Why only the minimum widgets rebuild

I use Provider’s selection APIs (Selector) to ensure widgets only rebuild when the specific data they depend on changes.

• [Completed count selection] lib/views/todo_view.dart:

  • Selector<AppStateViewModel, int>(selector: (context, vm) => vm.completedTodosCount, ...)
  • Depends only on a primitive int derived from todos. Only the count Text rebuilds when the computed value changes.

• [List length selection] lib/views/todo_view.dart:

  • Selector<AppStateViewModel, int>(selector: (context, vm) => vm.appState.todos.length, ...)
  • The ListView container rebuilds only when the length changes (add/remove). Toggling a todo’s completed flag does not change length, so the container does not rebuild.

• [Per-row selection] lib/views/todo_view.dart:

  • Each row is wrapped with Selector<AppStateViewModel, Todo>(selector: (context, vm) => vm.appState.todos[index], ...)
  • Only the row whose Todo instance changes rebuilds (e.g., when its completed flips).
  • Rows are keyed with ValueKey(todo.id), preserving identity across insert/remove operations and minimizing row re-mounting.

• [Stateless item + non-listening actions] lib/views/custom_widget/todo_card.dart:

  • TodoCard is stateless and receives primitive props (task, completed).
  • Actions use context.read<AppStateViewModel>(), which does not listen, so dispatching actions does not trigger rebuilds.

• [Derived, not duplicated state] lib/view_models/app_state_view_model.dart:

  • completedTodosCount is a getter derived from todos instead of a manually maintained counter, preventing drift and extra rebuilds.

Together, these choices ensure only the smallest necessary subtree rebuilds:

• The count text when the computed count changes.
• The list container only when length changes.
• The specific TodoCard whose data changed.