Unlike Frontend, Input Method, and Module, there can be only one user interface running. way.

If there is no special requirement, it's not recommended to implement a new user interface. There are already three existing User Interface Addons in Fcitx, one is fcitx-classic-ui, which provides skin support and uses cairo, pango, and xlib for rendering; another one is fcitx-kimpanel-ui, which uses kimpanel protocol via DBus, and it can provides native look-feeling under with kdeplasma-addons-kimpanel, and even more, can provides advance skin support with kimtoy; The third one is fcitx-light-ui, which only use Xft and Xlib for rendering, it intends to require least dependencies and achieve better rendering spend on old machine.

In order to implement a new user interface, it would be better to use a kimpanel compatible protocol, but not to write a fcitx one, since kimpanel can also benefits other input framework, scim and ibus.