https://qiita.com/k3ntar0/items/f14b0b03fbc13d0ed925 (https://medium.com/flutter-jp/state-1daa7fd66b94)

https://github.com/mono0926/wdb106-flutter/blob/main/lib/model/entities/cart_item/cart_item.dart https://github.com/mono0926/wdb106-flutter/blob/main/lib/model/entities/cart_item/cart_item.freezed.dart

json

jsonの機能を使うには、jsonのライブラリをいれないとダメ！