Contents
description bug_report

More Strict Assertions in the Navigator and the Hero Controller Scope

Contents

Summary

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The framework throws an assertion error when it detects there are multiple navigators registered with one hero controller scope.

Context

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The hero controller scope hosts a hero controller for its widget subtree. The hero controller can only support one navigator at a time. Previously, there was no assertion to guarantee that.

Description of change

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If the code starts throwing assertion errors after this change, it means the code was already broken even before this change. Multiple navigators may be registered under the same hero controller scope, and they can not trigger hero animations when their route changes. This change only surfaced this problem.

Migration guide

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An example application that starts to throw exceptions.

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

void main() {
  runApp(
    MaterialApp(
      builder: (BuildContext context, Widget child) {
        // Builds two parallel navigators. This throws
        // error because both of navigators are under the same
        // hero controller scope created by MaterialApp.
        return Stack(
          children: <Widget>[
            Navigator(
              onGenerateRoute: (RouteSettings settings) {
                return MaterialPageRoute<void>(
                  settings: settings,
                  builder: (BuildContext context) {
                    return const Text('first Navigator');
                  }
                );
              },
            ),
            Navigator(
              onGenerateRoute: (RouteSettings settings) {
                return MaterialPageRoute<void>(
                  settings: settings,
                  builder: (BuildContext context) {
                    return const Text('Second Navigator');
                  }
                );
              },
            ),
          ],
        );
      }
    )
  );
}

You can fix this application by introducing your own hero controller scopes.

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

void main() {
  runApp(
    MaterialApp(
      builder: (BuildContext context, Widget child) {
        // Builds two parallel navigators.
        return Stack(
          children: <Widget>[
            HeroControllerScope(
              controller: MaterialApp.createMaterialHeroController(),
              child: Navigator(
                onGenerateRoute: (RouteSettings settings) {
                  return MaterialPageRoute<void>(
                    settings: settings,
                    builder: (BuildContext context) {
                      return const Text('first Navigator');
                    }
                  );
                },
              ),
            ),
            HeroControllerScope(
              controller: MaterialApp.createMaterialHeroController(),
              child: Navigator(
                onGenerateRoute: (RouteSettings settings) {
                  return MaterialPageRoute<void>(
                    settings: settings,
                    builder: (BuildContext context) {
                      return const Text('second Navigator');
                    }
                  );
                },
              ),
            ),
          ],
        );
      }
    )
  );
}

Timeline

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Landed in version: 1.20.0
In stable release: 1.20

References

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API documentation:

Relevant issue:

Relevant PR: