Drag and Drop

Basics

GG has drag and drop support that allows you to drag any Wnd and drop it onto any other Wnd. The results of this drop depend on the two Wnds involved. Each Wnd has a DragDropDataType() method that returns a string describing what kind of data the Wnd represents. The string can be a MIME type or any arbitrary string the user likes. Upon the drop, the Wnd receiving the drop has its AcceptDrop() method called, in which it can examine the data type or any other relevant aspects of the dropped Wnd and decide whether or not to accept it. If the receiver accepts, the dropped Wnd is removed from its parent, if it has one. Note that only Wnds with non-empty DragDropDataType() or Wnds that were created with the DRAGABLE (both not both!) may be drag-and-dropped. A DRAGABLE Wnd is draggable as long as it is not the child of another Wnd, and as it is dragged, it moves across the screen normally. A Wnd with a non-empty DragDropDataType() string does not move when drag-and-dropped; instead, it is rendered in its original place, and a copy of it is rendered as if the cursor were dragging it.

Advanced Use

There is an extensive set of notification functions that may be called during a drag and drop operation. The drop target and the orginal parent of the dragged Wnd(s), if any, may be notified of various events or queried (e.g. AcceptDrop()). See the Wnd documentation for details.

Sometimes it is desirable to have the drop-receiving Wnd emit a signal to which clients may attach slots. Further, it may be desirable to allow the attached client slots to veto the drop. If this is desired, the recommended means of accomplishing this is to follow the pattern of ListBox. ListBox catches ListBox::DontAcceptDrop exceptions that are thrown in slots attached to its DroppedSignal signal. The signal is emitted after the drop is completed, and if DontAcceptDrop is caught, the stack is unwound in two steps. First, the insertion of the dropped item is rolled back, and then the exception is rethrown so that AcceptDrop() can return false. See the ListBox implementation for further details.

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