DidChangeDependencies may be unsafe

According to the documentation, didChangeDependencies can be overridden to perform expensive work that shouldn't be done on every build call.

But the way the life-cycle works right now, didChangeDependencies doesn't guarantee that build will be called.

This causes important issues, because the life-cycle may start some expensive work that will never be needed.

As an example, consider the widget Foo that depends on the InheritedWidget MyInherited through didChangeDependencies to perform some expensive work.

Then consider the following widget tree:

MyInherited(
  value: 0,
  child: Foo(),
)

That is later rebuilt to:

MyInherited(
  // updateShouldNotify will return true
  value: 42,
  // Foo is removed from the tree
  child: Container(),
)

then in that situation, Foo.didChangeDependencies will get called even although Foo gets removed from the widget tree and disposed.

For State.didChangeDependencies, the fix is relatively simple:

class StatefulElement extends ComponentElement {
  bool _didChangeDependencies = true;

  @override
  void didChangeDependencies() {
    _didChangeDependencies = true;
    super.didChangeDependencies();
  }

  @override
  Widget build() {
    if (_didChangeDependencies) {
      _didChangeDependencies = false;
      state.didChangeDependencies();
    }
    return super.build();
  }
}

It is probably reasonable to not change the behavior of Element didChangeDependencies and only update State.didChangeDependencies.

@Hixie

@Hixie or @goderbauer may know why this method exists.

Full test case:

import 'package:flutter/material.dart';
import 'package:flutter_test/flutter_test.dart';

void main() {
  testWidgets('Counter increments smoke test', (WidgetTester tester) async {
    GlobalKey key = GlobalKey();
    print('First');
    await tester.pumpWidget(MyInherited(key, 10, MyStatefulWidget()));
    print('Second');
    await tester.pumpWidget(MyInherited(key, 20, Container(color: Colors.blue)));
    print('Done');
  });
}

class MyInherited extends InheritedWidget {
  const MyInherited(Key key, this.value, Widget child) : super(key: key, child: child);

  final int value;
  @override
  bool updateShouldNotify(MyInherited oldWidget) => oldWidget.value != value;
}

class MyStatefulWidget extends StatefulWidget {
  @override
  State<StatefulWidget> createState() => MyState();
}

class MyState extends State<MyStatefulWidget> {
  int didChangeDependenciesCount = 0;
  @override
  void didChangeDependencies() {
    super.didChangeDependencies();
    didChangeDependenciesCount++;
    assert(didChangeDependenciesCount == 1);
    print('didChangeDependencies');
  }

  @override
  Widget build(BuildContext context) {
    print('build');
    context.dependOnInheritedWidgetOfExactType<MyInherited>();
    return Container(color: Colors.red);
  }
}

This thread has been automatically locked since there has not been any recent activity after it was closed. If you are still experiencing a similar issue, please open a new bug, including the output of flutter doctor -v and a minimal reproduction of the issue.

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DidChangeDependencies may be unsafe #34130

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DidChangeDependencies may be unsafe #34130

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rrousselGit commented on Jun 9, 2019

eseidelGoogle commented on Jun 17, 2019

dnfield commented on Jan 27, 2020

dnfield commented on Jan 27, 2020

github-actions commented on Aug 23, 2021

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