Added new kb article treelist-expand-nodes-programmatically (#2452)

* Added new kb article treelist-expand-nodes-programmatically

* kb(TreeList): article and example polish

---------

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

Author: 3 people

knowledge-base/treelist-expand-nodes-programmatically.md

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+---
+title: How to Programmatically Expand or Collapse TreeList Nodes
+description: Learn how to programmatically expand or collapse nodes in a Telerik TreeList for Blazor by utilizing the TreeList state management.
+type: how-to
+page_title: How to Programmatically Expand or Collapse TreeList Nodes
+slug: treelist-kb-expand-nodes-programmatically
+tags: treelist, expand, collapse
+res_type: kb
+ticketid: 1663716, 1649445, 1548181
+---
+
+## Environment
+
+<table>
+<tbody>
+<tr>
+<td>Product</td>
+<td>TreeList for Blazor</td>
+</tr>
+</tbody>
+</table>
+
+## Description
+
+In the Telerik TreeList for Blazor, I load the TreeList in a collapsed mode. I need to programmatically expand and collapse TreeList nodes.
+
+This KB article also answers the following questions:
+- How can I control the expanded state of TreeList nodes in code?
+- Is it possible to expand a TreeList nodes without user interaction?
+- What approach should I take to programmatically adjust the visibility of TreeList nodes?
+
+## Solution
+
+To control the expanded or collapsed state of items in the TreeList programmatically, utilize the TreeList State. The state management feature allows for the adjustment of item visibility through code.
+
+Below is an example demonstrating how to use the TreeList State to expand or collapse items programmatically:
+
+````CSHTML
+<div>
+    <span>
+        <TelerikButton OnClick="@SetTreeListExpandedItems">Expand/Collapse All Items</TelerikButton>
+    </span>
+</div>
+<br />
+
+<TelerikTreeList @ref="TreeListRef"
+                 Data="@Data"
+                 ItemsField="@(nameof(Employee.DirectReports))"
+                 OnStateInit="((TreeListStateEventArgs<Employee> args) => OnStateInitHandler(args))"
+                 Reorderable="true"
+                 Resizable="true"
+                 Sortable="true"
+                 FilterMode="@TreeListFilterMode.FilterRow"
+                 Pageable="true"
+                 Width="850px">
+    <TreeListColumns>
+        <TreeListColumn Field="@nameof(Employee.Name)" Expandable="true" Width="320px" />
+        <TreeListColumn Field="@nameof(Employee.EmailAddress)" Width="220px" />
+        <TreeListColumn Field="@nameof(Employee.HireDate)" Width="220px" />
+    </TreeListColumns>
+</TelerikTreeList>
+
+@code {
+    private TelerikTreeList<Employee> TreeListRef { get; set; } = new TelerikTreeList<Employee>();
+    private List<Employee> Data { get; set; }
+    private int LastId { get; set; } = 1;
+
+    private async Task SetTreeListExpandedItems()
+    {
+        if (!(TreeListRef.GetState().ExpandedItems.Any()))
+        {
+            List<Employee> toExpand = new List<Employee>();
+            foreach (Employee item in Data)
+            {
+                toExpand.Add(item);
+                if (item.DirectReports.Any())
+                {
+                    foreach (Employee child in item.DirectReports)
+                    {
+                        toExpand.Add(child);
+                    }
+                }
+            }
+
+            var expandedState = new TreeListState<Employee>()
+                {
+                    ExpandedItems = new List<Employee>(toExpand)
+                };
+
+            await TreeListRef.SetStateAsync(expandedState);
+        }
+        else
+        {
+            var expandedState = new TreeListState<Employee>()
+                {
+                    ExpandedItems = new List<Employee>()
+                };
+
+            await TreeListRef.SetStateAsync(expandedState);
+        }
+
+    }
+
+    private async Task OnStateInitHandler(TreeListStateEventArgs<Employee> args)
+    {
+        var collapsedItemsState = new TreeListState<Employee>()
+            {
+                //collapse all items in the TreeList upon initialization of the state
+                ExpandedItems = new List<Employee>()
+            };
+
+        args.TreeListState = collapsedItemsState;
+    }
+
+    protected override async Task OnInitializedAsync()
+    {
+        Data = await GetTreeListData();
+    }
+
+    private async Task<List<Employee>> GetTreeListData()
+    {
+        List<Employee> data = new List<Employee>();
+
+        for (int i = 1; i < 15; i++)
+        {
+            Employee root = new Employee
+                {
+                    Id = LastId,
+                    Name = $"root: {i}",
+                    EmailAddress = $"{i}@example.com",
+                    HireDate = DateTime.Now.AddYears(-i),
+                    DirectReports = new List<Employee>(), // prepare a collection for the child items, will be populated later in the code
+                };
+            data.Add(root);
+            LastId++;
+
+            for (int j = 1; j < 4; j++)
+            {
+                int currId = LastId;
+                Employee firstLevelChild = new Employee
+                    {
+                        Id = currId,
+                        Name = $"first level child {j} of {i}",
+                        EmailAddress = $"{currId}@example.com",
+                        HireDate = DateTime.Now.AddDays(-currId),
+                        DirectReports = new List<Employee>(), // collection for child nodes
+                    };
+                root.DirectReports.Add(firstLevelChild); // populate the parent's collection
+                LastId++;
+
+                for (int k = 1; k < 3; k++)
+                {
+                    int nestedId = LastId;
+                    // populate the parent's collection
+                    firstLevelChild.DirectReports.Add(new Employee
+                        {
+                            Id = LastId,
+                            Name = $"second level child {k} of {j} and {i}",
+                            EmailAddress = $"{nestedId}@example.com",
+                            HireDate = DateTime.Now.AddMinutes(-nestedId)
+                        }); ;
+                    LastId++;
+                }
+            }
+        }
+
+        return await Task.FromResult(data);
+    }
+
+    public class Employee
+    {
+        public List<Employee> DirectReports { get; set; }
+
+        public int Id { get; set; }
+        public string Name { get; set; }
+        public string EmailAddress { get; set; }
+        public DateTime HireDate { get; set; }
+    }
+}
+````
+
+## See Also
+
+- [TreeList State Documentation]({%slug treelist-state%})
+- [TreeList Overview]({%slug treelist-overview%})