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

Carlos Garnacho carlosg at gnome.org
Fri Jan 15 12:11:39 PST 2016

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 v6:
  - Turned wl_data_offer.finish calls with 0/NULL state/mimetype an
    error, made it explicit that it will only result in
    wl_data_offer.dnd_finished being sent if successful.

Changes since v5:
  - Further rewording of wl_data_offer.finish and wl_data_offer.accept.
    Added error for untimely wl_data_offer.finish requests.

Changes since v4:
  - Applied rewording suggestions from Jonas Ådahl. Added new
    wl_data_offer.finish request to allow explicit finalization on the
    destination side.

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

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 | 87 ++++++++++++++++++++++++++++++++++++++++++++++++----
 1 file changed, 81 insertions(+), 6 deletions(-)

diff --git a/protocol/wayland.xml b/protocol/wayland.xml
index 8a62190..dc05993 100644
--- a/protocol/wayland.xml
+++ b/protocol/wayland.xml
@@ -408,7 +408,7 @@
-  <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
@@ -418,12 +418,27 @@
       data directly from the source client.
+    <enum name="error">
+      <entry name="invalid_finish" value="0"
+	     summary="finish request was called untimely"/>
+    </enum>
     <request name="accept">
       <description summary="accept one of the offered mime types">
 	Indicate that the client can accept the given mime type, or
 	NULL for not accepted.
-	Used for feedback during drag-and-drop.
+	For objects of version 2 or older, this request is used by the
+	client to give feedback whether the client can receive the given
+	mime type, or NULL if none is accepted; the feedback does not
+	determine whether drag-and-drop operation succeeds or not.
+	For objects of version 3 or newer, this request determines the
+	final result of the drag-and-drop operation. If the end result
+	is that no mime types was accepted, the drag-and-drop operation
+	will be cancelled and the corresponding drag source will receive
+	wl_data_source.cancelled. Clients may still use this event in
+	conjunction with wl_data_source.action for feedback.
       <arg name="serial" type="uint"/>
@@ -442,6 +457,11 @@
 	The receiving client reads from the read end of the pipe until
 	EOF and then closes its end, at which point the transfer is
+	This request may happen multiple times for different mimetypes,
+	both before and after wl_data_device.drop. Drag-and-drop destination
+	clients may preemptively fetch data or examine it more closely to
+	determine acceptance.
       <arg name="mime_type" type="string"/>
       <arg name="fd" type="fd"/>
@@ -461,9 +481,26 @@
       <arg name="mime_type" type="string"/>
+    <!-- Version 3 additions -->
+    <request name="finish" since="3">
+      <description summary="the offer will no longer be used">
+	Notifies the compositor that the drag destination successfully
+	finished the drag-and-drop operation.
+	Upon receiving this request, the compositor will emit
+	wl_data_source.dnd_finished on the drag source client.
+	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.
+      </description>
+    </request>
-  <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 +547,52 @@
     <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
+	  wl_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 (e.g. compositor
+	  dependent timeouts to avoid stale drag-and-drop transfers).
 	The client should clean up and destroy this data source.
+	For objects of version 2 or older, wl_data_source.cancelled will
+	only be emitted if the data source was replaced by another data
+	source.
+      </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, wl_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.
+    <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 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 +734,7 @@
-  <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

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