Widget test a widget which uses WidgetsBinding.instance.addPostFrameCallback()

Question

I'm trying to do a widget test in flutter to one widget which uses WidgetsBinding.instance.addPostFrameCallback() to show a dialog at the very beggining of the widget lifecycle.

The widget works fine, but I cannot figure out how to do a widget test, because the very first expect() of the test is executed before this post frame is triggered.

Anyway to "tell" the test to wait until all the post frames are dispatched?

PD: tester.pump(), tester.pumpAndSettle(), etc. are not working.

I have the same issue and sorry to say I have no idea ;-/ – christianalfoni Sep 02 '19 at 13:48 — christianalfoni, Sep 02 '19 at 13:48

score 8 · Accepted Answer · answered Sep 02 '19 at 13:51

8

But what do you know... suddenly it popped up:

    tester.binding.scheduleWarmUpFrame();

Run that :-)

Taken from here: https://chromium.googlesource.com/external/github.com/flutter/flutter/+/v0.9.2/packages/flutter/test/widgets/binding_test.dart

answered Sep 02 '19 at 13:51

christianalfoni

994
8
12

1

warmUpFrame is not exactly what you need, bc it has some initialization overhead – Andrei Lesnitsky Jul 18 '21 at 21:58

score 3 · Answer 2 · answered Jul 18 '21 at 21:58

To schedule a frame (and force postFrameCallback to be called) in flutter test environment you need to call tester.binding.scheduleFrame() before tester.pump().

Alternatively, if you don't care about the whole widget tree being rebuilt, you could call tester.pumpWidget(widget) instead of tester.pump()

Example test:

testWidgets('testing frames', (tester) async {
  final w = Container();
  int framesCount = 0;

  tester.binding.addPersistentFrameCallback((timeStamp) {
    framesCount++;
  });

  await tester.pumpWidget(w);
  expect(framesCount, equals(1));

  // pumpWidget calls [scheduleFrame]
  await tester.pumpWidget(w);
  expect(framesCount, equals(2));

  await tester.pump();
  // no frame was scheduled, so framesCount is still 2
  expect(framesCount, equals(2));

  tester.binding.scheduleFrame(); // --
  await tester.pump();            //   |
  expect(framesCount, equals(3)); // <-

  // pumpWidget calls [scheduleFrame]
  await tester.pumpWidget(w);
  expect(framesCount, equals(4));
});

chess0045 · Answer 3 · 2019-09-13T22:27:34.407

This worked for me:

// press button
await tester.tap(find.text("Save"));
// await the flushbar or snackbar to appear
await tester.pumpAndSettle(const Duration(seconds: 1));
// find by text or key
expect(find.text("Error saving note"), findsOneWidget);
// this prevents this error:
// A Timer is still pending even after the widget tree was disposed.
// the flushbar in my code is displayed for 5 seconds. So we wait for it to 
// go away.
await tester.pumpAndSettle(const Duration(seconds: 5));

I dont know if this is the proper way to do. But it works. Good luck!

Widget test a widget which uses WidgetsBinding.instance.addPostFrameCallback()

3 Answers3