When comparing Kivy vs FLTK, the Slant community recommends Kivy for most people. In the question“What are the best cross-platform GUI toolkits?” Kivy is ranked 8th while FLTK is ranked 12th. The most important reason people chose Kivy is:
Kv design language is designed to work well with scalable graphical user interfaces (GUIs) as one of its primary focuses. It also segregates the logic from the design elements, making them easier to visually distinguish from each other.
Specs
Ranked in these QuestionsQuestion Ranking
Pros
Pro Kv design language
Kv design language is designed to work well with scalable graphical user interfaces (GUIs) as one of its primary focuses. It also segregates the logic from the design elements, making them easier to visually distinguish from each other.
Pro Multitouch gestures
Pro Simplicity
It's simple design and lack of more advanced C++ features makes it easy for beginners.
Pro Fast
Well-designed widgets, coded with careful attention to rendering/execution speed.
Pro Stable
FLTK code developed more than 10 years ago still compiles and runs perfectly, without changes.
Pro Lightweight
Uses a limited and lightweight design and restricts itself to solely GUI functionality. Because of this restriction, the FLTK hello world example is only about 100 KiB.
Pro GUI designer
Fast Light User-Interface Designer (FLUID) included.
Cons
Con Documentation & examples are terrible
Terrible documentation & not too many resources can be found.
Con Very difficult to deploy to various platforms, like Android
Con Can be too simple for some projects
FLTK offers far fewer widgets than most other toolkits.
Con Does not look like a native application
Because it uses non-native widgets, it doesn't look like a native application on any platform.