Q: Accept URLs Using
Drag-and-Drop
I'd like to allow users to drag a link from Internet Explorer into
certain controls on my VB form. How can I offer this in a way that
provides both appropriate feedback to the user, and allows me to know
the precise URL being dropped?
A: An often overlooked, and certainly
under-appreciated, method offered by VB forms and controls is OLE
drag-and-drop. When the user drags something from most Microsoft
applications (including from Internet Explorer), the application
packages that something in an OLE DataObject. Your application can then
examine this DataObject as the user drags it over your form or controls,
and can offer feedback to the user indicating whether you'll accept it
being dropped.
I've always found the most flexibility in taking direct control over
operations such as this. Set the OLEDropMode of the objects that accept
drops to vbOLEDropManual. With this setting, VB fires the associated
object's OLEDragOver event whenever the user drags a DataObject over the
object. You can test the DataObject in the OLEDragOver event to see if
it contains data you're interested in. For a URL, use the DataObject's
GetData method to retrieve its raw text contents.
The component that packages the DataObject initially also sets what
"effect" the user sees when over an OLE drop target (see Figure
1). If you don't find what you're looking for in the DataObject when
OLEDragOver fires—in this case, a URL—set Effect to vbDropEffectNone
and return from the event. If the DataObject appears to contain
something of interest (a URL), your first instinct might be to return
vbDropEffectCopy as the desired Effect. However, this might not be
appropriate in some cases.
How you set Effect determines what feedback the user receives. But
you can't set Effect to a value not set initially by the DataObject's
packager. OLE's IDropTarget interface offers one type not exposed by
VB's type library: DROPEFFECT_LINK, which has a value of 4. You can test
for this type specifically, or you can test the text itself to see if it
"looks good" (see Listing
1). The Effect parameter is a bitfield, and you can't set it to
values not turned on already, so the easiest answer is to leave it alone
as long as the DataObject seems to contain appropriate data. —K.E.P.
About the Author
Karl E. Peterson is a GIS analyst with a regional
transportation-planning agency and serves as a member of the Visual
Studio Magazine Technical Review and Editorial Advisory Boards.
Online, he's a Microsoft MVP and a section leader on several VSM
forums. Find more of Karl's VB samples at www.mvps.org/vb.
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