This entire book is outdated and no longer maintained. I am keeping it online, in case anyone still finds any of the information in it useful. To adapt to newer versions of Bevy, please consult Bevy's migration guides.

React to Changes with Asset Events

If you need to perform specific actions when an asset is created, modified, or removed, you can make a system that reacts to AssetEvent events.

#[derive(Resource)]
struct MyMapImage {
    handle: Handle<Image>,
}

fn fixup_images(
    mut ev_asset: EventReader<AssetEvent<Image>>,
    mut assets: ResMut<Assets<Image>>,
    map_img: Res<MyMapImage>,
) {
    for ev in ev_asset.iter() {
        match ev {
            AssetEvent::Created { handle } => {
                // a texture was just loaded or changed!

                // WARNING: this mutable access will cause another
                // AssetEvent (Modified) to be emitted!
                let texture = assets.get_mut(handle).unwrap();
                // ^ unwrap is OK, because we know it is loaded now

                if *handle == map_img.handle {
                    // it is our special map image!
                } else {
                    // it is some other image
                }
            }
            AssetEvent::Modified { handle } => {
                // an image was modified
            }
            AssetEvent::Removed { handle } => {
                // an image was unloaded
            }
        }
    }
}

Note: If you are handling Modified events and doing a mutable access to the data, the .get_mut will trigger another Modified event for the same asset. If you are not careful, this could result in an infinite loop! (from events caused by your own system)