We need a robust way to get the duration of a route transition in tests,
so that tests can remain independent of the type of route transition and
its duration.
With this PR, this will be the canonical way to pump until a page
transition has finished:
```dart
testWidgets('example', (WidgetTester tester) async {
final TransitionDurationObserver observer = TransitionDurationObserver();
await tester.pumpWidget(
MaterialApp(
navigatorObservers: <NavigatorObserver>[observer],
onGenerateRoute: (RouteSettings settings) { ... },
),
);
expect(find.text('Page 1'), findsOneWidget);
expect(find.text('Page 2'), findsNothing);
// Pump through the whole transition.
await tester.tap(find.text('Next'));
await observer.pumpPastTransition(tester);
expect(find.text('Page 1'), findsNothing);
expect(find.text('Page 2'), findsOneWidget);
// Or, pump through part of a transition with the duration.
await tester.tap(find.text('Back'));
await tester.pump(observer.transitionDuration ~/ 2);
expect(find.text('Page 1'), findsOneWidget);
expect(find.text('Page 2'), findsOneWidget);
});
```
This was spun out of https://github.com/flutter/flutter/pull/165832,
where updating the default route transition and its duration broke tests
in the framework, in customer tests, and in Google tests.
FYI @chrisbobbe
---------
Co-authored-by: Jing Shao <87506348+jingshao-code@users.noreply.github.com>
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