[PATCH wayland 1/2] protocol: Improve data source notification around DnD progress

[Carlos Garnacho]

Currently, there's no means for the DnD origin to know whether the
destination is actually finished with the DnD transaction, short of
finalizing it after the first transfer finishes, or leaking it forever.

But this poses other interoperation problems, drag destinations might
be requesting several mimetypes at once, might be just poking to find
out the most suitable format, might want to defer the action to a popup,
might be poking contents early before the selection was dropped...

In addition, data_source.cancelled is suitable for the situations where
the DnD operation fails (not on a drop target, no matching mimetypes,
etc..), but seems undocumented for that use (and unused in weston's DnD).

In order to improve the situation, the drag source should be notified
of all stages of DnD. In addition to documenting the "cancelled" event
for DnD purposes, The following 2 events have been added:

- wl_data_source.dnd_drop_performed: Happens when the operation has been
  physically finished (eg. the button is released), it could be the right
  place to reset the pointer cursor back and undo any other state resulting
  from the initial button press.
- wl_data_source.dnd_finished: Happens when the destination side destroys
  the wl_data_offer, at this point the source can just forget all data
  related to the DnD selection as well, plus optionally deleting the data
  on move operations.

Changes since v3:
  - Renamed dnd_performed to a more descriptive dnd_drop_performed,
    documented backwards compatible behavior on wl_data_offer.accept and
    wl_data_source.cancelled.

Changes since v2:
  - Minor rewording.

Changes since v1:
  - Renamed events to have a common "dnd" namespace. Made dnd_performed to
    happen invariably, data_device.cancelled may still happen afterwards.

Signed-off-by: Carlos Garnacho <carlosg at gnome.org>
Reviewed-by: Michael Catanzaro <mcatanzaro at igalia.com>
Reviewed-by: Jonas Ådahl <jadahl at gmail.com>
Reviewed-by: Mike Blumenkrantz <zmike at samsung.com>
---
 protocol/wayland.xml | 57 ++++++++++++++++++++++++++++++++++++++++++++++------
 1 file changed, 51 insertions(+), 6 deletions(-)

diff --git a/protocol/wayland.xml b/protocol/wayland.xml
index df8ed19..b54bcd0 100644
--- a/protocol/wayland.xml
+++ b/protocol/wayland.xml
@@ -408,7 +408,7 @@
   </interface>
 
 
-  <interface name="wl_data_offer" version="1">
+  <interface name="wl_data_offer" version="3">
     <description summary="offer to transfer data">
       A wl_data_offer represents a piece of data offered for transfer
       by another client (the source client).  It is used by the
@@ -423,7 +423,14 @@
 	Indicate that the client can accept the given mime type, or
 	NULL for not accepted.
 
-	Used for feedback during drag-and-drop.
+	This request determines the final result of the drag-and-drop
+	operation. If the end result is that no mimetype is accepted,
+	the drag source will receive drag_source.cancelled.
+
+	Prior to version 3, data_offer.accept was only considered for
+	client-side feedback, compositors wishing to preserve compatibility
+	with older versions should not emit data_source.cancelled as result
+	of a NULL target in those clients.
       </description>
 
       <arg name="serial" type="uint"/>
@@ -463,7 +470,7 @@
     </event>
   </interface>
 
-  <interface name="wl_data_source" version="1">
+  <interface name="wl_data_source" version="3">
     <description summary="offer to transfer data">
       The wl_data_source object is the source side of a wl_data_offer.
       It is created by the source client in a data transfer and
@@ -510,14 +517,52 @@
 
     <event name="cancelled">
       <description summary="selection was cancelled">
-	This data source has been replaced by another data source.
+	This data source is no longer valid. There are several reasons why
+	this could happen:
+
+	- The data source has been replaced by another data source.
+	- The drag-and-drop operation was performed, but the drop destination
+          did not accept any of the mimetypes offered through
+          data_source.target.
+	- The drag-and-drop operation was performed but didn't happen over a
+          surface.
+	- The compositor cancelled the drag-and-drop operation
+
 	The client should clean up and destroy this data source.
+
+	Prior to version 3, data_source.cancelled was documented as being
+	emitted in situations that made only sense on selection sources,
+	compositors wishing to preserve compatibility with older versions
+	should only emit this signal for selection sources in those clients.
+      </description>
+    </event>
+
+    <!-- Version 3 additions -->
+
+    <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
+	acceptance, data_source.cancelled may still be emitted afterwards
+	if the drop destination does not accept any mimetype.
+
+	This event might however not be received if the compositor cancelled
+	the drag-and-drop operation before this event could happen.
+
+	Note that the data_source may still be used further in the future and
+	should not be destroyed here.
       </description>
     </event>
 
+    <event name="dnd_finished" since="3">
+      <description summary="the drag-and-drop operation concluded">
+	The drop destination finished interoperating with this data
+	source, the client is now free to destroy this data source and
+	free all associated data.
+      </description>
+    </event>
   </interface>
 
-  <interface name="wl_data_device" version="2">
+  <interface name="wl_data_device" version="3">
     <description summary="data transfer device">
       There is one wl_data_device per seat which can be obtained
       from the global wl_data_device_manager singleton.
@@ -659,7 +704,7 @@
     </request>
   </interface>
 
-  <interface name="wl_data_device_manager" version="2">
+  <interface name="wl_data_device_manager" version="3">
     <description summary="data transfer interface">
       The wl_data_device_manager is a singleton global object that
       provides access to inter-client data transfer mechanisms such as
-- 
2.5.0