Easily navigate from CustomActivity in the designer to CustomActivity class file

If I double click on a CodeActivity step in the Workflow designer, visual studio navigates from the designer to the CodeActivity's handler.

If I double click on my custom Activity in the Workflow designer, nothing happens.

I've added an ActivityDesigner and located the OnMouseDoubleClick... but how do I tell VisualStudio to navigate to a file? I don't even know what object 开发者_运维技巧to start with.