How to load assets in tests that while using app.update ?

[Maykeye]

#[test]ing. I'm trying to test things with I/O and can't get it work without relying on app.run():

Here's dirty PoC.

#[test]
fn test_loading_image() {
    fn headless() -> PluginGroupBuilder {
        let mut winit = WinitPlugin::<WakeUp>::default();
        winit.run_on_any_thread = true;
        DefaultPlugins
            .set(RenderPlugin {
                render_creation: WgpuSettings {
                    backends: None,
                    ..default()
                }
                .into(),
                ..default()
            })
            .set(winit)
            .disable::<AudioPlugin>()
    }

    let mut app = App::new();
    app.add_plugins(headless());

    #[derive(Resource)]
    struct Timeout(Timer);
    #[derive(Resource)]
    struct Sample(Handle<Image>);
    app.insert_resource(Timeout(Timer::from_seconds(1.0, TimerMode::Once)));

    app.add_systems(Startup, |mut cmd: Commands, ass: Res<AssetServer>| {
        cmd.insert_resource(Sample(ass.load("sample.png")));
    });

    fn tick_timeout(
        mut t: ResMut<Timeout>,
        time: Res<Time<Real>>,
        sample: Res<Sample>,
        asset_serer: Res<AssetServer>,
        images: Res<Assets<Image>>,
    ) {
        let load_state = asset_serer.load_state(&sample.0);
        let from_load_state = matches!(load_state, bevy::asset::LoadState::Loaded);
        let from_assets = images.get(&sample.0).is_some();
        t.0.tick(time.delta());
        if t.0.just_finished() {
            println!(">>>>");
            println!(">>>> finished timer with {from_load_state} {from_assets}");
            println!(">>>>");
            exit(1);
        }
    }
    app.add_systems(Update, tick_timeout);

    //app.run();
    /*
     * WORKS:
     * >>>> finished timer with true true
     */

    /* DOES NOT WORK
     * >>>> finished timer with false false
     */
    for _ in 0..1000000 {
        app.update();
        std::thread::sleep(Duration::from_secs_f32(0.01));
    }
    assert_eq!(1,2);
}

I expect test to fail(duh) with report that image is loaded.
If I do it with app.run(), it does -- both assets and asset_server report it's true that image was loaded.

However just using app.update() doesn't seems to load anything. What to do?

[Maykeye]

And the answer is.....🥁🥁🥁 Clearing up!

    let mut chk = true;
    loop {
        if chk {
            let plugins_state = app.plugins_state();
            if plugins_state != PluginsState::Cleaned {
                println!(">>>>>>>> I'm {:?}", plugins_state);
                while app.plugins_state() == PluginsState::Adding {
                    bevy::tasks::tick_global_task_pools_on_main_thread();
                    println!("<<<<<< TICKITY I'm {:?}", plugins_state);
                }
                app.finish();
                app.cleanup();
            }
            chk = false;
        }
        app.update();
        std::thread::sleep(Duration::from_secs_f32(0.1));
    }

What it has to do with loading assets? Is there a point for default runner to not care of itself? Why it doesn't work if I tickle tick_global_task_pools_on_main_thread every frame? Mystery, which is better to leave aside as this work:

running 1 test
2025-12-27T08:08:09.997588Z  INFO bevy_diagnostic::system_information_diagnostics_plugin::internal: SystemInfo { os: "Linux (Garuda Linux Soaring)", kernel: "6.17.9-zen1-1-zen", cpu: "12th Gen Intel(R) Core(TM) i9-12900HK", core_count: "14", memory: "31.0 GiB" }
2025-12-27T08:08:10.000579Z ERROR bevy_render::extract_resource: Render app did not exist when trying to add `extract_resource` for <bevy_camera::clear_color::ClearColor>.
2025-12-27T08:08:10.023625Z  WARN bevy_gizmos: bevy_render feature is enabled but RenderApp was not detected. Are you sure you loaded GizmoPlugin after RenderPlugin?
>>>>>>>> I'm Ready
2025-12-27T08:08:10.023786Z  WARN bevy_render::texture: CompressedImageFormatSupport resource not found. It should either be initialized in finish() of RenderPlugin, or manually if not using the RenderPlugin or the WGPU backend.
2025-12-27T08:08:10.023877Z  WARN bevy_gltf: CompressedImageFormatSupport resource not found. It should either be initialized in finish() of RenderPlugin, or manually if not using the RenderPlugin or the WGPU backend.
>>>>
>>>> finished timer with true true at 2.042939
>>>>
>>> 574x270 //ive edited timeout to display image resolution

Not sure why run() should clear itself. Oh well. It seems it loads to the point of being able to tell dimensions at least, haven't tested further.