Move egui::util::cache to egui::cache; add FramePublisher

[emilk]

This moves egui::util::cache to egui::cache (the old path is deprecated, but still works).

It also adds the FramePublisher helper, which can be used to publish a value which will be retained for this frame and the next:

pub type MyPublisher = egui::cache::FramePublisher<MyKey, MyValue>;

// Publish:
ctx.memory_mut(|mem| {
    mem.caches.cache::<MyPublisher>().set(key, value);
});

// Retrieve:
let value: Option<MyValue> = ctx.memory_mut(|mem| {
    mem.caches
        .cache::<MyPublisher>()
        .get(key)
        .clone()
})

[github-actions]

Preview available at https://egui-pr-preview.github.io/pr/5426-emilk/cache-refactor
Note that it might take a couple seconds for the update to show up after the preview_build workflow has completed.

[zhatuokun]

If I call CacheStorage::cache somewhere, it will insert a Cache that implements the CacheTrait.

Although Cache will call evict_cache to clear the internal cache, even if the length of the internal cache of cache is 0, it still seems to exist in the CacheStorage.

So would it be better to delete the cache with a cache length of 0 in the CacheStorage::update?

[emilk]

Meta-cache eviction! Yeah, that would make sense, but hopefully there won't be that many cache types… but maybe.

[zhatuokun]

@emilk I tested this new feature and found that it works well in general, but when you have an Immediate Viewport, it will return None.

Here is my test code.

type MyPublisher = egui::cache::FramePublisher<String, u32>;

fn main() -> eframe::Result {
    eframe::run_simple_native("Example", eframe::NativeOptions::default(), |ctx, _| {
        let outer_key = String::from("outer_key");
        let value = ctx.memory_mut(|m| m.caches.cache::<MyPublisher>().get(&outer_key).copied());
        println!("outer {value:?}"); // None
        ctx.memory_mut(|m| m.caches.cache::<MyPublisher>().set(outer_key, 0));

        ctx.show_viewport_immediate(
            egui::ViewportId::from_hash_of("ExampleViewport"),
            Default::default(),
            |ctx, _| {
                let inner_key = String::from("inner_key");
                let value = ctx.memory_mut(|m| m.caches.cache::<MyPublisher>().get(&inner_key).copied());
                println!("inner: {value:?}"); // None
                ctx.memory_mut(|m| m.caches.cache::<MyPublisher>().set(inner_key, 0));
            },
        );
    })
}