Added KB: Validate Drop Target in TreeView

knowledge-base/treeview-validate-drop-target.md

@@ -0,0 +1,201 @@
+---
+title: Validate Drop Target in TreeView
+description: How to validate drop target in Telerik TreeView for Blazor and prevent drop for invalid target.
+type: how-to
+page_title: How to Validate Drop Target in TreeView
+slug: treeview-kb-validate-drop-target
+position: 
+tags: treeview, item, node, drop target, draggable, validate, invalid, prevent drop
+ticketid: 1592132
+res_type: kb
+---
+
+## Environment
+
+<table>
+    <tbody>
+        <tr>
+            <td>Product</td>
+            <td>TreeView for Blazor</td>
+        </tr>
+    </tbody>
+</table>
+
+
+## Description
+
+I have a draggable TreeView and I want to prevent the user from dropping items in specific locations or folders. How can I achieve this? 
+
+How to validate the drop target, so I don't allow the user to drop the file if the target is invalid?
+
+## Solution
+
+To validate the drop target and prevent the user from dropping on an invalid target, follow these steps:
+
+1. Handle the [`OnDrag`]({%slug treeview-events%}#drag-events) event of the TreeView to validate the target based on your requirements. In the example below, we are checking it the user tries to drop an item from the **Documents** folder into the **Pictures** folder - the **Pictures** folder is not a valid target in this case.
+1. Raise a flag if the target is not valid.
+1. Change the icon in the drag clue to indicate the target is not valid. Once the [Drag Clue Template](https://feedback.telerik.com/blazor/1501043-drag-clue-template) is available, you may use it to change the rendering as needed. At the time of writing, changing the icon is only possible with CSS as per the example below.
+1. Include a conditional logic in your [`OnDrop`]({%slug treeview-events%}#drag-events) handler to not perform any action if the target is invalid.
+1. Use CSS to hide the build-in drop hint for the invalid target.
+
+
+>caption Validate TreeView drop target and prevent drop for invalid target
+
+````CSHTML
+This TreeView does not allow the user to drop items from the Documents folder into the Pictures folder.
+
+<TelerikTreeView Data="@Data"
+                 @bind-ExpandedItems="@ExpandedItems"
+                 Draggable="true"
+                 DragThrottleInterval="150"
+                 OnDrag="@OnDrag"
+                 OnDrop="@OnItemDrop">
+</TelerikTreeView>
+
+@if (IsPointerOverInvalidTarget)
+{
+    <style>
+        .k-drag-clue .k-svg-icon svg path {
+            d: path("M256 32c-50.3 0-96.8 16.6-134.1 44.6-17.2 12.8-32.4 28.1-45.3 45.3C48.6 159.2 32 205.7 32 256c0 123.7 100.3 224 224 224 50.3 0 96.8-16.6 134.1-44.6 17.2-12.8 32.4-28.1 45.3-45.3 28-37.4 44.6-83.8 44.6-134.1 0-123.7-100.3-224-224-224m0 384c-88.2 0-160-71.8-160-160 0-32.6 9.8-62.9 26.6-88.2l221.6 221.6C318.9 406.2 288.6 416 256 416m133.4-71.8L167.8 122.6C193.1 105.8 223.4 96 256 96c88.2 0 160 71.8 160 160 0 32.6-9.8 62.9-26.6 88.2");
+        }
+
+        .k-drop-hint {
+            display: none;
+        }
+    </style>
+}
+
+@code {
+    private TreeItem DestinationItem { get; set; }
+    private List<TreeItem> Data { get; set; } = new List<TreeItem>();
+    private IEnumerable<object> ExpandedItems { get; set; } = Enumerable.Empty<object>();
+    private bool IsPointerOverInvalidTarget { get; set; }
+
+    private void OnDrag(TreeViewDragEventArgs args)
+    {
+        if (args.DestinationItem != null)
+        {
+            var draggedItem = args.Item as TreeItem;
+            var destinationItem = args.DestinationItem as TreeItem;
+
+            if (draggedItem.ParentId == 1 && (destinationItem.ParentId == 7 || destinationItem.Id == 6 || destinationItem.Id == 7))
+            {
+                IsPointerOverInvalidTarget = true;
+            }
+            else
+            {
+                IsPointerOverInvalidTarget = false;
+            }
+        }
+    }
+
+    private void OnItemDrop(TreeViewDropEventArgs args)
+    {
+        if (IsPointerOverInvalidTarget)
+        {
+            return;
+        }
+        else
+        {
+            var item = args.Item as TreeItem;
+            var destinationItem = args.DestinationItem as TreeItem;
+
+            if (destinationItem != null && IsChild(item, destinationItem))
+            {
+                return;
+            }
+
+            Data.Remove(item);
+
+            if (item.ParentId != null && !Data.Any(x => item.ParentId == x.ParentId))
+            {
+                Data.FirstOrDefault(x => x.Id == item.ParentId).HasChildren = false;
+            }
+
+            if (args.DropPosition == TreeViewDropPosition.Over)
+            {
+                item.ParentId = destinationItem.Id;
+                destinationItem.HasChildren = true;
+
+                Data.Add(item);
+            }
+            else
+            {
+                var index = Data.IndexOf(destinationItem);
+
+                item.ParentId = destinationItem.ParentId;
+
+                if (args.DropPosition == TreeViewDropPosition.After)
+                {
+                    index++;
+                }
+
+                Data.Insert(index, item);
+            }
+
+            // Refresh data
+            Data = new List<TreeItem>(Data);
+        }
+    }
+
+    private bool IsChild(TreeItem item, TreeItem destinationItem)
+    {
+        if (destinationItem?.ParentId == null || item == null)
+        {
+            return false;
+        }
+        else if (destinationItem.ParentId?.Equals(item.Id) == true)
+        {
+            return true;
+        }
+
+        var parentDestinationItem = Data.FirstOrDefault(e => e.Id.Equals(destinationItem.ParentId));
+
+        return IsChild(item, parentDestinationItem);
+    }
+
+    protected override void OnInitialized()
+    {
+        LoadData();
+
+        base.OnInitialized();
+    }
+
+
+    private void LoadData()
+    {
+        Data = new List<TreeItem>()
+        {
+            new TreeItem(1, null, "Documents", SvgIcon.Folder, true),
+                new TreeItem(2, 1, "report.xlsx", SvgIcon.FileExcel, false),
+                new TreeItem(3, 1, "status.docx", SvgIcon.FileWord, false),
+                new TreeItem(4, 1, "conferences.xlsx", SvgIcon.FileExcel, false),
+                new TreeItem(5, 1, "performance.pdf", SvgIcon.FilePdf, false),
+            new TreeItem(6, null, "Pictures", SvgIcon.Folder, true),
+                new TreeItem(7, 6, "Camera Roll", SvgIcon.Folder, true),
+                    new TreeItem(8, 7, "team.png", SvgIcon.FileImage, false),
+                    new TreeItem(9, 7, "team-building.png", SvgIcon.FileImage, false),
+                    new TreeItem(10, 7, "friends.png", SvgIcon.FileImage, false),
+        };
+        ExpandedItems = Data.ToList();
+    }
+
+    public class TreeItem
+    {
+        public int Id { get; set; }
+        public int? ParentId { get; set; }
+        public string Text { get; set; }
+        public ISvgIcon Icon { get; set; }
+        public bool HasChildren { get; set; }
+
+        public TreeItem(int id, int? parent, string text, ISvgIcon icon, bool hasChildren)
+        {
+            Id = id;
+            ParentId = parent;
+            Text = text;
+            Icon = icon;
+            HasChildren = hasChildren;
+        }
+    }
+}
+````