performed either by tapping the expander view or in both expander view and content view by setting the ExpandActionTrigger property.

<treeView:SfTreeView x:Name="treeView"  ExpandActionTrigger="Node" />
// Expands by tapping both expander view and content view.
treeView.ExpandActionTrigger = ExpandActionTrigger.Node;

Auto Expand Mode

By default, the Treeview items will be in collapsed state. You can define how the nodes to be expanded while loading the TreeView by using AutoExpandMode property.

The AutoExpandMode property is only applicable for bound mode. For Unbound mode, you need to set IsExpanded property to true while creating the nodes, to be in expanded state while loading the TreeView.

  • None : All items are collapsed when loaded.
  • RootNodes : Expands only the root item when loaded.
  • AllNodes : Expands all the items when loaded.

Expand or collapse the nodes based on property of underlying data object

You can bind expand state of node to the bool property in underlying data object by using IsExpandedPropertyName property. TreeView updates the expanded of node when underlying data object property gets changed and vice versa.

<treeView:SfTreeView x:Name="treeView"
         ItemsSource="{Binding Folders}">
    <treeView:SfTreeView.HierarchyPropertyDescriptors>        
        <treeView:HierarchyPropertyDescriptor IsExpandedPropertyName="IsExpanded" ChildPropertyName="SubFiles" TargetType="local:FileManager" />
    </treeView:SfTreeView.HierarchyPropertyDescriptors>
</treeView:SfTreeView>
public class FileManager : INotifyPropertyChanged
{
    private string fileName;        
    private ObservableCollection<FileManager> subFiles;
    private bool isExpanded;

    public ObservableCollection<FileManager> SubFiles
    {
        get { return subFiles; }
        set
        {
            subFiles = value;
            RaisedOnPropertyChanged("SubFiles");
        }
    }

    public string ItemName
    {
        get { return fileName; }
        set
        {
            fileName = value;
            RaisedOnPropertyChanged("ItemName");
        }
    }

    public bool IsExpanded
    {
        get { return isExpanded; }
        set
        {
            isExpanded = value;
            RaisedOnPropertyChanged("IsExpanded");
        }
    }

    public event PropertyChangedEventHandler PropertyChanged;

    public void RaisedOnPropertyChanged(string _PropertyName)
    {
        if (PropertyChanged != null)
        {
            PropertyChanged(this, new PropertyChangedEventArgs(_PropertyName));
        }
    }
}


public class FileManagerViewModel
{
    private ObservableCollection<FileManager> folders;

    public FileManagerViewModel()
    {
        GenerateSource();            
    }

    public ObservableCollection<FileManager> Folders
    {
        get { return folders; }
        set { this.folders = value; }
    }

    private void GenerateSource()
    {
        var fileManager = new ObservableCollection<FileManager>();
        
        var doc = new FileManager() { ItemName = "Documents", IsExpanded = true };
        var download = new FileManager() { ItemName = "Downloads",  IsExpanded = false };
        
        var pollution = new FileManager() { ItemName = "Environmental Pollution.docx"};
        var globalWarming = new FileManager() { ItemName = "Global Warming.ppt" };
        var sanitation = new FileManager() { ItemName = "Sanitation.docx"};
        var socialNetwork = new FileManager() { ItemName = "Social Network.pdf" };
        var youthEmpower = new FileManager() { ItemName = "Youth Empowerment.pdf" };

        var games = new FileManager() { ItemName = "Game.exe" };
        var tutorials = new FileManager() { ItemName = "Tutorials.zip" };
        var TypeScript = new FileManager() { ItemName = "TypeScript.7z"};
        var uiGuide = new FileManager() { ItemName = "UI-Guide.pdf"};
        
        doc.SubFiles = new ObservableCollection<FileManager>
        {
            pollution,
            globalWarming,
            sanitation,
            socialNetwork,
            youthEmpower
        };
     
        download.SubFiles = new ObservableCollection<FileManager>
        {
            games,
            tutorials,
            TypeScript,
            uiGuide
        };
    
        fileManager.Add(doc);
        fileManager.Add(download);
     
        folders = fileManager;
    }
}

NOTE

IsExpandedPropertyName property is not supported for unbound mode and it accepts only boolean type property.

Programmatic Expand and Collapse

The TreeView allows programmatic expand and collapse based on the TreeViewNode and level by using following methods.

// Expands all the nodes of root level '0'
treeView.ExpandNodes(0);

// Collapses all the nodes of root level '0'
treeView.CollapseNodes(0);

// Expand a particular node.
treeView.ExpandNode(node);

// Collapse a particular node.
treeView.CollapseNode(node);

Expand and Collapse all the nodes

Expand and collapse all the TreeViewNode programmatically at runtime by using the TreeView.ExpandAll method and TreeView.CollapseAll method.

//Expands all the nodes
treeView.ExpandAll();

//Collapses all the nodes
treeView.CollapseAll();

Expand and Collapse using Keyboard

TreeView allows to expand and collapse the nodes by using right and left arrows keys. To expand a node, press the right arrow key and to collapse a node, press the left arrow key on the focused item.

Events

TreeView exposes following events to handle expanding and collapsing of items.

The expanding and collapsing interactions can be handled with the help of NodeCollapsing and NodeExpanding events and expanded and collapsed interactions can be handled with help of NodeCollapsed and NodeExpanded events.