You mean this?
The keys should be on 'air', no touching anything. The feedback should come only from audio or/and screen.
I'm pretty sure what you're wanting is impossible to make at this time.
There must be a way to cut off light so that if it doesn't pass it registers a stroke.
Yeah but the technology to have the laser beam "stop" in the air doesn't exist. You could focus the beam to a point in the air in front of you but without anything for the beam to focus on to you would just end up with an unfocused image somewhere behind that point.
Basically from what you're saying I assume you mean a hologram keyboard. Projected holograms are currently a pretty basic technology (you can do it with a tank filled with a semi-opaque gas, basically like smoke, and I seem to remember seeing one with a clear liquid) and we're a long way off the point where it'll be possible to construct a keyboard based on this.
Detecting the strokes isn't the issue, projecting the keyboard into air is.