ACC2000: Esempio di utilizzo delle capacità di trascinamento della selezione del controllo TreeView

'==================================================================
'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"
    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
    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)
    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
    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 = 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