Timer callback called after widget dispose

Why This Happens

In Flutter, if you create a Timer or periodic timer in a State and do not cancel it in dispose(), the callback continues firing after the widget is removed. This leads to setState-after-dispose errors. Always cancel timers in the dispose method.

The Problem

class _MyState extends State<MyWidget> {
  late Timer _timer;

  @override
  void initState() {
    super.initState();
    _timer = Timer.periodic(Duration(seconds: 1), (_) {
      setState(() {});
    });
  }
  // Missing dispose!
}

The Fix

class _MyState extends State<MyWidget> {
  late Timer _timer;

  @override
  void initState() {
    super.initState();
    _timer = Timer.periodic(Duration(seconds: 1), (_) {
      setState(() {});
    });
  }

  @override
  void dispose() {
    _timer.cancel();
    super.dispose();
  }
}

Step-by-Step Fix

1. 1

   Identify the error

   Look at the setState-after-dispose error triggered by a Timer callback. The timer is still firing after the widget was removed.

2. 2

   Find the cause

   Check if Timer or Timer.periodic instances are cancelled in the dispose() method of the State.

3. 3

   Apply the fix

   Cancel all timers in the dispose() method to prevent callbacks from firing after the widget is removed.