flutter_code_examples flutter flutter design pattern references https://medium.com/flutter-community/flutter-design-patterns-0-introduction-5e88cfff6792 todo references https://zenn.dev/ntaoo/articles/6641e846765da1 https://qiita.com/advent-calendar/2020/flutter https://qiita.com/advent-calendar/2020/flutter-2 output zenn qiita blog