Monitors the _active_ views, i.e.
Monitors the _active_ views, i.e. views which are intersecting with the current transport position.
The view must store a handle to its underlying model.
Registers a live observer with this observable.
Registers a live observer with this observable. The method is called with the
observing function which receives the observable's update message of type A, and the
method generates an opaque Disposable instance, which may be used to
remove the observer eventually (through the dispose method).
Returns the set of _active_ views, i.e.
Returns the set of _active_ views, i.e. views which are intersecting with the current transport position.