Why does WidgetTester.pump not work for navigation changes where WidgetTester.pumpAndSettle does?

[Kent Boogaart]

Hi,

I'm writing some widget tests. In one particular test, I possibly nav from one page to the next when the user taps something (the same test code is checking two different scenarios - one where nav is expected, one where it isn't). The current page has an infinite animation, so I can't use pumpAndSettle after performing the tap, because if no nav occurs then it will just time out. And I can't just call pump without a small duration because navigation takes time. However, I thought calling pump with a reasonable duration (e.g. 3 seconds) would work, but it doesn't. Even if I set it to an hour, it doesn't.

I have repro'd this as follows:

import 'package:flutter/material.dart';

void main() => runApp(new MyApp());

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return new MaterialApp(
      title: 'Flutter Demo',
      theme: new ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: new Page1(),
    );
  }
}

class Page1 extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    var body = new Column(
      children: <Widget>[
        const Text("The first page"),
        new MaterialButton(child: const Text("NAVIGATE"), onPressed: () => Navigator.of(context).push(new MaterialPageRoute(builder: (_) => new Page2()))),
      ],
    );

    var scaffold = new Scaffold(
      body: body,
      appBar: new AppBar(title: const Text("Page 1")),
    );

    return scaffold;
  }
}

class Page2 extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    var body = const Text("The second page");

    var scaffold = new Scaffold(
      body: body,
      appBar: new AppBar(title: const Text("Page 2")),
    );

    return scaffold;
  }
}

import 'package:flutter_test/flutter_test.dart';
import 'package:repro_nav_widget_test/main.dart';

void main() {
  testWidgets('repro', (WidgetTester tester) async {
    await tester.pumpWidget(new MyApp());

    expect(find.text("Page 1"), findsOneWidget);

    await tester.tap(find.text("NAVIGATE"));

    // THIS WORKS
    //await tester.pumpAndSettle();
    
    // THIS DOES NOT
    await tester.pump(const Duration(seconds: 3));

    expect(find.text("Page 2"), findsOneWidget);
  });
}

Can anyone tell me why pumpAndSettle works where pump(Duration(seconds: 3)) does not?

Thanks,
Kent

[Ian Hickson]

"pump" works by running one frame, pretending that there was the given duration of jank before the frame.

For a navigation, you need to pump two frames: the first is to begin showing everything, the second is to finish the animations and get everything into its final place.

So generally, you want to do something like:

    await tester.pump();

    await tester.pump(const Duration(seconds: 3));

This is actually the preferred pattern in general; pumpAndSettle is something I would generally discourage from use except when you specifically need it. Mostly it's useful as a way to count the number of frames that were required, to verify that there weren't excessive frames pumped.

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Ian Hickson

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