Time for another status update on libei, the transport layer for bouncing emulated input events between applications and Wayland compositors . And this time it's all about portals and how we're about to use them for libei communication. I've hinted at this in the last post, but of course you're forgiven if you forgot about this in the... uhm.. "interesting" year that was 2022. So, let's recap first:
Our basic premise is that we want to emulate and/or capture input events in the glorious new world that is Wayland (read: where applications can't do whatever they want, whenever they want). libei is a C library  that aims to provide this functionality. libei supports "sender" and "receiver" contexts and that just specifies which way the events will flow. A sender context (e.g. xdotool) will send emulated input events to the compositor, a "receiver" context will - you'll never guess! - receive events from the compositor. If you have the InputLeap  use-case, the server-side will be a receiver context, the client side a sender context. But libei is really just the transport layer and hasn't had that many changes since the last post - most of the effort was spent on trying to figure out how to exchange the socket between different applications. And for that, we have portals!
In particular, we have a PR for the RemoteDesktop portal to add that socket exchange. In particular, once a RemoteDesktop session starts your application can request an EIS socket and send input events over that. This socket supersedes the current NotifyButton and similar DBus calls and removes the need for the portal to stay in the middle - the application and compositor now talk directly to each other. The compositor/portal can still close the session at any time though, so all the benefits of a portal stay there. The big advantage of integrating this into RemoteDesktop is that the infrastructucture for that is already mostly in place - once your compositor adds the bits for the new ConnectToEIS method you get all the other pieces for free. In GNOME this includes a visual indication that your screen is currently being remote-controlled, same as from a real RemoteDesktop session.
Now, talking to the RemoteDesktop portal is nontrivial simply because using DBus is nontrivial, doubly so for the way how sessions and requests work in the portals. To make this easier, libei 0.4.1 now includes a new library "liboeffis" that enables your application to catch the DBus. This library has a very small API and can easily be integrated with your mainloop (it's very similar to libei). We have patches for Xwayland to use that and it's really trivial to use. And of course, with the other Xwayland work we already had this means we can really run xdotool through Xwayland to connect through the XDG Desktop Portal as a RemoteDesktop session and move the pointer around. Because, kids, remember, uhm, Unix is all about lots of separate pieces.
On to the second mode of libei - the receiver context. For this, we also use a portal but a brand new one: the InputCapture portal. The InputCapture portal is the one to use to decide when input events should be captured. The actual events are then sent over the EIS socket.
Right now, the InputCapture portal supports PointerBarriers - virtual lines on the screen edges that, once crossed, trigger input capture for a capability (e.g. pointer + keyboard). And an application's basic approach is to request a (logical) representation of the available desktop areas ("Zones") and then set up pointer barriers at the edge(s) of those Zones. Get the EIS connection, Enable() the session and voila - the compositor will (hopefully) send input events when the pointer crosses one of those barriers. Once that happens you'll get a DBus signal in InputCapture and the events will start flowing on the EIS socket. The portal itself doesn't need to sit in the middle, events go straight to the application. The portal can still close the session anytime though. And the compositor can decide to stop capturing events at any time.
There is actually zero Wayland-y code in all this, it's display-system acgnostic. So anyone with too much motivation could add this to the X server too. Because that's what the world needs...
The (currently) bad news is that this needs to be pulled into a lot of different repositories. And everything needs to get ready before it can be pulled into anything to make sure we don't add broken API to any of those components. But thanks to a lot of work by Olivier Fourdan, we have this mostly working in InputLeap (tbh the remaining pieces are largely XKB related, not libei-related). Together with the client implementation (through RemoteDesktop) we can move pointers around like in the InputLeap of old (read: X11).
Our current goal is for this to be ready for GNOME 45/Fedora 39.
 eventually a protocol but we're not there yet
 It doesn't actually have to be a compositor but that's the prime use-case, so...
 or barrier or synergy. I'll stick with InputLeap for this post