The Google-developed open source UI framework was released in May 2017. Flutter is used for both mobile, desktop and web app development. Flutter is created using the object-oriented programming language, Dart. Flutter is developed with its material design guidelines.
Flutter is not as easy to learn as React Native. You need some background in Dart or any other object-oriented programming language to code in Flutter. The increased popularity of Flutter is the reason why many developers are picking up courses in Flutter.
Compared to React Native or Flutter, Ionic also has great documentation. While React Native is translated into native code, it is preferred by developers working on cross-platform apps.