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GBIC >> VB >> Tutorials >> Drag & Drop
Drag & Drop
You see it in just about every Windows application, so how can you do it in your VB application? Fortunately VB provides built-in tools to make it easy for you. While the code you write to respond to a drag and drop operation can be fairly complex, the mechanics of a drag and drop operation are fairly simple and logical. VB especially handles well the graphical tasks of a drag and drop operation - creating and moving the icons displayed during a drag and drop operation.


Drag and Drop

Ok, a short description first. Put the mouse cursor over an object on a form, then press and hold the left mouse button. Without releasing the button, move the mouse cursor to a new location. Then release the mouse button.

Was that fun? Either way, what you just did was a drag and drop operation. How your program responded to what you did was entirely up to the programmer of the application. In programs which support drag and drop operations, the cursor changes to an icon (indicating a drag operation is taking place) and when the mouse is released the icon reverts back to a normal cursor (indicating that the drop operation is now over).

VB offers both an automated and a manual way to allow users to perform a drag and drop operation. In either case, just like with common dialog windows, a drap and drop operation is just a method of having a user give you instructions/data, which you must use in code before anything happens.

The visible, outward sign of a drag and drop operation is the appearance and then disappearance of an icon during the operation. However, if you don't have code in the appropriate event, absolutely nothing else will take place in your application.

Drag and Drop Events
By way of clarification, a drag and drop event takes places between two objects. At one object a drag is initiated and at the second object a drop takes place.

When the drag begins, the first object's does not experience an event. VB silently handles the creation and movement of the drag icon. When the mouse is released, signalling a drop operation, the DragDrop event of the receiving control is fired and any code in that event is executed. If the DragDrop event has no code, then nothing else happens.

A second event is possible, which is the DragOver event. After a drag is initiated and at the moment the drag icon is moved over another control, that control's DragOver event is fired. It's only when the mouse button is released that the DragDrop event takes place on the receiving control.

The dragdrop can be very useful, users know how to use it and users see it in most of the popular shrinkwrap applications. With that background, how can you not put it into your own application? Well, first of all, not every application can be made easier to use by simply tossing in some drag and drop features. In my own experience, I use drag and drop in only 1-2 out of ten programs I write. It has to add significant value to be worth the time it takes to program.

There are only two event to worry about, the DragDrop and the DragOver, so it won't take long to cover them both. In this example, the events of a form (Form1) are examined:

Private Sub Form1_DragDrop(Source As Control, _
                      X As Single, Y As Single)
Private Sub Form1_DragOver(Source As Control, _
                      X As Single, Y As Single, State As Integer)

Both events share the common arguments Source, X, and Y, whereas the DragOver events includes the additional argument State.

  • Source
    This is a reference to the control which is being dragged. For example, if you drag a command button onto a form, Source refers to the command button. You would use Source.caption to refer to the caption of the command button. You can access other properties or methods of the command button the same way, by using the argument in place of the actual command button's name.
  • X/Y
    X and Y are the position at which the drop took place. You may use that information in a variety of ways, but if it is used at all, it is typically used to reposition the control from which the drag and drop operation began.
  • State
    An integer value corresponding to whether the dragged object is entering or leaving the control or has simply moved its location within the control.

Drag and Drop Properties

The only two properties of a control which affect a drag and drop operation are:

  • DragMode There are two settings - automatic or manual. If automatic is chosen, then all you do is drag a control (hold down the mouse while moving the cursor) and VB will automatically create the drag icon. If manual is selected, then you must invoke the .Drag method of the control for to begin the drag operation (including the display of the drag icon).
  • DragIcon VB provides a default icon (a small outline image) but you may also define the icon to be used while the drag is taking place.

Drag and Drop Methods

  • Drag
    The only method needed is appropriately called Drag. To use it, simply use code such as:
    command1.drag
    
    This will initiate a drag operation but as in the automatic case you still must write any code that is executed when the drop action occurs.