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

Tue Dec 15 17:40:47 PST 2015

On Tue, Dec 15, 2015 at 06:56:24PM +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 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>

Sorry for taking my sweet time, but anyhow, overall I think it looks
good, but obligatory bikeshedding follows.

> ---
>  protocol/wayland.xml | 129 +++++++++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 129 insertions(+)
> 
> diff --git a/protocol/wayland.xml b/protocol/wayland.xml
> index b54bcd0..ab64762 100644
> --- a/protocol/wayland.xml
> +++ b/protocol/wayland.xml
> @@ -468,6 +468,56 @@
>  
>        <arg name="mime_type" type="string"/>
>      </event>
> +
> +    <!-- Version 3 additions -->

bikeshed: For consistency, I think added requests should always come before
added events in the XML.

> +
> +    <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 data_device.enter, or anytime the source

wl_data_device.enter.

> +	side changes its offered actions through 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
> +	data_source.set_actions), destination side changes (through
> +	data_offer.set_actions), and as pointer enters/leaves surfaces.

wl_data_source.set_actions and wl_data_offer.set_actions.

> +
> +	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.

I think it would be good to mention the last action emitted before the
wl_data_device.drop event is the action that should be used when the
completing the operation. Also that no "action" event will be emitted
after the "wl_data_device.drop" event.

> +      </description>
> +      <arg name="dnd_action" type="uint"/>
> +    </event>
> +
> +    <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
> +	data_source.action and data_offer.action events if the compositor

wl_data_source.action and wl_data_offer.action.

> +	needs changing the selected action.
> +
> +	This request can be called multiple times throughout the
> +	drag-and-drop operation, typically in response to data_device.enter
> +	or data_device.motion events.

wl_data_device.enter and wl_data_device.motion.

> +
> +	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 drag_source.cancelled.

wl_data_source.cancelled.

> +      </description>
> +      <arg name="dnd_actions" type="uint"/>
> +      <arg name="preferred_action" type="uint"/>
> +    </request>
>    </interface>
>  
>    <interface name="wl_data_source" version="3">
> @@ -524,6 +574,9 @@
>  	- The drag-and-drop operation was performed, but the drop destination
>            did not accept any of the mimetypes offered through
>            data_source.target.

I know this is the wrong patch for commenting on this, but here was
spaces instead of tabs as well (the same for the point below the added
one).

> +	- The drag-and-drop operation was performed, but the drop destination
> +          did not select any action present in the mask offered through
> +	  data_source.action.

Mixed tabs and spaces (indentation).

>  	- The drag-and-drop operation was performed but didn't happen over a
>            surface.
>  	- The compositor cancelled the drag-and-drop operation
> @@ -558,8 +611,44 @@
>  	The drop destination finished interoperating with this data
>  	source, the client is now free to destroy this data source and
>  	free all associated data.
> +
> +	Clients can also trigger the deletion of source-side data on
> +	"move" drag-and-drop operations.

This wording seems a bit off to me. At first it sounded like I was back
at the wl_data_offer side and it could trigger the deletion of client
side things, but then I remembered what interface it was in. A
suggestion:

	If the action used to perform the operation was "move", the
	source can now delete the transferred data.

A side thought; how is this supposed to work when move does not require
any deletion? I.e. a file moved within a filesystem.

> +      </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 source side changes (through
> +        data_source.set_actions), destination side changes (through
> +        data_offer.set_actions), and as pointer enters/leaves surfaces.

wl_data_source.set-actions and wl_data_offer.set_actions.

and

"..., and as the pointer enter/leaves ...".

> +
> +        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.
> +
> +        Clients can trigger cursor surface changes from this point, so
> +        they reflect the current action.

Here as well I think it'd be good to add a note about what action should
be used to perform the operation, i.e. the last action emitted before
the "dnd_drop_performed" event. Maybe even specify that no events will
ever be emitted after the "dnd_drop_performed" event.

>        </description>
> +      <arg name="dnd_action" type="uint"/>
>      </event>
> +
> +    <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 a data_source.action event and
> +        data_offer.action events if the compositor needs changing the

wl_data_source.action and wl_data_offer.action.

> +        selected action.
> +      </description>
> +      <arg name="dnd_actions" type="uint"/>
> +    </request>
>    </interface>
>  
>    <interface name="wl_data_device" version="3">
> @@ -727,6 +816,46 @@
>        <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" since="3">
> +      <description summary="drag and drop actions">
> +        This is a bitmask of the available/preferred actions in a
> +        drag-and-drop operation.
> +
> +        The current reserved ranges are:
> +
> +        0x0000 - 0x00FF: Reserved for the wayland core protocol.
> +        0x01FF - 0xFFFF: Reserved for future toolkit-specific use. Slots
> +                         may be reserved.

This means we'll have 8 possible core actions, and 24 that may be
reserved. 8 I suppose is enough for core actions, but is 24 really
enough for all eternity if they are meant to be allocated by different
toolkits and applications? If we're to make it reservable it seems to be
better to make it more long term sustainable, i.e. not making it a bit
mask but a regular enum.

This means that some of the events and requests would have to use array
instead of uint, which doesn't seem that bad.

> +
> +        In the compositor, the selected action comes out as a result of
> +        matching the actions offered by the source and destination sides,

Full stop here.

> +        "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 & dest_actions).

s/&/∩/ (we're using UTF-8 so can just use the unicode character for
intersection here).

I also think there is no need variable name like things, i.e. I think it
can just be (source actions ∩ destination actions).

Jonas

> +
> +        In addition, compositors may also pick different actions in
> +        reaction to key modifiers being pressed, one common ground that
> +        has been present 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 (eg. "ask", or an hypothetical toolkit-specific action).
> +      </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">
> -- 
> 2.5.0
> 
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