关于流程变量更新的Activiti事件监听器

To any developers familiar with Activiti, essentially what I am looking for and currently I cannot find in their user guide for the product, whether an event exists, much the like the other start task event listener or end task event listener, a listener which is fired on update to a process variable? So the task is very much in progress and I want to react to a change of state of one of the process variables..To any developers familiar with Activiti, essen