Get properties of a widget in a widget test

get_widget_props_test.md

Sometimes we need to test that a widget is doing a particular thing based on a provided parameter, or something similar. We can do this by using tester.widget and TypeMatcher's having() function.

Let's say we have a custom app bar that, using package:badges, should show a badge on the leading widget (which shows if there is a drawer) if the int badgeCount passed to it is greater than zero. Simply testing for the existence of the Badge widget isn't correct, because Badge is always being built - we need to test if the badge's showBadge property is false. We'd write our test like so:

import 'package:badges/badges.dart';
import 'package:flutter/material.dart' hide Badge;
import 'package:flutter_test/flutter_test.dart';
import 'package:my_package/src/widgets/custom_app_bar.dart';

void main () {
  testWidgets('Get widget property', (tester) async {
    await tester.pumpWidget(
      MaterialApp(
        home: Scaffold(
          appBar: CustomAppBar(
            title: Text('AppBar'),
            badgeCount: 0,
          ),
          drawer: Drawer(),          
        ),
      ),
    );
    final widget = tester.widget(find.byType(CustomAppBar));
    expect(widget, isA<CustomAppBar>().having((p) => p.showBadge, 'showBadge', false);
  });
}

Breakdown:

• Instead of finding our app bar by directly calling find (like final widget = find.byType(CustomAppBar)), we call tester.widget(), and do our find call inside it.

• We can then expect that:

  A. our widget is of the type we expect it to be by using isA<CustomAppBar>(), which is a TypeMatcher<T>.

  B. our widget has the property we expect. We must specify the following three things:

  1. we reference the showBadge property itself by calling p.showBadge
  2. we provide the name of the property as a String (this must match the property referenced immediately prior), which in this case is 'showBadge'.
  3. we provide the actual value that we expect the property to be, which in our case is false.

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Credit to Remi on this StackOverflow answer.