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How to use TreeView control Drag-and-Drop capabilities in Access 2000

This article was previously published under Q209898
For a Microsoft Access 97 version of this article, see 165993.
Advanced: Requires expert coding, interoperability, and multiuser skills.

This article demonstrates how to use drag-and-drop functionality in the Microsoft TreeView control, version 6.0.

The Microsoft TreeView control, version 6.0 ships with Microsoft Office 2000 Developer.

Microsoft provides programming examples for illustration only, without warranty either expressed or implied. This includes, but is not limited to, the implied warranties of merchantability or fitness for a particular purpose. This article assumes that you are familiar with the programming language that is being demonstrated and with the tools that are used to create and to debug procedures. Microsoft support engineers can help explain the functionality of a particular procedure, but they will not modify these examples to provide added functionality or construct procedures to meet your specific requirements.
The Microsoft TreeView control, version 6.0, features built-in drag-and-drop functionality that was not available in earlier versions. It supports drag-and-drop events, such as OLEStartDrag, OLEDragOver, OLECompleteDrag, and OLEDragDrop.

The following example creates a hierarchical list of employees in the Northwind sample database. The TreeView control displays employees according to whom they report to and it enables you to drag employee names to reorder the hierarchy and update the underlying table.

CAUTION: Following the steps in this example will modify the sample database Northwind.mdb. You may want to back up the Northwind.mdb file and perform these steps on a copy of the database.
  1. Start Microsoft Access and open the sample database Northwind.mdb.
  2. Create a new form not based on any table or query in Design view.
  3. On the Insert menu, click ActiveX Control.
  4. In the Insert ActiveX Control dialog box, select Microsoft TreeView Control, version 6.0, then click OK.
  5. Set the following properties for the TreeView control:
       Name: xTree   Width: 3"   Height: 2"					
  6. Double-click the TreeView control and set the following custom properties on the General tab of the TreeCtrl Properties dialog box:
       Style: 6 - tvwTreelinesPlusMinusText   LineStyle: 1 - tvwRootLines   OLEDragMode: 1 - ccOLEDragAutomatic   OLEDropMode: 1 - ccOLEDropManual					
  7. Click OK.
  8. Save the form as TreeViewTest.
  9. On the View menu, click Code.
  10. Type the following procedures:
    '=================================================================='This procedure populates the TreeView control when the form opens.'==================================================================Private Sub Form_Load()    On Error GoTo ErrForm_Load    Dim db As Database    Dim rst As Recordset    Dim nodCurrent As Node, nodRoot As Node    Dim objTree As TreeView    Dim strText As String, bk As String    Set db = CurrentDb    'Open the Employees table.    Set rst = db.OpenRecordset("Employees", dbOpenDynaset, dbReadOnly)    'Create a reference to the TreeView Control.    Set objTree = Me!xTree.Object    'Find the first employee who is a supervisor.    rst.FindFirst "[ReportsTo] Is Null"    'Build the TreeView list of supervisors and their employees.    Do Until rst.NoMatch        'Extract the supervisor's name.        strText = rst![LastName] & (", " + rst![FirstName])        'Add a root level node to the tree for the supervisor.        Set nodCurrent = objTree.Nodes.Add(, , "a" & rst!EmployeeID, _            strText)        'Use a placeholder to save this place in the recordset.        bk = rst.Bookmark        'Run a recursive procedure to add all the child nodes        'for employees who report to this supervisor.        AddChildren nodCurrent, rst        'Return to your placeholder.        rst.Bookmark = bk        'Find the next supervisor.        rst.FindNext "[ReportsTo] Is Null"    LoopExitForm_Load:    Exit SubErrForm_Load:    MsgBox Err.Description, vbCritical, "Form_Load"    Resume ExitForm_LoadEnd Sub'==================================================================='This procedure adds child nodes to the tree for all employees who'report to a particular supervisor, and calls itself recursively'to add child nodes for all other employees they supervise.''Note that this procedure accepts the open Employees recordset by'reference so you do not have to open a new recordset for each call.'===================================================================Sub AddChildren(nodBoss As Node, rst As Recordset)    On Error GoTo ErrAddChildren    Dim nodCurrent As Node    Dim objTree As TreeView    Dim strText As String, bk As String    'Create a reference to the TreeView control.    Set objTree = Me!xTree.Object    'Find the first employee who reports to the supervisor for this node.    rst.FindFirst "[ReportsTo] =" & Mid(nodBoss.Key, 2)    'Build the list of employees who report to this supervisor.    Do Until rst.NoMatch        'Extract the employee's name.        strText = rst![LastName] & (", " + rst![FirstName])        'Add as a child node to the tree.        Set nodCurrent = objTree.Nodes.Add(nodBoss, tvwChild, "a" & _            rst!EmployeeID, strText)        'Save your place in the recordset.        bk = rst.Bookmark        'Add any employees for whom the current node is a supervisor.        AddChildren nodCurrent, rst        'Return to your place in the recordset and continue to search.        rst.Bookmark = bk        'Find the next employee who reports to this supervisor.        rst.FindNext "[ReportsTo]=" & Mid(nodBoss.Key, 2)    LoopExitAddChildren:    Exit SubErrAddChildren:    MsgBox "Can't add child:  " & Err.Description, vbCritical, _        "AddChildren(nodBoss As Node) Error:"    Resume ExitAddChildrenEnd Sub'=================================================================='This procedure in the OLEStartDrag event of the TreeView control'clears the selected node so you can choose a new one.'==================================================================Private Sub xTree_OLEStartDrag(Data As Object, AllowedEffects As _        Long)    Me!xTree.Object.SelectedItem = NothingEnd Sub'===================================================================='Use the OLEDragOver event of the TreeView control to select the'node to drag, and to highlight the target nodes where the drop will'occur when you release the mouse. This procedure sets the selected'node to drag once. After that, if a node is already selected, the'procedure assumes it was set during an earlier call in the dragging'process and it does not reset it.  The second half of this procedure'highlights the node you are dragging over.'====================================================================Private Sub xTree_OLEDragOver(Data As Object, Effect As Long, _        Button As Integer, Shift As Integer, x As Single, y As Single, _        State As Integer)    Dim oTree As TreeView    'Create a reference to the TreeView control.    Set oTree = Me!xTree.Object    'If no node is selected, select the first node you dragged over.    If oTree.SelectedItem Is Nothing Then        Set oTree.SelectedItem = oTree.HitTest(x, y)    End If    'Highlight the node being dragged over as a potential drop target.    Set oTree.DropHighlight = oTree.HitTest(x, y)End Sub'=================================================================='The OLEDragDrop event moves the selected node on the TreeView'control to its new location and changes the corresponding record in'the Employees table. The procedure first checks that the TreeView'has a selected node. If so, it continues to check if a drop target'node is highlighted. If no node is highlighted, then the user has'dragged the node off the tree and dropped it into a blank area, and'the procedure adds a branch to the root of the tree. If a node is'highlighted, the procedure modifies the Employee table's ReportTo'field accordingly and sets the selected node's parent property'to the node that has the drop highlight.'==================================================================Private Sub xTree_OLEDragDrop(Data As Object, Effect As Long, _        Button As Integer, Shift As Integer, x As Single, y As Single)    On Error GoTo ErrxTree_OLEDragDrop    Dim oTree As TreeView    Dim strKey As String, strText As String    Dim nodNew As Node, nodDragged As Node    Dim db As Database    Dim rs As Recordset    Set db = CurrentDb    'Open the Employees table for editing.    Set rs = db.OpenRecordset("Employees", dbOpenDynaset)    'Create a reference to the TreeView control.    Set oTree = Me!xTree.Object    'If nothing is selected for drag, do nothing.    If oTree.SelectedItem Is Nothing Then    Else        'Reference the selected node as the one being dragged.        Set nodDragged = oTree.SelectedItem        'If the node was dragged to an empty space, update the        'Employees table and make this employee a root node.        If oTree.DropHighlight Is Nothing Then            'Save the key and the text to use when you re-add the node.            strKey = nodDragged.Key            strText = nodDragged.Text            'Delete the current node for the employee.            oTree.Nodes.Remove nodDragged.Index            'Locate the record in the Employees table and update it.            rs.FindFirst "[EmployeeID]=" & Mid(strKey, 2)            rs.Edit            rs![ReportsTo] = Null            rs.Update            'Add this employee as a root node.            Set nodNew = oTree.Nodes.Add(, , strKey, strText)            'Add all the child nodes for this employee.            AddChildren nodNew, rs            'If you are not dropping the node on itself.        ElseIf nodDragged.Index <> oTree.DropHighlight.Index Then            'Set the drop target as the selected node's parent.            Set nodDragged.Parent = oTree.DropHighlight            'Locate the record in the Employees table and update it.            rs.FindFirst "[EmployeeID]=" & Mid(nodDragged.Key, 2)            rs.Edit            rs![ReportsTo] = Mid(oTree.DropHighlight.Key, 2)            rs.Update        End If    End If    'Deselect the node    Set nodDragged = Nothing    'Unhighlight the nodes.    Set oTree.DropHighlight = NothingExitxTree_OLEDragDrop:    Exit SubErrxTree_OLEDragDrop:    'If you create a circular branch.    If Err.Number = 35614 Then        MsgBox "A supervisor cannot report to a subordinate.", _            vbCritical, "Move Cancelled"    Else        MsgBox "An error occurred while trying to move the node.  " & _            "Please try again." & vbCrLf & Error.Description    End If    Resume ExitxTree_OLEDragDropEnd Sub					
  11. On the Debug menu, click Compile Northwind.
  12. Close the Microsoft Visual Basic Editor and switch the form to Form view. Practice dragging and dropping employee names in the TreeView control and note how both the tree and the Employees table are updated with the new information.
For more information about using the TreeView control, in the Visual Basic Editor, click Microsoft Visual Basic Help on the Help menu, type TreeView control in the Office Assistant or the Answer Wizard, and then click Search to view the topic.

For additional information about the TreeView control, please see the following article in the Microsoft Knowledge Base:
162523 Migrating from Data Outline Control to TreeView Control
For more information about using recursion in Microsoft Access, in the Visual Basic Editor, click Microsoft Visual Basic Help on the Help menu, type recursive procedures in the Office Assistant or the Answer Wizard, and then click Search to view the topic.
DragDrop StartDrag DragOver Recursive

Article ID: 209898 - Last Review: 10/11/2006 18:52:55 - Revision: 4.3

  • Microsoft Access 2000 Standard Edition
  • Microsoft Office 2000 Developer Edition
  • kbsampledatabase kbhowto kbprogramming KB209898