In Animate CC , if I click New>Actionscript 3 and create an OOP Actionscript 3 package in the Action window, I get an error message : Packages cannot be nested. How can you use OOP in Animate CC?
All AS3 package files are external .as files. You would start one by choosing File/New ActionScript 3.0 Class. You give the Class a name, and start typing away.
Back in the FLA you would select the associated symbol in the Library, and click the i button to show the symbol's properties. In there you can show the Advanced settings, and click on the Export for ActionScript box. Once you've done that you can enter in the same Class name you used before, to connect the symbol with its Class file.