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Showing content from https://docs.unity3d.com/Manual/sprite/../UIE-uxml-element-TreeView.html below:

Unity - Manual: TreeView

Use TreeView to display hierarchical data in a tree-like structure. It’s a powerful and flexible control that’s commonly used to present data with parent-child relationships or nested structures.

If you need a simple list of items (not a hierarchical tree), consider using the ListView control.

You can create a TreeView with UI Builder, UXML, and C#. The following C# example creates a TreeView:

var treeView = new TreeView();

To refresh the collection view, in general, call the RefreshItems or RefreshItem method. However, in the following cases, call Rebuild instead to refresh the collection view:

• You change the type of the data source, such as changing from a List<float> to a List<Vector3>.
• You make changes to makeItem or destroyItem.

Note: If you call Rebuild, the collection view is completely rebuilt, which can be expensive. If you call RefreshItems or RefreshItem, the collection view is only refreshed, which is less expensive.

You can bind the TreeView to a data source. The data source can be a list of objects or a list of strings.

The following example binds a TreeView to a custom type.

treeView.makeItem = () => new VisualElement();

var item = new VisualElement();
treeView.bindItem = (item, i) =>
{
    var c = treeView.GetItemDataForIndex<MyCustomType>(i);
};

Note: In the previouse example, the TreeView.bindItem receives the Index. You must use the Index (not the ID) with GetItemDataForIndex<T>.

Drag-and-drop is a common feature in UI design. To implement the drag-and-drop operations, override the following methods:

• To enable dragging items, override canStartDrag.
• To set which items are dragged, override setupDragAndDrop.
• To get item status changes, override dragAndDropUpdate. You can perform certain actions based on the drag position or other conditions.
• To control the drag-and-drop behavior, override handleDrop.

During the drag-and-drop operation, you can enable the reordering of items by dragging. To enable, set the reorderable attribute to true in UI Builder, UXML, and C#.

Refer to Create a drag-and-drop list and tree views between windows for an example.

To set up the hierarchy of the TreeView, declare TreeViewItemData<T> where T is the type of data for the tree, then set the source through the SetRootItems API. For example, assume you have the following UXML structure:

<UXML xmlns="UnityEngine.UIElements">
    <TreeView class="the-uxml-treeview" fixed-item-height="20" />
</UXML>

You can set up the hierarchy of the TreeView as follows:

/// <sample>
// Create some list of data, here simply numbers in a few foldouts
var items = new List<TreeViewItemData<string>>(10);
for (var i = 0; i < 10; i++)
{
    var itemIndex = i * 10 + i;

    var treeViewSubItemsData = new List<TreeViewItemData<string>>(10);
    for (var j = 0; j < 10; j++)
        treeViewSubItemsData.Add(new TreeViewItemData<string>(itemIndex + j + 1, $"Data {i+1}-{j+1}"));

    var treeViewItemData = new TreeViewItemData<string>(itemIndex, $"Data {i+1}", treeViewSubItemsData);
    items.Add(treeViewItemData);
};

// The "makeItem" function will be called as needed
// when the TreeView needs more items to render
Func<VisualElement> makeItem = () => new Label();

// As the user scrolls through the list, the TreeView object
// will recycle elements created by the "makeItem"
// and invoke the "bindItem" callback to associate
// the element with the matching data item (specified as an index in the list)
Action<VisualElement, int> bindItem = (e, i) =>
{
    var item = treeView.GetItemDataForIndex<string>(i);
    var id = treeView.GetIdForIndex(i);
    ((Label)e).text = $"ID {id} - {item}";
};

treeView.SetRootItems(items);
treeView.makeItem = makeItem;
treeView.bindItem = bindItem;
treeView.selectionType = SelectionType.Multiple;
treeView.Rebuild();

// Callback invoked when the user double clicks an item
treeView.itemsChosen += (selectedItems) =>
{
    Debug.Log("Items chosen: " + string.Join(", ", selectedItems));
};

// Callback invoked when the user changes the selection inside the TreeView
treeView.selectedIndicesChanged += (selectedIndices) =>
{
    var log = "IDs selected: ";
    foreach (var index in selectedIndices)
    {
        log += $"{treeView.GetIdForIndex(index)}, ";
    }
    Debug.Log(log.TrimEnd(',', ' '));
};
/// </sample>

To try this example live in Unity, go to Window > UI Toolkit > Samples.

The following are some frequently asked questions about the TreeView control:

Can I get the indices of the rows that are visible on the screen?

There are no dedicated APIs for this. You can use the bindItem and unbindItem callbacks to track these indices.

Can I get the list of rows that are visible in the view?

There are no dedicated APIs for this. You can use UQuery to retrieve the elements of interest.

Is it mandatory for any of the overridden functions of a view controller to call base.Method?

Call this method only if you want to extend its default behavior.

I’ve added a Toggle to my row. Why doesn’t the selection jump to that row when the user selects it?

By default, the row is selected only if the mouse down event is not consumed by the row’s contents. In this case, your Toggle stops the event propagation. To fix this, register a PointerDownEvent callback with TrickleDown on the Toggle to call SetSelection.

How do I receive a callback when a user changes their selection in the view?

It is recommended to use the selectedIndicesChanged callback to retrieve data by index when needed. While you can also use selectionChanged, be aware that it returns a list of objects, which can cause boxing allocations when used with value types.

How do I convert from ID to index and vice versa?

Use BaseTreeViewController.GetIndexForId and BaseTreeViewController.GetIdForIndex.

Can I have a horizontal TreeView?

The TreeView control doesn’t support horizontal layout and virtualization. It’s recommended to use a ScrollView with flex-direction: row to layout elements horizontally.

• Create list and tree views: Use ListView, TreeView, MultiColumnListView, and MultiColumnTreeView to create list and tree views.
• Create a drag-and-drop list and tree views between windows: Use ListView, TreeView, and MultiColumnListView to create a drag-and-drop UI between windows.

C# class: TreeView
Namespace: UnityEngine.UIElements
Base class: BaseTreeView

This element has the following member attributes:

This element inherits the following attributes from its base class:

This property must be set when using the virtualizationMethod is set to FixedHeight, for the collection view to function. If set when virtualizationMethod is DynamicHeight, it serves as the default height to help calculate the number of items necessary and the scrollable area, before items are laid out. It should be set to the minimum expected height of an item.

The default value is false which allows the user to drag items to and from other views when you implement canStartDrag, setupDragAndDrop, dragAndDropUpdate, and handleDrop. Set this value to true to allow the user to reorder items in the list.

The default value is SelectionType.Single. When you set the collection view to disable selections, any current selection is cleared.

If set to true, a border appears around the ScrollView that the collection view uses internally.

The default value is FixedHeight. When using fixed height, specify the fixedItemHeight property. Fixed height is more performant but offers less flexibility on content. When using DynamicHeight, the collection will wait for the actual height to be computed. Dynamic height is more flexible but less performant.

This element also inherits the following attributes from VisualElement:

This information is only used by the UI Builder as a hint to provide some completion to the data source path field when the effective data source cannot be specified at design time.

Setting the languageDirection to RTL adds basic support for right-to-left (RTL) by reversing the text and handling linebreaking and word wrapping appropriately. However, it does not provide comprehensive RTL support, as this would require text shaping, which includes the reordering of characters, and OpenType font feature support. Comprehensive RTL support is planned for future updates, which will involve additional APIs to handle language, script, and font feature specifications.

To enhance the RTL functionality of this property, users can explore available third-party plugins in the Unity Asset Store and make use of ITextElementExperimentalFeatures.renderedText

Use this property to write USS selectors that target a specific element. The standard practice is to give an element a unique name.

This key is used to save and load the view data from the view data store. If you don’t set this key, the persistence is disabled for the associated VisualElement. For more information, refer to View data persistence.

The following table lists all the C# public property names and their related USS selector.

Unity adds this USS class to every instance of the TreeView element. Any styling applied to this class affects every TreeView located beside, or below the stylesheet in the visual tree.

Unity adds this USS class to every item element of the TreeView. Any styling applied to this class affects every item located beside, or below the stylesheet in the visual tree.

Unity adds this USS class to every item toggle element of the TreeView. Any styling applied to this class affects every item located beside, or below the stylesheet in the visual tree.

Unity adds this USS class to every indent container element of the TreeView. Any styling applied to this class affects every item located beside, or below the stylesheet in the visual tree.

Unity adds this USS class to every indent element of the TreeView. Any styling applied to this class affects every item located beside, or below the stylesheet in the visual tree.

Unity adds this USS class to every item container element of the TreeView. Any styling applied to this class affects every item located beside, or below the stylesheet in the visual tree.

Unity adds this USS class to every instance of the BaseVerticalCollectionView element. Any styling applied to this class affects every BaseVerticalCollectionView located beside, or below the stylesheet in the visual tree.

Unity adds this USS class to an instance of the BaseVerticalCollectionView element if the instance’s BaseVerticalCollectionView.showBorder property is set to true. Any styling applied to this class affects every such BaseVerticalCollectionView located beside, or below the stylesheet in the visual tree.

Unity adds this USS class to every item element the BaseVerticalCollectionView contains. Any styling applied to this class affects every item element located beside, or below the stylesheet in the visual tree.

Unity adds this USS class to the bar that appears when the user drags an item in the list. Any styling applied to this class affects every BaseVerticalCollectionView located beside, or below the stylesheet in the visual tree.

Unity adds this USS class to the bar that appears when the user drags an item in the list. Any styling applied to this class affects every BaseVerticalCollectionView located beside, or below the stylesheet in the visual tree.

Unity adds this USS class to the item element when it’s being dragged. Any styling applied to this class affects every BaseVerticalCollectionView item located beside, or below the stylesheet in the visual tree.

Unity adds this USS class to every selected element in the BaseVerticalCollectionView. The BaseVerticalCollectionView.selectionType property decides if zero, one, or more elements can be selected. Any styling applied to this class affects every BaseVerticalCollectionView item located beside, or below the stylesheet in the visual tree.

Unity adds this USS class to every odd-numbered item in the BaseVerticalCollectionView when the BaseVerticalCollectionView.showAlternatingRowBackgrounds property is set to ContentOnly or All. When the showAlternatingRowBackgrounds property is set to either of those values, odd-numbered items are displayed with a different background color than even-numbered items. This USS class is used to differentiate odd-numbered items from even-numbered items. When the showAlternatingRowBackgrounds property is set to None, the USS class is not added, and any styling or behavior that relies on it’s invalidated.

Unity adds this USS class to the BaseVerticalCollectionView’s scroll view. Any styling applied to this class affects every BaseVerticalCollectionView scroll view located beside, or below the stylesheet in the visual tree.

You can also use the Matching Selectors section in the Inspector or the UI Toolkit Debugger to see which USS selectors affect the components of the VisualElement at every level of its hierarchy.

• MultiColumnTreeView
• MultiColumnListView
• ScrollViewA UI Control which displays a large set of Controls in a viewable area that you can see by using a scrollbar. More info
  See in Glossary
• ListView

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