Google’s UI toolkit for building cross-platform apps is Flutter.
Flutter is an open-source software development kit created by Google for building applications from a shared codebase. It supports mobile, web, desktop, and Google’s Fuchsia platform, allowing developers to target Android, iOS, Linux, macOS, and Windows with much of the same code.
Flutter was first described in 2015 under the early project name Sky and released in alpha in 2017. Flutter 1.0 arrived on December 4, 2018. The toolkit uses the Dart programming language and has its own rendering engine, which helps it display a consistent interface across different operating systems.
Its basic building blocks are called widgets. Flutter includes widgets styled for Google’s Material Design and Apple’s Cupertino design language. React Native and Xamarin are also cross-platform development tools, while Kotlin Multiplatform focuses on sharing code across platforms rather than providing Flutter’s complete widget-and-rendering framework.