Step 1 - Inventory

• List types: plain data vs IO vs orchestration vs string/date helpers.
• Note which files import dart:io or http (repository side) vs only dart:core (models, many utils).

Step 2 - Target layout

lib/
  main.dart
  models/
    todo.dart
    user.dart
  repositories/
    todo_file_repository.dart
  services/
    todo_service.dart
  utils/
    json_helpers.dart

• Names match your domain; extra folders (cli/, config/) are fine if they clarify.

Step 3 - Move rules

• Models: classes that mostly hold data and toJson/fromJson; no File or sockets.
• Repositories: read/write boundaries; return models or failures, not raw String everywhere if you can avoid it.
• Services: methods like addTodo, completeTodo, loadAll that call repositories and enforce rules.
• Utils: pure helpers used in two or more places; otherwise keep next to the single caller.

Step 4 - Fix imports

• Run dart analyze after each batch of moves.
• Use package:your_package/... imports inside lib/ for clarity in multi-file packages.

Step 5 - Verify

• dart run or tests still pass.
• dart format . once at the end.

Practice tasks

• Add a fake TodoRepository in-memory next to the file implementation; inject one or the other from main.
• Move JSON decode error handling into utils/ or keep it on the repository—pick one and write one sentence why.
• Add README.md section “Structure” with the folder list (only if your repo already uses a root README).