code: public void update(float deltaTime){
  		if (updating) {
  			throw new IllegalStateException("Cannot call update() on an Engine that is already updating.");
  		}
  		
  		updating = true;
  		ImmutableArray<EntitySystem> systems = systemManager.getSystems();
  		try {
  			for (int i = 0; i < systems.size(); ++i) {
  				EntitySystem system = systems.get(i);
  				
  				if (system.checkProcessing()) {
  					system.update(deltaTime);
  				}
  	
  				while(componentOperationHandler.hasOperationsToProcess() || entityManager.hasPendingOperations()) {
  					componentOperationHandler.processOperations();
  					entityManager.processPendingOperations();
  				}
  			}
  		}
  		finally {
  			updating = false;
  		}	
  	}
docs: Updates all the systems in this Engine.
  @param deltaTime The time passed since the last frame.