Using StreamBuilder in Flutter；https://medium.com/@sidky/using-streambuilder-in-flutter-dcc2d89c2eae BloC Architecture in Flutter: a Modern Architectural Approach and How We Use it at Jimdo：https://dev.jimdo.com/bloc-architecture-in-flutter/

https://www.didierboelens.com/2018/08/reactive-programming---streams---bloc/