How to mock function in flutter test - flutter

Question

How can I mock a function in flutter and verify it has been called n times?
Ive tried implementing Mock from mockito but it only throws errors:
class MockFunction extends Mock {
call() {}
}
test("onListen is called once when first listener is registered", () {
final onListen = MockFunction();
// Throws: Bad state: No method stub was called from within `when()`. Was a real method called, or perhaps an extension method?
when(onListen()).thenReturn(null);
bloc = EntityListBloc(onListen: onListen);
// If line with when call is removed this throws:
// Used on a non-mockito object
verify(onListen()).called(1);
});
});
As a workaround I am just manually tracking the calls:
test("...", () {
int calls = 0;
bloc = EntityListBloc(onListen: () => calls++);
// ...
expect(calls, equals(1));
});
So is there a way I can create simple mock functions for flutter tests?

Answer 1

What you could do is this:
class Functions {
void onListen() {}
}
class MockFunctions extends Mock implements Functions {}
void main() {
test("onListen is called once when first listener is registered", () {
final functions = MockFunctions();
when(functions.onListen()).thenReturn(null);
final bloc = EntityListBloc(onListen: functions.onListen);
verify(functions.onListen()).called(1);
});
}

Answer 2

The accepted answer is correct, but it doesn't represent a real-life scenario where you will probably want to substitute a top-level function with a mock or a fake. This article explains how to include top-level functions in your dependency injection composition so that you can substitute those functions with mocks.
You can compose dependency injection like this and point to top-level functions such as launchUrl with ioc_container.
IocContainerBuilder compose() => IocContainerBuilder(
allowOverrides: true,
)
..addSingletonService<LaunchUrl>(
(url, {mode, webOnlyWindowName, webViewConfiguration}) async =>
launchUrl(
url,
mode: mode ?? LaunchMode.platformDefault,
webViewConfiguration:
webViewConfiguration ?? const WebViewConfiguration(),
webOnlyWindowName: webOnlyWindowName,
),
)
..add((container) => MyApp(launchUrl: container<LaunchUrl>()));
Then, you can use the technique mentioned in the answer here to mock with Mocktail.
import 'package:fafsdfsdf/main.dart';
import 'package:flutter_test/flutter_test.dart';
import 'package:mocktail/mocktail.dart';
import 'package:url_launcher/url_launcher.dart';
import 'package:flutter/material.dart';
class LaunchMock extends Mock {
Future<bool> call(
Uri url, {
LaunchMode? mode,
WebViewConfiguration? webViewConfiguration,
String? webOnlyWindowName,
});
}
void main() {
testWidgets('Test Url Launch', (tester) async {
//These allow default values
registerFallbackValue(LaunchMode.platformDefault);
registerFallbackValue(const WebViewConfiguration());
//Create the mock
final mock = LaunchMock();
when(() => mock(
flutterDevUri,
mode: any(named: 'mode'),
webViewConfiguration: any(named: 'webViewConfiguration'),
webOnlyWindowName: any(named: 'webOnlyWindowName'),
)).thenAnswer((_) async => true);
final builder = compose()
//Replace the launch function with a mock
..addSingletonService<LaunchUrl>(mock);
await tester.pumpWidget(
builder.toContainer()<MyApp>(),
);
//Tap the icon
await tester.tap(
find.byIcon(Icons.favorite),
);
await tester.pumpAndSettle();
verify(() => mock(flutterDevUri)).called(1);
});
}