Chapter 2 · Practice 5 of 5

Dispose every controller, subscription and focus node you create

Why it matters

Controllers such as TextEditingController, AnimationController, ScrollController and FocusNode, plus StreamSubscriptions, hold resources and listeners. If a State creates one, that State must dispose of it in dispose().

Forgetting this leaks memory and keeps tickers or streams running after the screen is gone. A common symptom is "setState() called after dispose()" errors, which usually means a stream or future callback outlived its widget.

For async work started by a widget, check mounted before using context or calling setState() after an await.

Pair every controller created in initState() with a call in dispose()
Check mounted after an await before touching context

Avoid

Creating controllers inside build()

Example

Dart

Future<void> _save() async {
  await repository.save(_form);
  if (!mounted) return;
  ScaffoldMessenger.of(context).showSnackBar(
    const SnackBar(content: Text('Saved')),
  );
}