VB Shell Programming

Drop Handler Interfaces

Now that you know how a drop handler works, let’s discuss the interfaces involved in a little more detail.

IDropTarget

As Table 7.1 shows, IDropTarget contains four methods: DragEnter, DragOver, DragLeave, and Drop.

Table 7-1. IDropTarget

Method	Description
DragEnter	Determines whether a drop is valid.
DragOver	Provides drop information as object is dragged over the target.
DragLeave	Called when object leaves drop target.
Drop	Called when object is dropped on the target.

DragEnter

DragEnter is used to determine whether a drop is acceptable. For instance, if the file being dropped is not of a particular type, the operation could be cancelled (we will not do this here). This method also provides us with some minimal keyboard state information, such as whether the Alt, Shift, or Ctrl keys were pressed during the drop. Left- and right-mouse button states are also provided.

The syntax of the DragEnter method is as follows:

HRESULT DragEnter(IDataObject *pDataObj,
                  DWORD grfKeyState,
                  POINTL pt,
                  DWORD *pdwEffect);

The method’s parameters are:

pDataObj

[in] A pointer to an IDataObject interface on a data object. This object contains the data being transferred via the drag-and-drop operation.

grfKeyState

[in] The current state of the keyboard. It can contain one or more of the following values:

Constant	Description
MK_LBUTTON	Left mouse button is pressed.
MK_RBUTTON	Right mouse button is pressed.
MK_SHIFT	Shift key is pressed.
MK_CONTROL ...