Martin Gräßlin has written today on his blog about QML support for window decorations.
For the key information relevant to KDE developers and users with this change:
In the current state the decoration consists of 370 lines of QML code and I expect to need an additional 100 lines to finish the buttons (they are already functional, that is the close button closes the window) and add some of the configuration options. The same API in C++ consists of 1500 lines of code. So we do not only get fewer lines of code but also a more readable and easier to maintain codebase. For something like a window decoration a declarative approach is much better suited than the imperative C++ way of painting elements.
Overall the new QML API will provide the same powerful features as the KDecoration API, but also provides convenience functionality as KCommonDecoration, e.g. a button group taking care of laying out the decoration buttons. To make development of QML based decorations quite simple support is currently being added to Plasmate as part of the Google Summer of Code project. So you will be able to design and also test the decoration directly from inside Plasmate. This is also quite some improvement as with C++ the only way to test a decoration is to use it in KWin, which can be quite nasty during development.
The API will need some more love till I’m quite confident to provide it’s usage but there is enough time till 4.10 :-) So I hope that we can provide a better decoration experience in 4.10. I have a few ideas concerning the usage of QML based decorations in KWin and are looking forward to experiment with them.