code: public static void addObserver(TaskObserver<?> observer) {
  		if(observer != null && tls.get().observers == null) {
  			tls.get().observers = new ArrayList<TaskObserver<?>>();
  		}
  		tls.get().observers.add(observer);
  	}
docs: Adds an observer to the current thread's observers of task execution life
  cycle.
  
  @param observer