Proposal: initialize fields at the declaration site

before

class _MyState extends State<MyWidget> with SingleTickerProviderMixin {
  late final int _value;
  AnimationController? _controller;

  @override
  void initState() {
    _value = widget.value;
    _controller = AnimationController(
      duration: Durations.medium1,
      vsync: this,
    );
  }

  // ...
}

after

class _MyState extends State<MyWidget> with SingleTickerProviderMixin {
  late final int _value = widget.value;

  late final AnimationController _controller = AnimationController(
    duration: Durations.medium1,
    vsync: this,
  );

  // ...
}

This has a few advantages:

More concise
Less jumping around to see how fields are initialized
Less prone to late initialization errors
The prefer final & unnecessary nullable lints will kick in

I thought it was this way because of some code path where the state object could be created. But never actually mounted. Therefore would not be dispose()ed. However I think you're right upon looking into it.... this should be safe

This is not always a safe change. late finals are not initialized until accessed. Simple example, the following code will never update.

class FooState extends State<Foo> {
  late final timer = Timer.periodic(Duration(milliseconds: 10), (timer) {
    setState(() {
      count++;
    });
  });
  int count = 0;

  Widget build(_) => Text('Count $cound');
}

Yeah, forgot to mention that, but it's a good point—there was one time when I tried subscribing to a Firebase stream using a late final variable, and never actually subscribed to it 😒

`_InteractiveViewerState` code cleanup (#153645)

some code path where the state object could be created but never actually mounted… However I think you're right upon looking into it

I wanted to double-check as well 🙂
Fortunately, it does look like every State object is mounted when it's created:

flutter/packages/flutter/lib/src/widgets/framework.dart

Lines 4483 to 4488 in 1633c47

    
           final Element newChild = newWidget.createElement(); 
        
           assert(() { 
        
             _debugCheckForCycles(newChild); 
        
             return true; 
        
           }()); 
        
           newChild.mount(this, newSlot);

But recently I learned that I'd missed something important—there are actually 2 potential dangers involving late fields.

late initialization errors

Happens when a late variable is referenced before being initialized; can be avoided by initializing at the declaration site.

calling `createTicker()` inside `dispose()`

AnimationController calls vsync.createTicker() in its constructor, so if a late controller isn't referenced until it's being disposed of, it can result in an error.

For example, you could have a method that references the controller, but the controller won't be initialized until if/when the method is called:

class _MyState extends State<MyWidget> with TickerProviderMixin {
  late final _controller = AnimationController(vsync: this, duration: Durations.long1)
    ..addListener(() => setState(() {}));

  void _open() {
    _controller.forward();
  }

  @override
  void dispose() {
    _controller.dispose(); // dangerous
    super.dispose();
  }

  // ...
}

`_InteractiveViewerState` code cleanup (flutter#153645)

@nate-thegrate

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@nate-thegrate

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Update TickerMode.getValuesNotifier to handle initialization during S…

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Proposal: initialize fields at the declaration site #153644

before

after

late initialization errors

calling `createTicker()` inside `dispose()`

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	final Element newChild = newWidget.createElement();
	assert(() {
	_debugCheckForCycles(newChild);
	return true;
	}());
	newChild.mount(this, newSlot);

Proposal: initialize fields at the declaration site #153644

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before

after

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moffatman commented on Aug 18, 2024

jonahwilliams commented on Aug 19, 2024

nate-thegrate commented on Aug 19, 2024

nate-thegrate commented on Aug 23, 2024

late initialization errors

calling createTicker() inside dispose()

flutter-triage-bot commented on Feb 1, 2025

flutter-triage-bot commented on Mar 10, 2025

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calling `createTicker()` inside `dispose()`