Difference In Event Handling Between ActionScript 2 and ActionScript 3

In the previous versions there were several methods of event handling like on() method which could be placed directly on movie clip or button instances, onClipEvent() method which could be used directly on movie clip instances. These method has disadvantages. As the code is placed directly on the movie clip or button, it becomes difficult to find out the code for the big projects.
To overcome these problem Single Event Handling system is developed which is based on DOM (Document Object Model) level 3 specifications.