Interface for classes allowing the registration of event observers.
A class implementing this interface will usually delegate to other classes
implementing the ObservableEventSource interface.
The orientation sensor device specific sample data, which are heading (angle
between the magnetic north direction and the y-axis, 0=North, 90=East,
180=South, 270=West ), pitch ( rotation around x-axis,-180 to 180, with
positive values when the z-axis moves toward the y-axis) and roll ( rotation
around y-axis, -90 to 90, with positive values when the x-axis moves toward
the z-axis ).