Container with color optimization

A new ColoredBox widget has been added to the framework, and the Container widget has been optimized to use it if a user specifies a color instead of a decoration.

It is very common to use the Container widget as follows:

dart

return Container(color: Colors.red);

Previously, this code resulted in a widget hierarchy that used a BoxDecoration to actually paint the background color. The BoxDecoration widget covers many cases other than just painting a background color, and is not as efficient as the new ColoredBox widget, which only paints a background color.

Widget tests that wanted to assert based on the color of a container in the widget tree would previously have to find the BoxDecoration to actually get the color of the container. Now, they are able to check the color property on the Container itself, unless a BoxDecoration was explicitly provided as the decoration property. It is still an error to supply both color and decoration to Container.

Tests that assert on the color of a Container or that expected it to create a BoxDecoration need to be modified.

Code before migration:

darttestWidgets('Container color', (WidgetTester tester) async {
  await tester.pumpWidget(Container(color: Colors.red));

  final Container container = tester.widgetList<Container>().first;
  expect(container.decoration.color, Colors.red);
  // Or, a test may have specifically looked for the BoxDecoration, e.g.:
  expect(find.byType(BoxDecoration), findsOneWidget);
});

Code after migration:

darttestWidgets('Container color', (WidgetTester tester) async {
  await tester.pumpWidget(Container(color: Colors.red));

  final Container container = tester.widgetList<Container>().first;
  expect(container.color, Colors.red);
  // If your test needed to work directly with the BoxDecoration, it should
  // instead look for the ColoredBox, e.g.:
  expect(find.byType(BoxDecoration), findsNothing);
  expect(find.byType(ColoredBox), findsOneWidget);
});