[PATCH wayland v8 2/2] protocol: Add DnD actions

[Carlos Garnacho]

Hey Jonas,

On Mon, Jan 11, 2016 at 9:12 AM, Jonas Ådahl <jadahl at gmail.com> wrote:
> On Thu, Dec 24, 2015 at 03:21:11AM +0100, Carlos Garnacho wrote:
>> 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 at gmail.com>
>>
>> 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 at gnome.org>
>> Reviewed-by: Michael Catanzaro <mcatanzaro at igalia.com>
>> Reviewed-by: Mike Blumenkrantz <zmike at samsung.com>
>
> Looks good to me; this is Reviewed-by: Jonas Ådahl <jadahl at gmail.com>
> now. I have one comment asking for clarifications inline, and a few
> style nits.

Thanks! The style nits are fixed in the patch I've just sent.

>
>> ---
>>  protocol/wayland.xml | 163 ++++++++++++++++++++++++++++++++++++++++++++++++++-
>>  1 file changed, 162 insertions(+), 1 deletion(-)
>>
>> diff --git a/protocol/wayland.xml b/protocol/wayland.xml
>> index fb81542..4690e0e 100644
>> --- a/protocol/wayland.xml
>> +++ b/protocol/wayland.xml
>> @@ -421,6 +421,10 @@
>>      <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"/>
>>      </enum>
>>
>>      <request name="accept">
>> @@ -495,9 +499,81 @@
>>       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
>> +     needs changing 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.
>> +      </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, mainly in response to source side changes (through
>> +     wl_data_source.set_actions), destination side changes (through
>> +     wl_data_offer.set_actions), and as the pointer enters/leaves
>> +     surfaces.
>> +
>> +     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, the drag-and-drop destination is expected to
>> +     possibly 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_action and wl_data_offer.accept requests
>
> s/set_action/set_actions/. I think it should be pointed out that only a
> single action must be set via set_action at this point and that no more
> wl_data_offer.action event will be emitted, but wl_data_source.action
> will be, for that change.

The desired action should be provided through preferred_action, which
is already forced to be 1-bit, but I agree with us defining/enforcing
better what's the expected behavior. I've added further comments
around the "ask" action and documented some more protocol error
situations.

Cheers,
  Carlos