This document describes some of the internals of the DND handling code. Organization ============ The DND code is split between a lowlevel part - gdkdnd.c and a highlevel part - gtkdnd.c. To put it simply, gdkdnd.c contain the portions of DND code that are easiest to do in raw X, while gtkdnd.c contains the portions of DND that are easiest to do with an event loop and high level selection handling. Except for a few details of selection handling, most of the dependencies on the DND protocol are confined to gdkdnd.c. There are two or three supported protocols - Motif DND, Xdnd and a pseudo-protocol ROOTWIN, which is used for drops on root windows that aren't really accepting drops. gdkdnd.c divides into 4 pieces: 1) Utility functions (finding client windows) 2) Motif specific code (the biggest chunk) 3) Xdnd specific code 4) The public interfaces The code in gtkdnd.c roughly consists of three parts 1) General utility functions 2) Destination side code 3) Source side code. Both on the source and dest side, there is some division between the low level layers and the default handlers, though they are rather mixed in many cases. Structures and Memory Management ================================ Information about source sites and drop sites is stored in the structures GtkSourceSite and GtkDestSite. Information about in-progress drags and drops is stored in the structures GtkSourceInfo and GtkDestInfo. The GtkSourceInfo structure is created when the drag begins, and persists until the drag either completes or times out. A pointer to it is stored in dataset-data for the GdkDragContext, however there is no ownership. If the SourceInfo is destroyed before the context, the field is simply cleared. A GtkDestInfo is attached to each GdkDragContext that is received for an incoming drag. In contrast to the SourceInfo the DestInfo is "owned" by the context, and when the context is destroyed, destroyed. The GDK API =========== It is expect that the GDK DND API will never be used by anything other than the DND code in GTK+. /* Drag and Drop */ GdkDragContext * gdk_drag_context_new (void); These create and refcount GdkDragContexts in a straightforward manner. /* Destination side */ void gdk_drag_status (GdkDragContext *context, GdkDragAction action, guint32 time); void gdk_drag_finish (GdkDragContext *context, gboolean success, guint32 time); GdkAtom gdk_drag_get_selection (GdkDragContext *context); /* Source side */ GdkDragContext * gdk_drag_begin (GdkSurface *window, GList *targets, GdkDragAction actions); gboolean gdk_drag_get_protocol (guint32 xid, GdkDragProtocol *protocol); void gdk_drag_find_window (GdkDragContext *context, GdkSurface *drag_surface, gint x_root, gint y_root, GdkSurface **dest_surface, GdkDragProtocol *protocol); gboolean gdk_drag_motion (GdkDragContext *context, GdkSurface *dest_surface, GdkDragProtocol protocol, gint x_root, gint y_root, GdkDragAction action, guint32 time); void gdk_drag_drop (GdkDragContext *context, guint32 time); void gdk_drag_abort (GdkDragContext *context, guint32 time); GdkAtom gdk_drag_get_selection (GdkDragContext *context); Retrieves the selection that will be used to communicate the data for the drag context (valid on both source and dest sides) Cursors and window hierarchies ============================== The DND code, when possible (and it isn't possible over Motif window) uses a shaped window as a drag icon. Because the cursor may fall inside this window during the drag, we actually have to figure out which window the cursor is in _ourselves_ so we can ignore the drag icon properly. (Oh for OutputOnly windows!) To avoid obscene amounts of server traffic (which are only slightly observable locally, but would really kill a session over a slow link), the code in GDK does XGetWindowAttributes for every child of the root window at the beginning of the drag, then selects with SubstructureNotifyMask on the root window, so that it can update this list. It probably would be easier to just reread the entire list when one of these events occurs, instead of incrementally updating, but updating the list in sync was sort of fun code, so I did it that way ;-) There is also a problem of trying to follow the mouse cursor as well as possible. Currently, the code uses PointerMotionHint, and an XQueryPointer on MotionNotify events. This results in pretty good syncing, but may result in somewhat poor accuracy for drops. (Because the coordinates of the drop are the coordinates when the server receives the button press, which might actually be before the XQueryPointer for the previous MotionNotify event is done.) Probably better is doing MotionNotify compression and discarding MotionNotify events when there are more on the queue before the next ButtonPress/Release. Proxying ======== A perhaps rather unusual feature of GTK's DND is proxying. A dest site can be specified as a proxy drop site for another window. This is most needed for the plug-socket code - the socket needs to pass on drags to the plug since the original source only sees toplevel windows. However, it can also be used as a user visible proxy - i.e., dragging to buttons on the taskbar. Internally, when the outer drag enters a proxy dest site, a new source drag is created, with SourceInfo and GdkDragContext. From the GDK side, it looks much like a normal source drag; on the GTK+ side, most of the code is disjoint. The need to pass in a specific target window is the reason why the GDK DND API splits gdk_drag_find_window() and gdk_drag_motion(). For proxy drags, the GtkDestInfo and GtkSourceInfo for the drag point at each other. Because the abstraction of the drag protocol is at the GDK level, a proxy drag from Motif to Xdnd or vice versa happens pretty much automatically during the drag, though the drop can get complicated. For Xdnd <-> Motif, Motif <-> Xdnd, or Motif <-> Motif drags, it is necessary to for the Proxy to retrieve the data and pass it on to the true destination, since either the selection names differ or (Motif<->Motif), the proxy needs to know about the XmDRAG_SUCCESS/FAILURE selection targets. Further Reading: ================ Xdnd: The spec is at: http://www.cco.caltech.edu/~jafl/xdnd/ Motif: The Motif DND protocol is best described in the Hungry Programmers _Inside Lesstif_ book, available from: http://www.igpm.rwth-aachen.de/~albrecht/hungry.html Harald Albrecht and Mitch Miers have done a far better job at documenting the DND protocol then anything the OpenGroup has produced. Owen Taylor otaylor@redhat.com Oct 18, 1998