ACC97: Example Using TreeView Control Drag-and-Drop Capabilities (165993)

MORE INFORMATION

The Microsoft TreeView control, version 5.0 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 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 97 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 the Microsoft TreeView Control, version 5.0, and then click OK.
5. Set the following properties for the TreeView control:

   Name: xTree
   Width: 3"
   Height" 2"

6. Double-click the TreeView control to set its custom properties on the General tab of the TreeCtrl Properties dialog box:

   Style: 6 - tvwTreelinesPlusMinusText
   LineStyle: 1 - tvwRootLines
   OLEDragMode: 1 - ccOLEDragAutomatic
   OLEDropMode: 1 - ccOLEDropManual

   Click OK.
7. Save the form as TreeViewTest.
8. On the View menu, click Code.
9. 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, rst As Recordset, nodCurrent As Node
         Dim objTree As TreeView, strText As String, nodRoot As Node
         Dim 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"
         Loop
   
         ExitForm_Load:
            Exit Sub
         ErrForm_Load:
            MsgBox Err.Description, vbCritical, "Form_Load"
            Resume ExitForm_Load
         End 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, 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)
         Loop
   
         ExitAddChildren:
            Exit Sub
         ErrAddChildren:
            MsgBox "Can't add child:  " & Err.Description, vbCritical, _
               "AddChildren(nodBoss As Node) Error:"
            Resume ExitAddChildren
         End 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 = Nothing
         End 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, strKey As String, strText As String
         Dim nodNew As Node, nodDragged As Node
         Dim db As Database, 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 = Nothing
         ExitxTree_OLEDragDrop:
            Exit Sub
         ErrxTree_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_OLEDragDrop
         End Sub
   					

10. On the Debug menu, click "Compile and Save All Modules."
11. Close the form's class module 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.