Drag and Drop Operations

When a user drags one object from one location to another, your application needs to know about it.

Drag-and-drop is a process where the user, with the mouse, clicks an object, holds down the mouse button, and drags that object from one location to another location.

VB supports two kinds of drag-and-drop operations:

•  Automatic 
•  Manual 

The Automatic drag-n-drop is the simplest. The form's DragDrop event controls the placement of the drop. To set up the drag, you only need to change the control's DragMode property to 1-Automatic. When the user moves the control, VB displays an outline of the control, but it doesn't actually move the object. The following code performs automatic drag-and-drop:

Private Sub frmDemo_DragDrop (Source As Control, _
X As Single, Y As Single)

   ' the code receives the actual control
   ' that the user dragged as an argument

   'move the control to the dropped location
   Source.Move X, Y

End Sub

The Move method moves the control from its original location to the desired location (where the user releases the mouse button).

The manual drag-n-drop works like the automatic drag-n-drop, with the following differences:

• You must set the DragMode property to 0-Manual
• The Manual setting also allows you to recognize a MouseDown event before dragging starts, so that you can record the mouse position.
• You must add code to the mouse Down event procedure to invoke the drag.

To enable dragging from code, leave DragMode in its default setting (0-Manual). Then use the Drag method whenever you want to begin or stop dragging an object. Use the following Visual Basic constants to specify the action of the Drag argument.

The MouseDown event procedure can perform the special Drag method on the object is you want to continue the drag-n-drop operation.

Private Sub Image1_MouseDown(Button As Integer, _
Shift As Integer, X As Single, Y As Single)

   txtStore.Text = "Clicked over the image at " & X & ", " & Y
   Image1.Drag

End Sub

The Drag method initiates dragging of the control.

Use the manual drag-and-drop operations if you want to set up drag-and-drop limitations before and during the drag-and-drop process.

One of the foundations of Windows applications is that they respond to the mouse. Today's lesson explained how to integrate the mouse into your Visual Basic application. The mouse events inform your program of clicks, double clicks, and the buttons pressed. Moreover, we explained how to implement drag-and-drop operations.