Incomplete or missing didUpdateWidget method

[jacob314]

Flutter developers are unable to hot reload as many changes to the parameters of StatefulWidgets as they should because didUpdateWidget is often not implement or not implement fully. Apps can also suddenly start failing in confusing ways due to changes at a distance that cuase Flutter to reuse State objects where it didn’t before exposing these bugs.

Describe the rule you'd like to see implemented
The lint should trigger if a Flutter State object's initState method stores values from fields of a widget but didUpdateWidget does not at minimum have a code path that reads those same fields. This will require flow analysis within the State class as it is common to have helper methods called from both initState and didUpdateWidget that update the State object to match the widget. We will need to carefully track how many false positives this lint triggers. If the flow analysis is robust there should be few very few positives but there may be some false negatives where the lint thinks didUpdateWidget methods are fine even though they have bugs as we cannot ensure the logic in didUpdateWidget is correct only that it is not neglecting members from the widget. If it doesn't introduce significant false positives, we could warn about absolutely any member use in initState that is not also matched by member use in didUpdateWidget.

Examples

All of these examples are members of a State object

 @override
 void initState() {
   super.initState();
   _controller = AnimationController(vsync: tickerProvider);
   _updateAnimationController(); 
 }
 

 
 @override
 void didUpdateWidget(SingleTimerWidget oldWidget) {
   super.didUpdateWidget(oldWidget);
  // Lint if this block is missing as _timer is referenced in updateAnimationController that is called from initState
   if (widget._timer != oldWidget._timer) { // P2 probably false positive prone lint would be to warn if there aren't checks for if the oldWidget and newWidget have an equal field.
   _updateAnimationController()
   }
  }
 
  void _updateAnimationController() {
    final remainingProgress = _toProgress(widget._timer); 
  }
 }

No lint error in this case. There is an initState but no didUpdateWidget method but that does not indicate a bug as initState does not read any data off the widget.

 @override
 void initState() {
   super.initState();
   _controller = AnimationController(vsync: tickerProvider);
 }

Ideally, provide motivating examples w/ rationale.

Additional context
We find in practice that users very frequently neglect to implement didUpdateWidget as issues tend to only show up on a hot reload or rare cases where the same Widget object

[jacob314]

Fyi @pq filed an issue for this case we have been discussing. I expect this lint should find a very large number of real issues.

[stan-at-work]

They could be realy great