protocol: Add DnD actions

These 2 requests have been added:

- wl_data_source.set_actions: Notifies the compositor of the available
  actions on the data source.
- wl_data_offer.set_actions: Notifies the compositor of the available
  actions on the destination side, plus the preferred action.

Out of the data from these requests, the compositor can determine the action
both parts agree on (and let the user play a role through eg. keyboard
modifiers). The chosen option will be notified to both parties
through the following two requests:

- wl_data_source.action
- wl_data_offer.action

In addition, the destination side can peek the source side actions through
wl_data_offer.source_actions.

Compared to the XDND protocol, there's two notable changes:

- XDND lets the source suggest an action, whereas wl_data_device lets
  the destination prefer a given action. The difference is subtle here,
  it comes off as convenience because it is the drag destination which
  receives the motion events (unlike in X) and can perform action updates.

  The drag destination seems also in a better position to update the
  preferred action based on things like the data being transferred, the
  place being dropped, and whether the drag is client-local.

- That same source-side preferred action is used in XDND to convey the
  modifier-induced action to the drag destination, which would then ack
  it, or reply with another action that's accepted (or none), this makes
  the XdndPosition/XdndStatus messaging very verbose, and synchronous
  because the drag source always needs to know the latest status/action
  for every position+action sent.

  Here it's the compositor which takes care of modifiers and matching
  available/accepted actions, this allows for the signaling to happen
  only whenever the actions/modifiers change for real.

Roughly based on previous work by Giulio Camuffo <giuliocamuffo@gmail.com>

Changes since v10:
- Narrow down the situations where wl_data_source/offer.accept requests
  are supposed to happen.

Changes since v9:
- Deferred the protocol errors to .finish after some IRC chat with Jonas,
  added further errors if actions API is used on selection sources/offers.

Changes since v8:
- Defined further the expected behavior on "ask", described the protocol
  errors that may happen. Fix more spaces vs tabs issues.

Changes since v7:
- Misc changes after updating the progress notification patch.

Changes since v6:
- Further explanations on wl_data_source/offer.set_actions, including a
  description of "ask" actions. Added protocol errors for unknown action
  values.

Changes since v5:
- Applied rewording suggestions from Jonas Ådahl. Dropped slot reservation
  scheme for actions. Fixed indentation and other minor formatting issues.

Changes since v4:
- Minor rewording.

Changes since v3:
- Splitted from DnD progress notification changes.
- Further rationales in commit log.

Changes since v2:
- Renamed notify_actions to set_actions on both sides, seems more consistent
  with the rest of the protocol.
- Spelled out better which events may be triggered on the compositor side
  by the requests, the circumstances in which events are emitted, and
  what are events useful for in clients.
- Defined a minimal common ground wrt compositor-side action picking and
  keybindings.
- Acknowledge the possibility of compositor/toolkit defined actions, even
  though none are used at the moment.
Changes since v1:
- Added wl_data_offer.source_actions to let know of the actions offered
  by a data source.
- Renamed wl_data_source.finished to "drag_finished" for clarity
- Improved wording as suggested by Bryce

Signed-off-by: Carlos Garnacho <carlosg@gnome.org>
Reviewed-by: Michael Catanzaro <mcatanzaro@igalia.com>
Reviewed-by: Mike Blumenkrantz <zmike@samsung.com>
Reviewed-by: Jonas Ådahl <jadahl@gmail.com>
Reviewed-by: Bryce Harrington <bryce@osg.samsung.com>

1 files changed, 204 insertions, 1 deletions

diff --git a/protocol/wayland.xml b/protocol/wayland.xml
index 7330b6f..b223bb4 100644
--- a/protocol/wayland.xml
+++ b/protocol/wayland.xml
@@ -421,6 +421,12 @@
     <enum name="error">
       <entry name="invalid_finish" value="0"
 	     summary="finish request was called untimely"/>
+      <entry name="invalid_action_mask" value="1"
+	     summary="action mask contains invalid values"/>
+      <entry name="invalid_action" value="2"
+	     summary="action argument has an invalid value"/>
+      <entry name="invalid_offer" value="3"
+	     summary="offer doesn't accept this request"/>
     </enum>
 
     <request name="accept">
@@ -495,9 +501,98 @@
 	It is a client error to perform other requests than
 	wl_data_offer.destroy after this one. It is also an error to perform
 	this request after a NULL mime type has been set in
-	wl_data_offer.accept.
+	wl_data_offer.accept or no action was received through
+	wl_data_offer.action.
       </description>
     </request>
+
+    <request name="set_actions" since="3">
+      <description summary="set the available/preferred drag-and-drop actions">
+	Sets the actions that the destination side client supports for
+	this operation. This request may trigger the emission of
+	wl_data_source.action and wl_data_offer.action events if the compositor
+	need to change the selected action.
+
+	This request can be called multiple times throughout the
+	drag-and-drop operation, typically in response to wl_data_device.enter
+	or wl_data_device.motion events.
+
+	This request determines the final result of the drag-and-drop
+	operation. If the end result is that no action is accepted,
+	the drag source will receive wl_drag_source.cancelled.
+
+	The dnd_actions argument must contain only values expressed in the
+	wl_data_device_manager.dnd_actions enum, and the preferred_action
+	argument must only contain one of those values set, otherwise it
+	will result in a protocol error.
+
+	While managing an "ask" action, the destination drag-and-drop client
+	may perform further wl_data_offer.receive requests, and is expected
+	to perform one last wl_data_offer.set_actions request with a preferred
+	action other than "ask" (and optionally wl_data_offer.accept) before
+	requesting wl_data_offer.finish, in order to convey the action selected
+	by the user. If the preferred action is not in the
+	wl_data_offer.source_actions mask, an error will be raised.
+
+	If the "ask" action is dismissed (e.g. user cancellation), the client
+	is expected to perform wl_data_offer.destroy right away.
+
+	This request can only be made on drag-and-drop offers, a protocol error
+	will be raised otherwise.
+      </description>
+      <arg name="dnd_actions" type="uint"/>
+      <arg name="preferred_action" type="uint"/>
+    </request>
+
+    <event name="source_actions" since="3">
+      <description summary="notify the source-side available actions">
+	This event indicates the actions offered by the data source. It
+	will be sent right after wl_data_device.enter, or anytime the source
+	side changes its offered actions through wl_data_source.set_actions.
+      </description>
+      <arg name="source_actions" type="uint"/>
+    </event>
+
+    <event name="action" since="3">
+      <description summary="notify the selected action">
+	This event indicates the action selected by the compositor after
+	matching the source/destination side actions. Only one action (or
+	none) will be offered here.
+
+	This event can be emitted multiple times during the drag-and-drop
+	operation in response to destination side action changes through
+	wl_data_offer.set_actions.
+
+	This event will no longer be emitted after wl_data_device.drop
+	happened on the drag-and-drop destination, the client must
+	honor the last action received, or the last preferred one set
+	through wl_data_offer.set_actions when handling an "ask" action.
+
+	Compositors may also change the selected action on the fly, mainly
+	in response to keyboard modifier changes during the drag-and-drop
+	operation.
+
+	The most recent action received is always the valid one. Prior to
+	receiving wl_data_device.drop, the chosen action may change (e.g.
+	due to keyboard modifiers being pressed). At the time of receiving
+	wl_data_device.drop the drag-and-drop destination must honor the
+	last action received.
+
+	Action changes may still happen after wl_data_device.drop,
+	especially on "ask" actions, where the drag-and-drop destination
+	may choose another action afterwards. Action changes happening
+	at this stage are always the result of inter-client negotiation, the
+	compositor shall no longer be able to induce a different action.
+
+	Upon "ask" actions, it is expected that the drag-and-drop destination
+	may potentially choose different a different action and/or mime type,
+	based on wl_data_offer.source_actions and finally chosen by the
+	user (e.g. popping up a menu with the available options). The
+	final wl_data_offer.set_actions and wl_data_offer.accept requests
+	must happen before the call to wl_data_offer.finish.
+      </description>
+      <arg name="dnd_action" type="uint"/>
+    </event>
   </interface>
 
   <interface name="wl_data_source" version="3">
@@ -508,6 +603,13 @@
       to requests to transfer the data.
     </description>
 
+    <enum name="error">
+      <entry name="invalid_action_mask" value="0"
+	     summary="action mask contains invalid values"/>
+      <entry name="invalid_source" value="1"
+	     summary="source doesn't accept this request"/>
+    </enum>
+
     <request name="offer">
       <description summary="add an offered mime type">
 	This request adds a mime type to the set of mime types
@@ -554,6 +656,9 @@
 	- The drag-and-drop operation was performed, but the drop destination
 	  did not accept any of the mimetypes offered through
 	  wl_data_source.target.
+	- The drag-and-drop operation was performed, but the drop destination
+	  did not select any of the actions present in the mask offered through
+	  wl_data_source.action.
 	- The drag-and-drop operation was performed but didn't happen over a
 	  surface.
 	- The compositor cancelled the drag-and-drop operation (e.g. compositor
@@ -569,6 +674,25 @@
 
     <!-- Version 3 additions -->
 
+    <request name="set_actions" since="3">
+      <description summary="set the available drag-and-drop actions">
+	Sets the actions that the source side client supports for this
+	operation. This request may trigger wl_data_source.action and
+	wl_data_offer.action events if the compositor needs to change the
+	selected action.
+
+	The dnd_actions argument must contain only values expressed in the
+	wl_data_device_manager.dnd_actions enum, otherwise it will result
+	in a protocol error.
+
+	This request must be made once only, and can only be made on sources
+	used in drag-and-drop, so it must be performed before
+	wl_data_device.start_drag. Attempting to use the source other than
+	for drag-and-drop will raise a protocol error.
+      </description>
+      <arg name="dnd_actions" type="uint"/>
+    </request>
+
     <event name="dnd_drop_performed" since="3">
       <description summary="the drag-and-drop operation physically finished">
 	The user performed the drop action. This event does not indicate
@@ -588,7 +712,41 @@
 	The drop destination finished interoperating with this data
 	source, so the client is now free to destroy this data source and
 	free all associated data.
+
+	If the action used to perform the operation was "move", the
+	source can now delete the transferred data.
+      </description>
+    </event>
+
+    <event name="action" since="3">
+      <description summary="notify the selected action">
+	This event indicates the action selected by the compositor after
+	matching the source/destination side actions. Only one action (or
+	none) will be offered here.
+
+	This event can be emitted multiple times during the drag-and-drop
+	operation, mainly in response to destination side changes through
+	wl_data_offer.set_actions, and as the data device enters/leaves
+	surfaces.
+
+	It is only possible to receive this event after
+	wl_data_source.dnd_drop_performed if the drag-and-drop operation
+	ended in an "ask" action, in which case the final wl_data_source.action
+	event will happen immediately before wl_data_source.dnd_finished.
+
+	Compositors may also change the selected action on the fly, mainly
+	in response to keyboard modifier changes during the drag-and-drop
+	operation.
+
+	The most recent action received is always the valid one. The chosen
+	action may change alongside negotiation (e.g. an "ask" action can turn
+	into a "move" operation), so the effects of the final action must
+	always be applied in wl_data_offer.dnd_finished.
+
+	Clients can trigger cursor surface changes from this point, so
+	they reflect the current action.
       </description>
+      <arg name="dnd_action" type="uint"/>
     </event>
   </interface>
 
@@ -705,6 +863,17 @@
       <description summary="end drag-and-drag session successfully">
 	The event is sent when a drag-and-drop operation is ended
 	because the implicit grab is removed.
+
+	The drag-and-drop destination is expected to honor the last action
+	received through wl_data_offer.action, if the resulting action is
+	"copy" or "move", the destination can still perform
+	wl_data_offer.receive requests, and is expected to end all
+	transfers with a wl_data_offer.finish request.
+
+	If the resulting action is "ask", the action will not be considered
+	final. The drag-and-drop destination is expected to perform one last
+	wl_data_offer.set_actions request, or wl_data_offer.destroy in order
+	to cancel the operation.
       </description>
     </event>
 
@@ -757,6 +926,40 @@
       <arg name="id" type="new_id" interface="wl_data_device"/>
       <arg name="seat" type="object" interface="wl_seat"/>
     </request>
+
+    <!-- Version 3 additions -->
+
+    <enum name="dnd_action" bitfield="true" since="3">
+      <description summary="drag and drop actions">
+	This is a bitmask of the available/preferred actions in a
+	drag-and-drop operation.
+
+	In the compositor, the selected action is a result of matching the
+	actions offered by the source and destination sides.  "action" events
+	with a "none" action will be sent to both source and destination if
+	there is no match. All further checks will effectively happen on
+	(source actions ∩ destination actions).
+
+	In addition, compositors may also pick different actions in
+	reaction to key modifiers being pressed, one common design that
+	is used in major toolkits (and the behavior recommended for
+	compositors) is:
+
+	- If no modifiers are pressed, the first match (in bit order)
+	  will be used.
+	- Pressing Shift selects "move", if enabled in the mask.
+	- Pressing Control selects "copy", if enabled in the mask.
+
+	Behavior beyond that is considered implementation-dependent.
+	Compositors may for example bind other modifiers (like Alt/Meta)
+	or drags initiated with other buttons than BTN_LEFT to specific
+	actions (e.g. "ask").
+      </description>
+      <entry name="none" value="0"/>
+      <entry name="copy" value="1"/>
+      <entry name="move" value="2"/>
+      <entry name="ask" value="4"/>
+    </enum>
   </interface>
 
   <interface name="wl_shell" version="1">