Add drag and drop section to spec

spec/main.tex

@@ -13,9 +13,10 @@
 
 \section{Wayland Overview}
 
- - wayland is a protocol for a new display server.  
-
- - wayland is an implementation
+\begin{itemize}
+\item wayland is a protocol for a new display server.  
+\item wayland is an implementation
+\end{itemize}
 
 \subsection{Replacing X11}
 
@@ -68,28 +69,33 @@ the type of event.  Events are generated both in repsonse to a request
 (in which case the request and the event constitutes a round trip) or
 spontanously when the server state changes.
 
-    - state is broadcast on connect, events sent out when state
-      change.  client must listen for these changes and cache the state.
-      no need (or mechanism) to query server state.
+\begin{itemize}
+\item state is broadcast on connect, events sent out when state
+  change.  client must listen for these changes and cache the state.
+  no need (or mechanism) to query server state.
 
-    - server will broadcast presence of a number of global objects,
-      which in turn will broadcast their current state
+\item server will broadcast presence of a number of global objects,
+  which in turn will broadcast their current state
+\end{itemize}
 
 \subsection{Connect Time}
 
- - no fixed format connect block, the server emits a bunch of events
-   at connect time
-
- - presence events for global objects: output, compositor, input devices
-
+\begin{itemize}
+\item no fixed format connect block, the server emits a bunch of
+  events at connect time
+\item presence events for global objects: output, compositor, input
+  devices
+\end{itemize}
 \subsection{Security and Authentication}
 
- - mostly about access to underlying buffers, need new drm auth
-   mechanism (the grant-to ioctl idea), need to check the cmd stream?
+\begin{itemize}
+\item mostly about access to underlying buffers, need new drm auth
+  mechanism (the grant-to ioctl idea), need to check the cmd stream?
 
- - getting the server socket depends on the compositor type, could be
-   a system wide name, through fd passing on the session dbus. or the
-   client is forked by the compositor and the fd is already opened.
+\item getting the server socket depends on the compositor type, could
+  be a system wide name, through fd passing on the session dbus. or
+  the client is forked by the compositor and the fd is already opened.
+\end{itemize}
 
 \subsection{Creating Objects}
 
@@ -124,7 +130,7 @@ created by the client
 global object
 
 \begin{itemize}
-\item - input group, keyboard, mouse
+\item input group, keyboard, mouse
 \item keyboard map, change events
 \item pointer motion
 \item enter, leave, focus
@@ -135,39 +141,192 @@ global object
 
 \subsection{Output}
 
- - global objects
- - a connected screen
- - laid out in a big coordinate system
- - basically xrandr over wayland
+\begin{itemize}
+\item global objects
+\item a connected screen
+\item laid out in a big coordinate system
+\item basically xrandr over wayland
+\end{itemize}
+
+\subsection{Drag and Drop}
+
+Multi-device aware. Orthogonal to rest of wayland, as it is its own
+toplevel object.  Since the compositor determines the drag target, it
+works with transformed surfaces (dragging to a scaled down window in
+expose mode, for example).
+
+Issues: 
+
+\begin{itemize}
+\item we can set the cursor image to the current cursor + dragged
+  object, which will last as long as the drag, but maybe an request to
+  attach an image to the cursor will be more convenient?
+
+\item Should drag.send() destroy the object?  There's nothing to do
+  after the data has been transferred.
+
+\item How do we marshall several mime-types?  We could make the drag
+  setup a multi-step operation: dnd.create, drag.offer(mime-type1,
+  drag.offer(mime-type2), drag.activate().  The drag object could send
+  multiple offer events on each motion event.  Or we could just
+  implement an array type, but that's a pain to work with.
+
+\item Middle-click drag to pop up menu?  Ctrl/Shift/Alt drag?
+
+\item Send a file descriptor over the protocol to let initiator and
+  source exchange data out of band?
+
+\item Action?  Specify action when creating the drag object? Ask
+  action?
+\end{itemize}
+
+New objects, requests and events:
+
+  - New toplevel dnd global.  One method, creates a drag object:
+
+	dnd.start(new object id, surface, input device, mime types), 
+
+    Starts drag for the device, if it's grabbed by the surface. drag
+    ends when button is released.  Caller is responsible for
+    destroying the drag object.
+
+  - Drag object methods:
+
+	drag.destroy(id), destroy drag object.
+
+	drag.send(id, data), send drag data.
+
+	drag.accept(id, mime type), accept drag offer, called by
+	target surface.
+
+  - drag object events:
+
+	drag.offer(id, mime-types), sent to potential destination
+	surfaces to offer drag data.  If the device leaves the window
+	or the originator cancels the drag, this event is sent with
+	mime-types = NULL.
+
+	drag.target(id, mime-type), sent to drag originator when a
+	target surface has accepted the offer. if a previous target
+	goes away, this event is sent with mime-type = NULL.
+
+	drag.data(id, data), sent to target, contains dragged data.
+	ends transaction on the target side.
+
+Sequence of events:
+
+\begin{itemize}
+\item The initiator surface receives a click (which grabs the input
+  device to that surface) and then enough motion to decide that a drag
+  is starting.  Wayland has no subwindows, so it's entirely up to the
+  application to decide whether or not a draggable object within the
+  surface was clicked.
+
+\item The initiator creates a drag object by calling the create\_drag
+  method on the dnd global object.  As for any client created object,
+  the client allocates the id.  The create\_drag method also takes the
+  originating surface, the device that's dragging and the mime-types
+  supported.  If the surface has indeed grabbed the device passed in,
+  the server will create an active drag object for the device.  If the
+  grab was released in the meantime, the drag object will be
+  in-active, that is, the same state as when the grab is released.  In
+  that case, the client will receive a button up event, which will let
+  it know that the drag finished.  To the client it will look like the
+  drag was immediately cancelled by the grab ending.
+
+  The special mime-type application/x-root-target indicates that the
+  initiator is looking for drag events to the root window as well.
+
+\item To indicate the object being dragged, the initiator can replace
+  the pointer image with an larger image representing the data being
+  dragged with the cursor image overlaid.  The pointer image will
+  remain in place as long as the grab is in effect, since no other
+  surfaces receive enter/leave events.
+
+\item As long as the grab is active (or until the initiator cancels
+  the drag by destroying the drag object), the drag object will send
+  "offer" events to surfaces it moves across. As for motion events,
+  these events contain the surface local coordinates of the device as
+  well as the list of mime-types offered.  When a device leaves a
+  surface, it will send an offer event with an empty list of
+  mime-types to indicate that the device left the surface.
+
+\item If a surface receives an offer event and decides that it's in an
+  area that can accept a drag event, it should call the accept method
+  on the drag object in the event.  The surface passes a mime-type in
+  the request, picked from the list in the offer event, to indicate
+  which of the types it wants.  At this point, the surface can update
+  the appearance of the drop target to give feedback to the user that
+  the drag has a valid target.  If the offer event moves to a
+  different drop target (the surface decides the offer coordinates is
+  outside the drop target) or leaves the surface (the offer event has
+  an empty list of mime-types) it should revert the appearance of the
+  drop target to the inactive state.  A surface can also decide to
+  retract its drop target (if the drop target disappears or moves, for
+  example), by calling the accept method with a NULL mime-type.
+
+\item When a target surface sends an accept request, the drag object
+  will send a target event to the initiator surface.  This tells the
+  initiator that the drag currently has a potential target and which
+  of the offered mime-types the target wants.  The initiator can
+  change the pointer image or drag source appearance to reflect this
+  new state.  If the target surface retracts its drop target of if the
+  surface disappears, a target event with a NULL mime-type will be
+  sent.
+
+  If the initiator listed application/x-root-target as a valid
+  mime-type, dragging into the root window will make the drag object
+  send a target event with the application/x-root-target mime-type.
+
+\item When the grab is released (indicated by the button release
+  event), if the drag has an active target, the initiator calls the
+  send method on the drag object to send the data to be transferred by
+  the drag operation, in the format requested by the target.  The
+  initiator can then destroy the drag object by calling the destroy
+  method.
+
+\item The drop target receives a data event from the drag object with
+  the requested data.
+\end{itemize}
+
+MIME is defined in RFC's 2045-2049. A registry of MIME types is
+maintained by the Internet Assigned Numbers Authority (IANA).
+
+ftp://ftp.isi.edu/in-notes/iana/assignments/media-types/
+
 
 \section{Types of compositors}
 
 \subsection{System Compositor}
 
- - ties in with graphical boot
- - hosts different types of session compositors
- - lets us switch between multiple sessions (fast user switching,
+\begin{itemize}
+\item ties in with graphical boot
+\item hosts different types of session compositors
+\item lets us switch between multiple sessions (fast user switching,
    secure/personal desktop switching)
- - multiseat
- - linux implementation using libudev, egl, kms, evdev, cairo
- - for fullscreen clients, the system compositor can reprogram the
+\item multiseat
+\item linux implementation using libudev, egl, kms, evdev, cairo
+\item for fullscreen clients, the system compositor can reprogram the
    video scanout address to source fromt the client provided buffer.
+\end{itemize}
 
 \subsection{Session Compositor}
 
- - nested under the system compositor.  nesting is feasible because
+\begin{itemize}
+\item nested under the system compositor.  nesting is feasible because
    protocol is async, roundtrip would break nesting
- - gnome-shell
- - moblin
- - compiz?
- - kde compositor?
- - text mode using vte
- - rdp session
- - fullscreen X session under wayland
- - can run without system compositor, on the hw where it makes
+\item gnome-shell
+\item moblin
+\item compiz?
+\item kde compositor?
+\item text mode using vte
+\item rdp session
+\item fullscreen X session under wayland
+\item can run without system compositor, on the hw where it makes
    sense
- - root window less X server, bridging X windows into a wayland
+\item root window less X server, bridging X windows into a wayland
    session compositor
+\end{itemize}
 
 \subsection{Embbedding Compositor}
 
@@ -188,8 +347,10 @@ the requests the embedded client needs to inform the host about buffer
 updates and a mechanism for forwarding input events from the host
 application.
 
- - firefox embedding flash by being a special purpose compositor to
+\begin{itemize}
+\item firefox embedding flash by being a special purpose compositor to
    the plugin
+\end{itemize}
 
 \section{Implementation}
 
@@ -199,45 +360,50 @@ what's currently implemented
 
 \texttt{libwayland-server.so}
 
- - implements protocol side of a compositor
-
- - minimal, doesn't include any rendering or input device handling
-
- - helpers for running on egl and evdev, and for nested wayland
+\begin{itemize}
+\item implements protocol side of a compositor
+\item minimal, doesn't include any rendering or input device handling
+\item helpers for running on egl and evdev, and for nested wayland
+\end{itemize}
 
 \subsection{Wayland Client Library}
 
 \texttt{libwayland.so}
 
- - minimal, designed to support integration with real toolkits such as
+\begin{itemize}
+\item minimal, designed to support integration with real toolkits such as
    Qt, GTK+ or Clutter.
 
- - doesn't cache state, but lets the toolkits cache server state in
+\item doesn't cache state, but lets the toolkits cache server state in
    native objects (GObject or QObject or whatever).
+\end{itemize}
 
 \subsection{Wayland System Compositor}
 
- - implementation of the system compositor
+\item implementation of the system compositor
 
- - uses libudev, eagle (egl), evdev and drm
+\item uses libudev, eagle (egl), evdev and drm
 
- - integrates with ConsoleKit, can create new sessions
+\item integrates with ConsoleKit, can create new sessions
 
- - allows multi seat setups
+\item allows multi seat setups
 
- - configurable through udev rules and maybe /etc/wayland.d type thing
+\item configurable through udev rules and maybe /etc/wayland.d type thing
+\end{itemize}
 
 \subsection{X Server Session}
 
- - xserver module and driver support
+\begin{itemize}
+\item xserver module and driver support
 
- - uses wayland client library
+\item uses wayland client library
 
- - same X.org server as we normally run, the front buffer is a wayland
+\item same X.org server as we normally run, the front buffer is a wayland
    surface but all accel code, 3d and extensions are there
 
- - when full screen the session compositor will scan out from the X
+\item when full screen the session compositor will scan out from the X
    server wayland surface, at which point X is running pretty much as it
    does natively.
+\end{itemize}
 
 \end{document}