Adding drag and drop to a plug-in.

Opticks supports drag and drop GUI interfaces within the Opticks application window.

There are two ways to add drag and drop to a plug-in's GUI depending on what items need to be handled.

Dropping session items into a custom window.

The Opticks plug-in API makes it easy for plug-in windows to accept drops of session items from the session explorer. Any Window sub-class (usually dock windows) can accept session item drops from the session explorer. A custom window needs to call the Window::enableSessionItemDrops() method to enable drop support. The method parameter is an object which implements the Window::SessionItemDropFilter interface. The Window::SessionItemDropFilter::accept() method will be called on a drag enter event for each SessionItem dragged into the custom window. This method should return true if the SessionItem is valid for a drop event. A typical implementation will look like this.

bool CustomWindow::accept(SessionItem *pItem) const
{
   return dynamic_cast<RasterElement*>(pItem) != NULL;
}

This code will accept any drag enter event which contains at least one RasterElement. If any SessionItem in the event is accepted and a drop occurs, the Window will emit Window::signalSessionItemDropped() and Window::signalSessionItemsDropped().

The first signal's argument is a SessionItem pointer and is emitted once for each SessionItem in the event, not just SessionItems accepted by the drop filter. The second signal's argument is a std::vector of SessionItem pointers containing all SessionItems in the event, not just SessionItems accepted by the drop filter.

An example event handler is presented below. This handler is attached to the Window::signalSessionItemDropped() signal.

void CustomWindow::dropSessionItem(Subject& subject, const string& signal, const boost::any& value)
{
   RasterElement* pRaster = dynamic_cast<RasterElement*>(boost::any_cast<SessionItem*>(value));
   if(pRaster != NULL)
   {
      watchNewRasterElement(pRaster);
   }
}

Custom drop handling.

If a plug-in needs to handle other drag and drop types or needs finer grained control over SessionItem drag and drop, a qt event filter can be established.

First, the drop target needs an event handler installed and drops need to be enabled. The drop target can be any QWidget, including widgets which the plug-in does not implement such as SpatialDataView as in the example below.

CustomWidget::CustomWidget()
{
   std::vector<Window*> windows;
   Service<DesktopServices>()->getWindows(SPATIAL_DATA_WINDOW, windows);
   for(std::vector<Window*>::iterator window = windows.begin(); window != windows.end(); ++window)
   {
      SpatialDataWindow* pWindow = static_cast<SpatialDataWindow*>(*window);
      if(pWindow != NULL)
      {
         pWindow->getWidget()->installEventFilter(this);
         pWindow->getWidget()->setAcceptDrops(true);
      }
   }
}

Don't forget to remove the event filter when your custom widget is destroyed.

CustomWidget::~CustomWidget()
{
   std::vector<Window*> windows;
   Service<DesktopServices>()->getWindows(SPATIAL_DATA_WINDOW, windows);
   for(std::vector<Window*>::iterator window = windows.begin(); window != windows.end(); ++window)
   {
      SpatialDataWindow* pWindow = static_cast<SpatialDataWindow*>(*window);
      if(pWindow != NULL)
      {
         pWindow->getWidget()->removeEventFilter(this);
      }
   }
}

The event filter should handle DragEnter and Drop events as in the below example.

bool CustomWidget::eventFilter(QObject *pObj, QEvent *pEvent)
{
   SpatialDataView* pView = dynamic_cast<SpatialDataView*>(pObj);
   switch(pEvent->type())
   {
   case QEvent::DragEnter:
      windowDragEnterEvent(pView, static_cast<QDragEnterEvent*>(pEvent));
      return pEvent->isAccepted();
   case QEvent::Drop:
      windowDropEvent(pView, static_cast<QDropEvent*>(pEvent));
      return pEvent->isAccepted();
   }
   return false;
}

void CustomWidget::windowDragEnterEvent(SpatialDataView *pView, QDragEnterEvent *pEvent)
{
   if(pEvent != NULL && pEvent->mimeData()->hasFormat("text/x-session-id"))
   {
      pEvent->acceptProposedAction();
   }
}

void CustomWidget::windowDropEvent(SpatialDataView *pView, QDropEvent *pEvent)
{
   VERIFYNRV(pEvent && pView);

   QString sessionId(pEvent->mimeData()->data("text/x-session-id"));
   SessionItem* pSessionItem = Service<SessionManager>()->getSessionItem(sessionId);
   if(pSessionItem != NULL)
   {
      watchSessionItem(pSessionItem);
      pEvent->accept();
   }
}

This example accepts drops with the text/x-session-id MIME type. This is the MIME type emitted by the session explorer on session item drags and is normally handled by Dropping session items into a custom window.. The MIME type accepted can be text/x-session-id, any standard Qt drag-and-drop MIME type, or a custom MIME type emitted by a plug-in widget. See the Qt documentation on drag-and-drop for information on generating drag events.