Improve background tab behaviour

[cBournhonesque]

The main functionality was merged here: #371

But some things could be improved:

• have more granular control over the interval. At a minimum make it configurable at start. Maybe even overridable?
• some plugins don't need to run when the tab is background (for example InputPlugin). They should be disabled while the app is hidden in wasm

[simbleau]

I think the general approach should be:

Use the Window winit events to listen for when the window is tabbed-off / minimized / etc.

Keep an attention state (active/inactive).
(eg)

#[derive(State)]
pub enum BevyWindowState {
    Inactive,
    Active,
}

Depending on that, send inputs/etc.

[simbleau]

I'm also torn whether or not that should be in bevy proper, not just a plugin. As it currently stands, the winit event for WindowUnfocused just straight up doesn't work, because it fires after it returns, firing the Unfocused and Refocused at the same time.

It's downright basically a bug, it's not delivering value to anyone.

[simbleau]

I've made a mention in bevy: bevyengine/bevy#13486

[simbleau]

I just thought of something else...

Let's imagine you have a system that is supposed to be an atomic counter.

Every system, the counter += 1.

If we do this WebPlugin, wouldn't we be double-counting every time the webworker runs the Main schedule? This would be a very difficult thing to triage.

Can we adjust the plugin so it only executes the Main schedule if (and only if) the window is not focused? Perhaps using a rolling track of whether the BevyWindowState is BevyWindowState::Inactive.

[cBournhonesque]

It does execute the Main schedule only if the window is not focused:

if window().unwrap().document().unwrap().hidden() {

Basically the solution is extremely unsafe: we take a raw pointer on the world, which we use only when the window is hidden.
But that is ok to do because we know that the bevy scheduler doesn't run when the tab is hidden (since there are no requestAnimationFrame calls), so nothing else is using the world.

So:

• when the tab is open: the Main schedule runs normally
• when the tab is hidden: the Main schedule runs at a set interval in the WebWorker, and does not run in main bevy thread

@Nul-led correct me if i'm wrong

[Nul-led]

Its technically safe tho since we are in wasm land and everything is single threaded anyways, so we could totally run this alongside winits event loop

[simbleau]

Note these window.unwrap()... calls will panic in a headless environment.

Primarily wasm-pack test --node or using headless browsers.

We should change this to

if window().and_then(w| w.document()).is_some_and(|d| d.hidden()) {
...
}

[Nul-led]

Note these window.unwrap()... calls will panic in a headless environment.

Primarily wasm-pack test --node or using headless browsers.

We should change this to

if window().and_then(w| w.document()).is_some_and(|d| d.hidden()) {
...
}

Wouldnt it be better to have a toggle for the "hidden" check instead, so we can enable / disable it on app launch

[Nul-led]

@simbleau
https://crates.io/crates/bevy_web_keepalive
hope this helps :)