elegant way of capturing a reference to an integer variable? - input

Question

I have this snippet:
let mut animation_index = 0 as usize;
let mut ptr : *mut usize = &mut animation_index as _;
{
io_context.window().add_key_callback(
Box::new(move |key_states| {
if key_states[KbKey::Space.to_index()] == KeyActionState::Press
{
unsafe {
*ptr += 1;
println!("{}", animation_index);
}
}
})
);
}
Basically it adds a callback such that if and when I press space, the integer variable animation_index goes up by 1. This works, but requires the use of mutable pointers and unsafe, which is very ugly.
I'd like to have the same logic but ideally do it with pure safe rust isntead.

Answer 1

It looks like you are trying to share a mutable value across threads.
Typically, this is done with atomics, Arc<Mutex<T>> or Arc<RwLock<T>>.
use std::synce::{Arc, RwLock};
let mut animation_index = Arc::new(RwLock::new(0usize));
{
// a clone of the counter that can be moved into the callback
let animation_index = animation_index.clone();
io_context.window().add_key_callback(
Box::new(move |key_states| {
if key_states[KbKey::Space.to_index()] == KeyActionState::Press
{
let index = animation_index.write().unwrap();
*index += 1;
println!("{}", index);
}
})
);
}
With atomics it would look something like this:
use std::sync::Arc;
use std::sync::atomic::{AtomicUsize, Ordering};
let mut animation_index = Arc::new(AtomicUsize::new(0));
{
// a clone of the counter that can be moved into the callback
let animation_index = animation_index.clone();
io_context.window().add_key_callback(
Box::new(move |key_states| {
if key_states[KbKey::Space.to_index()] == KeyActionState::Press
{
let index = animation_index.fetch_add(1, Ordering::SeqCst);
println!("{}", index);
}
})
);
}