where can I find the example that get a attribute's previous value and current value on the workitem?

You cannot get its previous value except from the server side plugin in Java.
there is some data saved in history, but I haven't looked for a way to navigate the history.

Work Items are IAuditables and they have up to two 'predecessor states'. You can use the Java API to look at the predecessor state and compare the values e.g.of an attribute.

In a participant, you get the new state and (if exists) the old state of a work item. Thus you are able to compare the attribute value and act on it. See https://rsjazz.wordpress.com/?s=participant for examples. This would be a server extension.