Function use_state 

pub fn use_state<T>(init: impl FnOnce() -> T) -> State<T>
where
    T: 'static,

Creates a reactive value initialized with the returned value of the init callback that we pass to it. A State<T> is a combination of Rc<RefCell<T>> + Copy and reactive semantics. It allow us to store a value into a reactive pointer and share this to other components to let them read or even write to it.

let mut state = use_state(|| 0);

rect()
    .expanded()
    .background((200, 200, 200))
    .on_press(move |_| {
        // Mutate and notify all subscribers
        *state.write() += 1;
    })
    // Read the state and subscribe to it
    .child(state.read().to_string())