Widgets: the building blocks of UI

Flutter widgets are built using a modern framework that takes inspiration from React: widgets describe what their view should look like given their current configuration and state, and when state changes the widget rebuilds its description, which the framework diffs against the previous description to determine minimal render tree changes. Flutter's UI layer has two tiers: the style-neutral core widgets library (package:flutter/widgets.dart) from the Flutter SDK, and design system packages material_ui and cupertino_ui on pub.dev. Use the widgets package for custom design systems, design-agnostic packages, or embedded devices where minimal bundle size is critical; material_ui implements Material 3, cupertino_ui Apple's Human Interface Guidelines. Authors commonly subclass StatelessWidget or StatefulWidget and implement build(), describing the widget in terms of lower-level widgets down to RenderObject. Scope: Flutter. Reviewed October 4, 2026.