ACC: How to Simulate Drag-And-Drop Capabilities (95/97)

MORE INFORMATION

The four procedures below, DragStart, DragStop, DropDetect, and DragDrop, are required to implement drag-and-drop capabilities between controls in Microsoft Access.

DragStart, DragStop

Enables a control to be dragged. DragStart and DragStop are called from the MouseDown and MouseUp events respectively for each control that you want to be capable of being dragged.

   Syntax:

      Sub DragStart (DragFrm As Form)
      Sub DragStop ()

      DragFrm: The form containing the control being dragged.

   Example:

      Private Sub MyControl_MouseDown (Button As Integer, Shift As ...
         DragStart Me
      End Sub

      Private Sub MyControl_MouseDown (Button As Integer, Shift As ...
         DragStop
      End Sub
				

DropDetect

Enables a control to be a drop target. DropDetect should be called from the MouseMove event for each control that you want to be a drop target for a control being dragged.

   Syntax:

      Sub DropDetect (DropFrm As Form, DropCtrl As Control,
                      Button As Integer, Shift As Integer,
                      X As Single, Y As Single)

      DropFrm: The form containing the control being dropped on.
      DropCtrl: The control being dropped on.
      Button, Shift, X, Y: The parameters from the MouseMove event.

   Example:

      Private Sub MyControl_MouseMove (Button As Integer, Shift As ...
         DropDetect Me, Me![Employee ID], Button, Shift, X, Y
      End Sub
				

NOTE: A control can be dragged and be a drop target by calling DragStart, DragStop, and DropDetect from the MouseDown, MouseUp, and MouseMove events respectively.

DragDrop

This procedure will be called in response to a drag-enabled control being dragged to a drop-enabled control. You will add your own code to this procedure to control what happens in response to a drop operation. For example you could copy the contents of the dragged control to the dropped control.

   Syntax:

      Sub DragDrop (DragFrm As Form, DragCtrl As Control, DropFrm As Form,
                    DropCtrl As Control, Button As Integer, Shift As _
                         Integer,
                    X As Single, Y As Single)

   DragFrm:  The form containing the control being dragged.
   DragCtrl: The control being dragged.
   DropFrm:  The form containing the control being dropped on.
   DropCtrl: The control being dropped on.
   Button:   The state of the mouse buttons when the drop occurred.
   Shift:    The state of the SHIFT, CTRL, ALT keys when the drop occurred.
   X, Y:     The x and y coordinates of the mouse where the drop occurred.

   NOTE: For more information on Button, Shift, X, Y arguments, search
   Help for the MouseMove event.
				

Example:

The following example will copy the contents of the drag control to the drop control, trapping and displaying any error that occurs:

   Sub DragDrop (DragFrm As Form, DragCtrl As Control, ...
      On Error Resume Next
      DropCtrl = DragCtrl
      If Err Then MsgBox Error$
   End Sub
				

EXAMPLE 1: Drag and Drop Between Controls on Two Forms

The following example demonstrates how to set the Salesperson combo box on the Northwind Orders form by dragging the EmployeeID field from the Employees form and dropping it into the Salesperson combo box.

1. Open the sample database Northwind.mdb.
2. Create a new module and type the following lines in the Declarations section:
   

          Option Explicit
   
          Dim DragFrm As Form
          Dim DragCtrl As Control
          Dim DropTime
   
          Const MAX_DROP_TIME = .1
   
          Dim CurrentMode As Integer
          Const NO_MODE = 0
          Const DROP_MODE = 1
          Const DRAG_MODE = 2
   						

3. Type the following three procedures:
   

          Sub DragStart (SourceFrm As Form)
             ' NOTE: You should not use Screen.ActiveForm in place of
             ' SourceFrm because you may be dragging from a subform.
             Set DragFrm = SourceFrm
             Set DragCtrl = Screen.ActiveControl
             CurrentMode = DRAG_MODE
          End Sub
   
          Sub DragStop ()
             CurrentMode = DROP_MODE
             DropTime = Timer
          End Sub
   
          Sub DropDetect (DropFrm As Form, DropCtrl As Control, _
                          Button As Integer, Shift As Integer, _
                          X As Single, Y As Single)
             ' If a drop hasn't happened, then exit.
             If CurrentMode <> DROP_MODE Then Exit Sub
             CurrentMode = NO_MODE
   
             ' The timer interval is permitted between the MouseUp event and
             ' the MouseMove event. This ensures that the MouseMove event does
             ' not invoke the Drop procedure unless it is the MouseMove event
             ' that Microsoft Access automatically fires for the Drop control
             ' following the MouseUp event of a drag control. Subsequent
             ' MouseMove events will fail the timer test and be ignored.
   
             If Timer - DropTime > MAX_DROP_TIME Then Exit Sub
   
             ' Did we drag/drop onto ourselves?
             If (DragCtrl.Name <> DropCtrl.Name) Or _
                (DragFrm.hWnd <> DropFrm.hWnd) Then
                ' If not, then a successful drag/drop occurred.
            DragDrop DragFrm, DragCtrl, DropFrm, DropCtrl, Button, Shift, X, Y
            End If
          End Sub
   						

4. Type the following DragDrop procedure.
   
   NOTE: This procedure does not have to appear in the same module as the procedures typed above. Because you will be customizing the contents of the DragDrop procedure to respond to a successful drag-and-drop operation, it may be better to place it in a separate module.
   

          Sub DragDrop (DragFrm As Form, DragCtrl As Control, _
                        DropFrm As Form, DropCtrl As Control, _
                        Button As Integer, Shift As Integer, _
                        X As Single, Y As Single)
             On Error Resume Next
             DropCtrl = DragCtrl
             If Err Then MsgBox Error$
          End Sub
   						

5. Open the Orders form in Design view and add the following OnMouseMove event procedure code for the Salesperson combo box:
   

          Sub EmployeeID_MouseMove (Button As Integer, Shift As Integer, _
             X As Single, Y As Single)
             DropDetect Me, Me![EmployeeID], Button, Shift, X, Y
          End Sub
   
        NOTE: This control will only be a drop target.
   						

6. Open the Employees form in Design view and set the Enabled property of the EmployeeID text box to Yes.
7. Add the following OnMouseDown event procedure code for the EmployeeID control:
   

          Sub EmployeeID_MouseDown (Button As Integer, Shift As Integer, _
          X As Single, Y As Single)
             DragStart Me
          End Sub
   
        NOTE: This control will only be a drag target.
   						

8. Add the following OnMouseUp event procedure code for the EmployeeID control:
   

          Sub EmployeeID_MouseUp (Button As Integer, Shift As Integer, _
          X As Single, Y As Single)
             DragStop
          End Sub
   						

9. View the Orders form in Form view and position it in the upper-left corner of the screen.
10. View the Employees form in Form view and position it on top of the Orders form, but to the right in such a way that the Salesperson combo box on the Orders form is still visible.
11. Hold down the mouse button in the EmployeeID text box in the Employees form, then move the mouse pointer (still holding the button down) over the Salesperson combo box on the Orders form, and then release the mouse button.

RESULT: The Salesperson combo box will contain the salesperson that you dragged from the Employees form. Try navigating to different Employee records and dragging the Employee ID to the Salesperson combo box.

EXAMPLE 2: Drag and Move Items from One List Box to Another List Box

The following example demonstrates how to present a list of items to a user in one list box and how to move these items back and forth to a second list box using a drag-and-drop operation.

1. Complete the first four steps of EXAMPLE 1 to create the Drag and Drop procedures in the Northwind database.
2. Open the Customers table in Design view and add the following new field:
   

         Field Name: Selected
         Data Type : Yes/No
   						

3. Create a new, blank form called List Box Example with the following properties:
   

         RecordSelectors: No
         NavigationButtons: No
         ScrollBars: Neither
   						

4. Add a list box to the form with the following properties:
   

         Name: List1
         RowSourceType: Table/Query
         RowSource: SELECT CustomerID, CompanyName FROM Customers WHERE
         Selected=False ORDER BY CompanyName;
         ColumnCount: 2
         ColumnWidths: 0
         Width: 1.5"
         Height: 1.5"
   						

5. Add a second list box next to the first list box with the following properties:
   

         Name: List2
         RowSourceType: Table/Query
         RowSource: SELECT CustomerID, CompanyName FROM Customers WHERE
         Selected=True ORDER BY CompanyName;
         ColumnCount: 2
         ColumnWidths: 0
         Width: 1.5"
         Height: 1.5"
   						

6. On the View menu click Code and enter the following event procedure code for the List1 and List2 mouse events:
   

         Private Sub List1_MouseDown (Button As Integer, Shift As Integer, _
         X As Single, Y As Single)
            DragStart Me
         End Sub
   
         Private Sub List1_MouseMove (Button As Integer, Shift As Integer, _
         X As Single, Y As Single)
            DropDetect Me, Me![List1], Button, Shift, X, Y
         End Sub
   
         Private Sub List1_MouseUp (Button As Integer, Shift As Integer, _
         X As Single, Y As Single)
            DragStop
         End Sub
   
         Private Sub List2_MouseDown (Button As Integer, Shift As Integer, _
         X As Single, Y As Single)
            DragStart Me
         End Sub
   
         Private Sub List2_MouseMove (Button As Integer, Shift As Integer, _
         X As Single, Y As Single)
            DropDetect Me, Me![List2], Button, Shift, X, Y
         End Sub
   
         Private Sub List2_MouseUp (Button As Integer, Shift As Integer, _
         X As Single, Y As Single)
            DragStop
         End Sub
   						

7. Modify the DragDrop procedure created in step 4 in Example 1 to read as follows:
   

         Sub DragDrop (DragFrm As Form, DragCtrl As Control, _
                       DropFrm As Form, DropCtrl As Control, _
                       Button As Integer, Shift As Integer, _
                       X As Single, Y As Single)
            ' Which form was dropped on?
            ' It is a good idea to use the DragDrop procedure to
            ' determine which drag-and-drop operation occurred; then call
            ' appropriate code to handle the special cases.
            Select Case DropFrm.Name
               Case "List Box Example"
                  ListBoxExample DragFrm, DragCtrl, DropFrm, DropCtrl, _
                                 Button, Shift, X, Y
               Case Else
                  ' For all other cases, copy contents of Drag to Drop
                  ' control.
                  On Error Resume Next
                  DropCtrl = DragCtrl
                  If Err Then MsgBox Error$
            End Select
         End Sub
   						

8. Type the following new procedure in the module with the DragDrop procedure:
   

         Sub ListBoxExample (DragFrm As Form, DragCtrl As Control, _
                             DropFrm As Form, DropCtrl As Control, _
                             Button As Integer, Shift As Integer, _
                             X As Single, Y As Single)
            Dim DB As Database
            Dim SQL As String
   
            Set DB = CurrentDB()
   
            ' Create SQL statement to update Selected field of
            ' .. drag/dropped list box item.
            SQL = "UPDATE Customers SET Selected="
   
            ' Drag from List1 toggle Selected=True, List2 toggles False.
            SQL = IIF(DragCtrl.Name = "List1", SQL & "True", SQL & "False")
            ' If CTRL key not used, alter dragged value only.
            If (Shift And CTRL_MASK) = 0 Then
                  SQL = SQL & " WHERE [CustomerID]='" & DragCtrl & "'"
            End If
   
            ' Run update query to toggle Selected field of Customer record(s).
            DB.Execute SQL
   
            ' Requery the list box controls to show update lists.
            DragCtrl.Requery
            DropCtrl.Requery
   
         End Sub
   						

9. View the List Box Example form in Form view, and then drag company names from one list to the other and vice versa. Press the CTRL key and drag to move ALL rows from one list to the other list.