With Plasma 5 our lock screen architecture changed significantly. For example we do no longer support screen saver hacks or widgets on top of the locked screen. Both are very unlikely to make a return in future releases. This means that bug reports against the old infrastructure might no longer apply to our current code base. Two weeks ago I went through all bug reports and feature requests to evaluate whether they still apply to our new infrastructure or should be closed.
This is something I do not like to do. I find it extremely sad to close bug reports because they are outdated. Especially if the bugs have been open for several years without any activity. After going through all those reports it is obvious that we offered too many possibilities to configure the screen locker with too few people caring about it. The number of available screen savers was just immense – especially if one considers that there are also 3rd party savers. While it’s easy to install them, there is basically nobody who cared about them. Some are decades old with the devs having moved on years ago.
Apart from that one could also notice that there were important features missing in our lock screen: audio and multimedia control. The problem is obvious: you suspend your notebook while audio is playing, resume in a place where it should not play audio (e.g. classes) and first need to unlock the screen before being able to mute the audio. An unpleasant experience.
The problem here is that media keys are not supposed to work. The lock screen grabs all keyboard input and prevents other applications to get the keyboard input. This includes our global keyboard infrastructure. We cannot just forward all keys to the global keyboard infrastructure as that could be used to create a key logger when the screen is locked. Even more: most short cuts shouldn’t be invokable when the screen is locked, e.g. you don’t want the desktop to switch.
After brooding over it for a few days I had an idea on how to resolve the problem: the lock screen needs to integrate with our globalshortcut handling. When the screen gets locked we get the available shortcuts from the daemon and map them against a white list of allowed shortcuts. Whenever a key is now pressed while the screen is locked, it’s verified against the fetched list. If we have a match the shortcut is invoked. Not all shortcuts are supported, though. The architecture ensures that one cannot abuse the infrastructure to turn it into a key logger. All alphanumeric keys are excluded. In addition it uses as mentioned a white list, which is not configurable, but hard coded on purpose. At the moment we support only a very limited set of global shortcuts: volume keys, brightness keys and media control keys.
Media control keys were also an interesting topic to work on. Our Plasma session didn’t have any global shortcuts for media handling, so there was nothing which our lock screen could do about it. It cannot figure out whether there is a media application running and then invoke an action on it.
So I stepped back and thought about whether there is a better way to solve it in a general way. Plasma supports the mpris2 interface allowing to control any mpris2-enabled media application. What if the mpris2 engine inside Plasma binds the multi media keys? They could forward to the currently running media player, give us a consistent way to interact with media players and allow us to expose it in the lock screen. So now we have by default mapped the media player controls as global shortcut delegating to any media application. If you press the “Play” button it will Play/Pause VLC, no matter whether it’s the active application or not. And in addition also in the lock screen.
Now there was just one problem to solve: pressing shortcuts and not having visual feedback is not that good. On the desktop we show an on-screen-display whenever the volume changes, but as the screen is locked, we cannot see it. So this architecture also needed enhancement. With a few more changes our lock screen is now able to listen to the requests for on-screen-display information and integrate them:
All just for supporting shortcuts in the lock screen.