Components / TreeView

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TreeView Events

This article explains the events available in the Telerik TreeView for Blazor:

CheckedItemsChanged

The CheckedItemsChanged event fires every time the user uses a checkbox to select a new item.

ExpandedItemsChanged

The ExpandedItemsChanged event fires every time the user expands or collapses a TreeView item.

OnExpand

The OnExpand event fires when the user expands or collapses a node (either with the mouse, or with the keyboard). You can use it to know that the user performed that action, and/or to implement load on demand.

If you set the ShouldRender field of the event arguments to true, the component will re-render after the event (it will call StateHasChanged()). This can be useful if you need to change its parameters or state during the event execution and especially if you need to execute async logic in the event handler.

The event is an EventCallback. It can be synchronous and return void, or asynchronous and return async Task. Do not use async void.

OnItemClick

The OnItemClick event fires when the user clicks, taps or presses Enter on a TreeView node (item). For example, use this event to react to user actions and load data on demand for another component.

The OnItemClick event handler receives a TreeViewItemClickEventArgs argument, which has the following properties.

Property Type Description
EventArgs EventArgs This object maps to MouseEventArgs or KeyboardEventArgs depending on the user action.
Field string The Field parameter of the clicked column.
Item object The data item. Cast the object to your model type to access its members.
ShouldRender bool Sets if the component will re-render after the event via StateHasChanged() call. This can be useful if you need to change the component parameters or state during the event execution, and especially if you need to execute async logic in the event handler.

OnItemContextMenu

The OnItemContextMenu event fires when the user right-clicks on a TreeView node, presses the context menu keyboard button, or taps-and-holds on a touch device.

The event handler receives a TreeViewItemContextMenuEventArgs argument, which has the following properties.

Property Type Description
EventArgs EventArgs This object maps to MouseEventArgs or KeyboardEventArgs depending on the user action.
Field string The Field parameter of the clicked column.
Item object The data item. Cast the object to your model type to access its members.
ShouldRender bool Sets if the component will re-render after the event via StateHasChanged() call. This can be useful if you need to change the component parameters or state during the event execution, and especially if you need to execute async logic in the event handler.

The OnItemContextMenu is used to integrate the Context menu to the TreeView node.

OnItemDoubleClick

The OnItemDoubleClick event fires when the user double-clicks or double-taps a TreeView node.

The event handler receives a TreeViewItemDoubleClickEventArgs argument, which has the following properties.

Property Type Description
EventArgs EventArgs This object maps to MouseEventArgs or KeyboardEventArgs depending on the user action.
Field string The Field parameter of the clicked column.
Item object The data item. Cast the object to your model type to access its members.
ShouldRender bool Sets if the component will re-render after the event via StateHasChanged() call. This can be useful if you need to change the component parameters or state during the event execution, and especially if you need to execute async logic in the event handler.

OnItemRender

The OnItemRender event fires when each node in the TreeView renders.

The event handler receives as an argument an TreeViewItemRenderEventArgs object that contains:

Property Description
Item The current item that renders in the TreeView.
Class The custom CSS class that will be added to the item. Renders on the <div> element that wraps the current Item.

SelectedItemsChanged

The SelectedItemsChanged event fires when the selection is enabled and the user clicks on a new item.

Drag Events

The TreeView implements the Drag and Drop functionality through the following drag events:

  • The OnDragStart event fires when the user starts dragging a node.
  • The OnDrag event fires continuously while a node is being dragged by the user.
  • The OnDrop event fires when the user drops a node into a new location. The event fires only if the new location is a Telerik component.
  • The OnDragEnd event fires when a drag operation ends. Unlike the OnDrop event, OnDragEnd will fire even if the new location is not a Telerik component.

For more details and examples, see the Treeview Drag and Drop article.

Example

Handle Blazor TreeView Events

<TelerikTreeView Data="@TreeViewData"
                 CheckBoxMode="@TreeViewCheckBoxMode.Single"
                 SelectionMode="@TreeViewSelectionMode.Single"
                 CheckedItems="@TreeViewCheckedItems"
                 CheckedItemsChanged="@((IEnumerable<object> items) => TreeViewCheckedItemsChanged(items))"
                 ExpandedItems="@TreeViewExpandedItems"
                 ExpandedItemsChanged="@TreeViewExpandedItemsChanged"
                 OnExpand="@OnTreeViewExpand"
                 OnItemClick="@OnTreeViewItemClick"
                 OnItemContextMenu="OnTreeViewItemContextMenu"
                 OnItemDoubleClick="@OnTreeViewItemDoubleClick"
                 OnItemRender="@OnTreeViewItemRender"
                 SelectedItems="@TreeViewSelectedItems"
                 SelectedItemsChanged="@((IEnumerable<object> item) => TreeViewSelectedItemsChanged(item))">
</TelerikTreeView>

@{
    <div>
                <span>Console</span>
                <span>
                    <TelerikButton OnClick="@OnClearClick" Icon="@SvgIcon.X">Clear</TelerikButton>
                </span>
                <div>
                    @(new MarkupString(EventLog))
                </div>
    </div>
}

<style>
    .bold-text-parent-items {
        font-weight: bold;
    }
</style>

@code {
    private string EventLog { get; set; } = string.Empty;
    private List<TreeItem> TreeViewData { get; set; }
    private IEnumerable<object> TreeViewCheckedItems { get; set; } = new List<object>();
    private IEnumerable<object> TreeViewExpandedItems { get; set; } = new List<TreeItem>();
    private IEnumerable<object> TreeViewSelectedItems { get; set; } = new List<object>();

    private void TreeViewCheckedItemsChanged(IEnumerable<object> items)
    {
        EventLog += "<div><strong>The CheckedItemsChanged event fired.</strong</div>";

        TreeViewCheckedItems = items;
    }

    private async Task TreeViewExpandedItemsChanged(IEnumerable<object> items)
    {
        EventLog += "<div>The <strong>ExpandedItemsChanged</strong> event fired.</div>";

        TreeViewExpandedItems = items;
    }

    private async Task OnTreeViewExpand(TreeViewExpandEventArgs args)
    {
        EventLog += "<div>The <strong>OnExpand</strong> event fired.</div>";

        TreeItem node = args.Item as TreeItem;
    }

    private async Task OnTreeViewItemClick(TreeViewItemClickEventArgs args)
    {
        EventLog += "<div><span>The <strong>OnItemClick</strong> event fired.</span>";

        TreeItem node = args.Item as TreeItem;

        if (args.EventArgs is KeyboardEventArgs keyboardEventArgs)
        {
            EventLog += $"<span>The user clicked {keyboardEventArgs.Key} on node {node.Text}</span></div>";
        }
        else if (args.EventArgs is MouseEventArgs mouseEventArgs)
        {
            EventLog += $"<span>The user clicked {mouseEventArgs.ClientX} {mouseEventArgs.ClientY} on node {node.Text}</span></div>";
        }
    }

    private void OnTreeViewItemContextMenu(TreeViewItemContextMenuEventArgs args)
    {
        EventLog += "<div><span>The <strong>OnItemContextMenu</strong> event fired.</span>";

        TreeItem node = args.Item as TreeItem;

        if (args.EventArgs is KeyboardEventArgs keyboardEventArgs)
        {
            EventLog += $"<span>The user clicked {keyboardEventArgs.Key} on node {node.Text}</span></div>";
        }
        else if (args.EventArgs is MouseEventArgs mouseEventArgs)
        {
            EventLog += $"<span>The user clicked {mouseEventArgs.ClientX} {mouseEventArgs.ClientY} on node {node.Text}</span></div>";
        }
    }

    private async Task OnTreeViewItemDoubleClick(TreeViewItemDoubleClickEventArgs args)
    {
        EventLog += "<div><span>The <strong>OnItemDoubleClick</strong> event fired.</span>";

        TreeItem node = args.Item as TreeItem;

        if (args.EventArgs is KeyboardEventArgs keyboardEventArgs)
        {
            EventLog += $"<span>The user clicked {keyboardEventArgs.Key} on node {node.Text}</span></div>";
        }
        else if (args.EventArgs is MouseEventArgs mouseEventArgs)
        {
            EventLog += $"<span>The user clicked {mouseEventArgs.ClientX} {mouseEventArgs.ClientY} on node {node.Text}</span></div>";
        }
    }

    private void OnTreeViewItemRender(TreeViewItemRenderEventArgs args)
    {
        EventLog += "<div>The <strong>OnItemRender</strong> event fired.</div>";

        TreeItem node = args.Item as TreeItem;

        if (node.ParentId == null && node.HasChildren)
        {
            args.Class = "bold-text-parent-items";
        }
    }

    private void TreeViewSelectedItemsChanged(IEnumerable<object> items)
    {
        EventLog += "<div>The <strong>SelectedItemsChanged</strong> event fired.</div>";

        TreeViewSelectedItems = items;
    }

    private void OnClearClick()
    {
        EventLog = string.Empty;
    }

    protected override void OnInitialized()
    {
        LoadFlatData();

        //provide initial checked item when the page is loaded
        var precheckedItem = TreeViewData.Where(x => x.Id == 3);

        TreeViewCheckedItems = new List<object>(precheckedItem);
    }

    #region Data Model

    public class TreeItem
    {
        public int Id { get; set; }
        public string Text { get; set; }
        public int? ParentId { get; set; }
        public bool HasChildren { get; set; }
        public ISvgIcon Icon { get; set; }
    }
    #endregion

    #region Data Generation

    private void LoadFlatData()
    {
        List<TreeItem> items = new List<TreeItem>();

        items.Add(new TreeItem()
            {
                Id = 1,
                Text = "Project",
                ParentId = null,
                HasChildren = true,
                Icon = SvgIcon.Folder
            });

        items.Add(new TreeItem()
            {
                Id = 2,
                Text = "Design",
                ParentId = 1,
                HasChildren = true,
                Icon = SvgIcon.Brush
            });
        items.Add(new TreeItem()
            {
                Id = 3,
                Text = "Implementation",
                ParentId = 1,
                HasChildren = true,
                Icon = SvgIcon.Folder
            });

        items.Add(new TreeItem()
            {
                Id = 4,
                Text = "site.psd",
                ParentId = 2,
                HasChildren = false,
                Icon = SvgIcon.FilePsd
            });
        items.Add(new TreeItem()
            {
                Id = 5,
                Text = "index.js",
                ParentId = 3,
                HasChildren = false,
                Icon = SvgIcon.Js
            });
        items.Add(new TreeItem()
            {
                Id = 6,
                Text = "index.html",
                ParentId = 3,
                HasChildren = false,
                Icon = SvgIcon.Html5
            });
        items.Add(new TreeItem()
            {
                Id = 7,
                Text = "styles.css",
                ParentId = 3,
                HasChildren = false,
                Icon = SvgIcon.Css
            });

        TreeViewData = items;
    }
    #endregion
}

See Also

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