Callback to track time when last event occured. Written in Rust for referential transparency.

event_timer_cb.rs

use std::boxed::Box;

type Callback<U, T> = Box<dyn Fn(&mut U, &T)>;
type Condition<U, T> = Box<dyn Fn(&U, &T) -> bool>;
type Effect<U, T> = Box<dyn Fn(&mut U, &T)>;

fn make_callback<U: 'static, T: 'static>(
    condition: Condition<U, T>,
    effect: Effect<U, T>,
) -> Callback<U, T> {
    let cb = move |u: &mut U, t: &T| {
        if condition(u, t) {
            effect(u, t)
        }
    };
    Box::new(cb)
}

fn step(u: &mut [f64; 2], t: &mut f64, callback: &Callback<[f64; 2], f64>) {
    callback(u, t);
    *t += 1.;
    u[0] *= 1.5;
}

fn main() {
    let u = &mut [1., 0.];
    let t = &mut 0.;

    let condition: Condition<[f64; 2], f64> = Box::new(|u: &[f64; 2], t: &f64| u[0] >= 3.);
    let effect: Effect<[f64; 2], f64> = Box::new(|u: &mut [f64; 2], t: &f64| {
        u[1] = *t;
        u[0] = 0.1
    });

    let cb = make_callback(condition, effect);

    for i in 1..20 {
        step(u, t, &cb);
        println!("u: [{0:.2}, {1:.2}], t: {2:.2}", u[0], u[1], t);
    }
}