Sprite won't be visible until the next sprite spawns

[nskins]

Hi! I'm running into a strange issue where the sprite will spawn "invisibly" and then only become visible once the next sprite spawns. I have code to reproduce this and have included a video. I am very much a beginner with Bevy, so I wouldn't know if this is a bug with my code or an issue with the engine.

In the video, watch when the score goes from 160 -> 165. You'll see that a banana appears to materialize in the middle of the screen. But it had spawned at the top of the screen like the rest of the bananas.

I would very much appreciate to hear any thoughts about this issue. Please let me know if I can be of any further assistance!

• Code example: https://github.com/nskins/banana_catch (branch: main)
• Relevant issue: Bananas can appear out of thin air for certain spawn timer values nskins/banana_catch#6
• Additional discussion: #3 - 10% Chance to Spawn Banana Bundles nskins/banana_catch#5 (review)

Screencast.from.03-28-2023.08.53.47.PM.webm

[guest-twenty-one]

This looks like an asset unloading bug:

1. at 0:14 the last single banana is caught and its entity is despawned
2. the asset server marks the banana.png image for removal as there's no sprite using it
3. ONE or TWO frames later, another single banana sprite is spawned using the same banana.png image
4. the asset server unloads the banana.png image (even though it shouldn't), and your banana sprite disappears from the screen (the entity is still there and moving down - you just don't see it)
5. it appears after another newly spawned sprite loads the banana.png image again

What's interesting is that, if the new sprite is spawned in the same frame or 3 or more frames after the last banana sprite is despawned (step 1), then everything works fine.

Anyway, you should cache your images/assets to stop them from unloading:

fn main() {
    App::new()
        // ...
        .add_startup_system(load_and_cache_images)
        // ...
}

#[derive(Resource)]
struct ImageCache {
	banana: Handle<Image>,
	bunch_of_bananas: Handle<Image>
}

fn load_and_cache_images(
    mut commands: Commands,
    asset_server: Res<AssetServer>,
) {
    commands.insert_resource(ImageCache {
        banana: asset_server.load("sprites/banana.png"),
        bunch_of_bananas: asset_server.load("sprites/bananabunch.png")
    });
}

then use the cached images when spawning sprites:

fn spawn_falling_objects_over_time(
    mut commands: Commands,
    window_query: Query<&Window, With<PrimaryWindow>>,
    falling_object_spawn_timer: Res<FallingObjectSpawnTimer>,
    image_cache: Res<ImageCache>,
) {
    if falling_object_spawn_timer.timer.finished() {
        let window = window_query.single();

        let bounds_width = window.width() - (2.0 * BOUND_SIZE);

        let random_x = (random::<f32>() * bounds_width) + BOUND_SIZE;

        let (kind, points, texture) = if random::<f32>() < 0.1 {
            (FallingObjectKind::BananaBunch, 5, image_cache.bunch_of_bananas.clone_weak())
        } else {
            (FallingObjectKind::Banana, 1, image_cache.banana.clone_weak())
        };	
		
        commands.spawn((
            SpriteBundle {
                transform: Transform::from_xyz(random_x, window.height(), 0.0),
                texture,
                ..default()
            },
            FallingObject {
                kind,
                points,
            },
        ));
    }
}

edit: This seems to be the same bug as: #6626

[nskins]

Hi @guest-twenty-one! Thank you so much for taking a look at my issue and providing such a detailed response. I appreciate the insight you've given me into how Bevy handles assets.

To add on to this, reading the Handle documentation was very informative. The fix above will keep a strong handle available and thus will not allow the assets to unload. Just want to throw that out there in case anyone else runs into this problem. 🙂