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gui (short for generic user interface) is a crate providing basic user interface functionality. It strives to be as independent as possible of the underlying system architecture. That is, it is compatible with windowing as well as terminal based systems and it does not rely on the specifics of any GUI toolkit.
Part of this story means that it is not an out-of-the-box replacement for your favorite user interface toolkit (think: GTK+, Qt, wxWidgets to name popular ones, although these are naturally much more complex and provide way more functionality -- they are just among the most well known), but that it should be seen more as a building block, providing certain hooks and basic functionality typically seen in a UI. The infrastructure for dispatching events to widgets is an example. To make proper use of it, the functionality provided by this crate needs to be glued with the underlying system.
The crate is to be considered an experiment more than anything else. It is used for exploring parts of the design space for user interface architecture using Rust. Design of UI systems in Rust is generally considered hard and to a large degree an unsolved problem, although there are various promising designs out there.
Given this current state, changes in API design are to be expected.