Added new kb article treelist-paging-pre-selected-node (#3115)

Co-authored-by: KB Bot <[email protected]>

Author: github-actions[bot]

knowledge-base/treelist-paging-pre-selected-node.md

@@ -0,0 +1,120 @@
+---
+title: How to Initially Display the Page with a Pre-Selected Node in TreeList
+description: Learn how to ensure a pre-selected node is visible by automatically navigating to its page in a Telerik TreeList with pagination enabled.
+type: how-to
+page_title: Navigate to Page Containing Pre-Selected Node in TreeList
+meta_title: Navigate to Page Containing Pre-Selected Node in TreeList
+slug: treelist-kb-paging-pre-selected-node
+tags: treelist, paging, pre-selected, node, navigation, page, size, blazor
+res_type: kb
+ticketid: 1690423
+---
+
+## Environment
+
+<table>
+    <tbody>
+        <tr>
+            <td>Product</td>
+            <td>TreeList for Blazor</td>
+        </tr>
+    </tbody>
+</table>
+
+## Description
+
+I have a Telerik [TreeList](slug:treelist-overview) with a checkbox for selection and pagination enabled. I pre-select a node programmatically and need the TreeList to automatically navigate to the page containing the first selected node.
+
+## Solution
+
+To automatically show the page that contains the first pre-selected node in your Telerik TreeList with paging enabled, you need to programmatically set the TreeList's `Page` property based on the index of the selected node in your data.
+
+1. Identify the index of the first selected node in your data source.
+2. Use the PageSize to determine on which page the node appears.
+3. Assign the calculated page number to the TreeList's Page parameter before rendering.
+
+````Razor
+<TelerikTreeList Data="@TreeListData"
+                 IdField="@nameof(Employee.Id)"
+                 ParentIdField="@nameof(Employee.ParentId)"
+                 SelectionMode="@TreeListSelectionMode.Multiple"
+                 @bind-SelectedItems="@SelectedEmployees"
+                 Pageable="true"
+                 @bind-PageSize="@PageSize"
+                 @bind-Page="@CurrentPage">
+    <TreeListColumns>
+        <TreeListCheckboxColumn Width="50px" />
+        <TreeListColumn Field="@nameof(Employee.FirstName)" Title="First Name" Width="350px" Expandable="true" />
+        <TreeListColumn Field="@nameof(Employee.LastName)" Title="Last Name" />
+        <TreeListColumn Field="@nameof(Employee.Position)" Title="Position" Width="200px" />
+    </TreeListColumns>
+</TelerikTreeList>
+
+@code {
+    private List<Employee> TreeListData { get; set; } = new();
+    private IEnumerable<Employee> SelectedEmployees { get; set; } = Enumerable.Empty<Employee>();
+    private int PageSize = 10;
+    private int CurrentPage = 1;
+
+    protected override void OnInitialized()
+    {
+        TreeListData = new List<Employee>();
+
+        for (int i = 1; i <= 59; i++)
+        {
+            TreeListData.Add(new Employee()
+            {
+                Id = i,
+                ParentId = i <= 3 ? null : i % 3 + 1,
+                FirstName = "First " + i,
+                LastName = "Last " + i,
+                Position = i <= 3 ? "Team Lead" : "Software Engineer"
+            });
+        }
+
+        SelectedEmployees = new List<Employee>() { TreeListData.ElementAt(2) };
+
+        var selectedId = SelectedEmployees.FirstOrDefault()?.Id;
+        if (selectedId != null)
+        {
+            // Step 1: Flatten the tree as it would appear expanded
+            var flatList = new List<Employee>();
+            foreach (var root in TreeListData.Where(e => e.ParentId == null))
+            {
+                FlattenHierarchy(root, flatList);
+            }
+
+            // Step 2: Find index of selected item in flattened list
+            var index = flatList.FindIndex(e => e.Id == selectedId);
+            if (index >= 0)
+            {
+                CurrentPage = (index / PageSize) + 1;
+            }
+        }
+    }
+
+    private void FlattenHierarchy(Employee node, List<Employee> result)
+    {
+        result.Add(node);
+        var children = TreeListData.Where(e => e.ParentId == node.Id);
+        foreach (var child in children)
+        {
+            FlattenHierarchy(child, result);
+        }
+    }
+
+    public class Employee
+    {
+        public int Id { get; set; }
+        public int? ParentId { get; set; }
+        public string FirstName { get; set; } = string.Empty;
+        public string LastName { get; set; } = string.Empty;
+        public string Position { get; set; } = string.Empty;
+    }
+}
+````
+
+## See Also
+
+* [TreeList Overview](slug:treelist-overview)
+* [TreeList Paging](slug:treelist-paging)