Clean Architecture in Flutter with BLoC: A Practical Guide

Clean architecture in Flutter is the single biggest reason the production apps I ship stay maintainable after a year of feature churn. Over 4+ years building iOS and Android apps, I've watched "just put the logic in the widget" turn setState spaghetti into a codebase nobody wants to touch. This guide walks through how I actually split a Flutter app into domain , data , and presentation layers with BLoC — using one concrete feature so you can copy the structure into your own project today. I'll build a small "Todos" feature end to end: an entity, a use case, a repository with a DTO mapper, and a Cubit that drives the UI. The point isn't the todo list — it's the boundaries between layers and why each one earns its keep. Why clean architecture in Flutter pays off The core idea is the dependency rule : source-code dependencies point inward . The UI knows about the domain; the domain knows about nothing. Your business rules never import Flutter, Firebase, Dio, or Supabase. That inversion buys three things I care about on every project: Testability. Domain logic runs in plain Dart unit tests — no widget pump, no emulator, no network. Swappable infrastructure. Move from REST to GraphQL, or Firestore to a local SQLite cache, by rewriting one data-layer class. The domain and UI don't change. Parallel work. Once the domain contract exists, one person builds the API client while another builds the screen against a fake. Here's the layer breakdown I use, and what's allowed to live in each: Layer Knows about Contains Depends on Domain Nothing external Entities, repository interfaces , use cases Pure Dart only Data Domain + the outside world DTOs, mappers, repository implementations , data sources Domain Presentation Domain Blocs/Cubits, states, widgets Domain Notice the data and presentation layers both depend on domain, and domain depends on neither. That's the whole game. Folder structure that scales I organise by feature first, then by layer . A flat models/ , services/ , scr