Polymer 1.0：如何正确绑定（注入/覆盖）dom-module的功能

I have a Polymer element of which the public API should allow to bind to a function which the user can define, i.e. it should allow to pass in a function implementation. I have tried quite a few approaches but only 1 worked. Now I'm wondering if this is the proper / correct way or not.I have a Polymer element of which the public AP