IDropTarget::DragOver

Provides target feedback to the user and communicates the drop's effect to the DoDragDrop function so the effect of the drop can be communicated through the DoDragDrop function back to the source.

HRESULT DragOver(

DWORD grfKeyState,	//Current state of keyboard modifier keys
POINTL pt,	//Current cursor coordinates
DWORD * pdwEffect	//Effect of the drag-and-drop operation
);

Parameters

grfKeyState

[in] Identifies the current state of the keyboard modifier keys on the keyboard. Valid values can be a combination of any of the flags MK_CONTROL, MK_SHIFT, MK_ALT, MK_BUTTON, MK_LBUTTON, MK_MBUTTON, and MK_RBUTTON.

pt

[in] Points to the current cursor coordinates in the coordinate space of the drop target window.

pdwEffect

[in, out] Specifies the current effect flag. Valid values are from the enumeration DROPEFFECT.

Return Values

S_OK

The function completed its task successfully.

E_OUTOFMEMORY

Out of memory.

E_INVALIDARG

One or more arguments are invalid.

E_UNEXPECTED

An unexpected error occurred.

Comments

The DoDragDrop function calls this method each time the user moves the mouse across a given target window. DoDragDrop exits the loop if the drag-and-drop operation is canceled, if the user drags the mouse out of the target window, or if the drop is completed.

In IDropTarget::DragOver, you must provide features similar to IDropTarget::DragEnter. You must determine the effect of dropping the data on the target by examining the formats and medium specified for the data object along with the state of the modifier keys. The mouse position may also play a role in determining the effect of a drop. The following modifier keys affect the result of the drop:

Key Combination	User-Visible Feedback	Drop Effect
CTRL + SHIFT	=	DROPEFFECT_LINK
CTRL	+	DROPEFFECT_COPY
No keys or SHIFT	None	DROPEFFECT_MOVE

You communicate the effect of the drop back to the source through DoDragDrop in pdwEffect. The DoDragDrop function then calls IDropSource::GiveFeedback so the source application can display the appropriate visual feedback to the user.

On entry to IDropTarget::DragOver, the pdwEffect parameter is set to the effects given to the DoDragDrop function. The IDropTarget::DragOver method must choose one of these effects or disable the drop.

Upon return, pdwEffect is set to either DROPEFFECT_COPY to copy the dragged data to the target, DROPEFFECT_LINK to create a link to the source data, or DROPEFFECT_MOVE to allow the dragged data to be permanently moved from the source application to the target.

You may also wish to provide appropriate visual feedback in the target window. There may be some target feedback already displayed from a previous call to IDropTarget::DragOver or from the initial IDropTarget::DragEnter. If this feedback is no longer appropriate, you should remove it.

For efficiency reasons, a data object is not passed in IDropTarget::DragOver. The data object passed in the most recent call to IDropTarget::DragEnter is available and can be used.

Notes to Implementors

This function is called frequently during the DoDragDrop loop so it makes sense to optimize your implementation of the DragOver method as much as possible.

See Also

DoDragDrop, IDropSource, IDropTarget, RegisterDragDrop, RevokeDragDrop

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