There are no events in Magento 2 web APIs, but it is possible to extend any public method in the system using befor/after/around [plugins][1]. Also see why it is [better to use plugins][2] instead of observers.


  [1]: http://devdocs.magento.com/guides/v2.0/extension-dev-guide/plugins.html
  [2]: http://magento.stackexchange.com/a/93964/697