KDE developer Martin Gräßlin has posted a road-map for Kwin on Qt 5. The release of Qt 5 has brought many system optimizations as well as 99% backwards compatibility. But applications that interacts with system needs to be ported on Qt 5. Kwin is one such application, it needs to be ported to Qt 5. To bring Kwin to Qt 5 developer needs to switch to XCB from Xlib.
Gräßlin writes on his blog:
As far as I have seen we would not be required to port everything to XCB, but overall it is a good idea to do so. It will make KWin faster and of course it’s better to have everything using XCB and using XLib only where – for legacy reasons – it is required: the GLX compositing backend (looks like our current EGL backend is also bound to XLib). Also the porting allows us to do some things “right”. Some KWin effects for example are interacting with X11 directly and that is something which should be tried to be abstracted properly so that we do not need to adjust the effects once we are running on Wayland.
Earlier the team had plans to start working on Wayland from this January. It has changed somewhat; they are planning to use Qt compositor – a neat Qt-like abstraction on top of the Wayland C libraries. Major drawback of Qt compositor is “that it absolutely requires Qt 5. So to be able to integrate it into KWin we need to first port to Qt 5. It is hardly any difference: we would have needed to go to Qt 5 anyway, when does not really matter, just that I now have a wonderful motivation to get there”, writes Gräßlin.