bevy/examples/games/loading_screen.rs

//! Shows how to create a loading screen that waits for assets to load and render.
use bevy::{ecs::system::SystemId, prelude::*};
use pipelines_ready::*;

// The way we'll go about doing this in this example is to
// keep track of all assets that we want to have loaded before
// we transition to the desired scene.
//
// In order to ensure that visual assets are fully rendered
// before transitioning to the scene, we need to get the
// current status of cached pipelines.
//
// While loading and pipelines compilation is happening, we
// will show a loading screen. Once loading is complete, we
// will transition to the scene we just loaded.

fn main() {
    App::new()
        .add_plugins(DefaultPlugins)
        // `PipelinesReadyPlugin` is declared in the `pipelines_ready` module below.
        .add_plugins(PipelinesReadyPlugin)
        .insert_resource(LoadingState::default())
        .insert_resource(LoadingData::new(5))
        .add_systems(Startup, (setup, load_loading_screen))
        .add_systems(
            Update,
            (update_loading_data, level_selection, display_loading_screen),
        )
        .run();
}

// A `Resource` that holds the current loading state.
#[derive(Resource, Default)]
enum LoadingState {
    #[default]
    LevelReady,
    LevelLoading,
}

// A resource that holds the current loading data.
#[derive(Resource, Debug, Default)]
struct LoadingData {
    // This will hold the currently unloaded/loading assets.
    loading_assets: Vec<UntypedHandle>,
    // Number of frames that everything needs to be ready for.
    // This is to prevent going into the fully loaded state in instances
    // where there might be a some frames between certain loading/pipelines action.
    confirmation_frames_target: usize,
    // Current number of confirmation frames.
    confirmation_frames_count: usize,
}

impl LoadingData {
    fn new(confirmation_frames_target: usize) -> Self {
        Self {
            loading_assets: Vec::new(),
            confirmation_frames_target,
            confirmation_frames_count: 0,
        }
    }
}

// This resource will hold the level related systems ID for later use.
#[derive(Resource)]
struct LevelData {
    unload_level_id: SystemId,
    level_1_id: SystemId,
    level_2_id: SystemId,
}

fn setup(mut commands: Commands) {
    let level_data = LevelData {
        unload_level_id: commands.register_system(unload_current_level),
        level_1_id: commands.register_system(load_level_1),
        level_2_id: commands.register_system(load_level_2),
    };
    commands.insert_resource(level_data);

    // Spawns the UI that will show the user prompts.
    let text_style = TextFont {
        font_size: 42.0,
        ..default()
    };
    commands
        .spawn((
            Node {
                justify_self: JustifySelf::Center,
                align_self: AlignSelf::FlexEnd,
                ..default()
            },
            BackgroundColor(Color::NONE),
        ))
        .with_child((Text::new("Press 1 or 2 to load a new scene."), text_style));
}

// Selects the level you want to load.
fn level_selection(
    mut commands: Commands,
    keyboard: Res<ButtonInput<KeyCode>>,
    level_data: Res<LevelData>,
    loading_state: Res<LoadingState>,
) {
    // Only trigger a load if the current level is fully loaded.
    if let LoadingState::LevelReady = loading_state.as_ref() {
        if keyboard.just_pressed(KeyCode::Digit1) {
            commands.run_system(level_data.unload_level_id);
            commands.run_system(level_data.level_1_id);
        } else if keyboard.just_pressed(KeyCode::Digit2) {
            commands.run_system(level_data.unload_level_id);
            commands.run_system(level_data.level_2_id);
        }
    }
}

// Marker component for easier deletion of entities.
#[derive(Component)]
struct LevelComponents;

// Removes all currently loaded level assets from the game World.
fn unload_current_level(
    mut commands: Commands,
    mut loading_state: ResMut<LoadingState>,
    entities: Query<Entity, With<LevelComponents>>,
) {
    *loading_state = LoadingState::LevelLoading;
    for entity in entities.iter() {
        commands.entity(entity).despawn();
    }
}

fn load_level_1(
    mut commands: Commands,
    mut loading_data: ResMut<LoadingData>,
    asset_server: Res<AssetServer>,
) {
    // Spawn the camera.
    commands.spawn((
        Camera3d::default(),
        Transform::from_xyz(155.0, 155.0, 155.0).looking_at(Vec3::new(0.0, 40.0, 0.0), Vec3::Y),
        LevelComponents,
    ));

    // Save the asset into the `loading_assets` vector.
    let fox = asset_server.load(GltfAssetLabel::Scene(0).from_asset("models/animated/Fox.glb"));
    loading_data.loading_assets.push(fox.clone().into());
    // Spawn the fox.
    commands.spawn((
        SceneRoot(fox.clone()),
        Transform::from_xyz(0.0, 0.0, 0.0),
        LevelComponents,
    ));

    // Spawn the light.
    commands.spawn((
        DirectionalLight {
            shadows_enabled: true,
            ..default()
        },
        Transform::from_xyz(3.0, 3.0, 2.0).looking_at(Vec3::ZERO, Vec3::Y),
        LevelComponents,
    ));
}

fn load_level_2(
    mut commands: Commands,
    mut loading_data: ResMut<LoadingData>,
    asset_server: Res<AssetServer>,
) {
    // Spawn the camera.
    commands.spawn((
        Camera3d::default(),
        Transform::from_xyz(1.0, 1.0, 1.0).looking_at(Vec3::new(0.0, 0.2, 0.0), Vec3::Y),
        LevelComponents,
    ));

    // Spawn the helmet.
    let helmet_scene = asset_server
        .load(GltfAssetLabel::Scene(0).from_asset("models/FlightHelmet/FlightHelmet.gltf"));
    loading_data
        .loading_assets
        .push(helmet_scene.clone().into());
    commands.spawn((SceneRoot(helmet_scene.clone()), LevelComponents));

    // Spawn the light.
    commands.spawn((
        DirectionalLight {
            shadows_enabled: true,
            ..default()
        },
        Transform::from_xyz(3.0, 3.0, 2.0).looking_at(Vec3::ZERO, Vec3::Y),
        LevelComponents,
    ));
}

// Monitors current loading status of assets.
fn update_loading_data(
    mut loading_data: ResMut<LoadingData>,
    mut loading_state: ResMut<LoadingState>,
    asset_server: Res<AssetServer>,
    pipelines_ready: Res<PipelinesReady>,
) {
    if !loading_data.loading_assets.is_empty() || !pipelines_ready.0 {
        // If we are still loading assets / pipelines are not fully compiled,
        // we reset the confirmation frame count.
        loading_data.confirmation_frames_count = 0;

        loading_data.loading_assets.retain(|asset| {
            asset_server
                .get_recursive_dependency_load_state(asset)
                .is_none_or(|state| !state.is_loaded())
        });

        // If there are no more assets being monitored, and pipelines
        // are compiled, then start counting confirmation frames.
        // Once enough confirmations have passed, everything will be
        // considered to be fully loaded.
    } else {
        loading_data.confirmation_frames_count += 1;
        if loading_data.confirmation_frames_count == loading_data.confirmation_frames_target {
            *loading_state = LoadingState::LevelReady;
        }
    }
}

// Marker tag for loading screen components.
#[derive(Component)]
struct LoadingScreen;

// Spawns the necessary components for the loading screen.
fn load_loading_screen(mut commands: Commands) {
    let text_style = TextFont {
        font_size: 67.0,
        ..default()
    };

    // Spawn the UI and Loading screen camera.
    commands.spawn((
        Camera2d,
        Camera {
            order: 1,
            ..default()
        },
        LoadingScreen,
    ));

    // Spawn the UI that will make up the loading screen.
    commands
        .spawn((
            Node {
                height: Val::Percent(100.0),
                width: Val::Percent(100.0),
                justify_content: JustifyContent::Center,
                align_items: AlignItems::Center,
                ..default()
            },
            BackgroundColor(Color::BLACK),
            LoadingScreen,
        ))
        .with_child((Text::new("Loading..."), text_style.clone()));
}

// Determines when to show the loading screen
fn display_loading_screen(
    mut loading_screen: Single<&mut Visibility, (With<LoadingScreen>, With<Node>)>,
    loading_state: Res<LoadingState>,
) {
    let visibility = match loading_state.as_ref() {
        LoadingState::LevelLoading => Visibility::Visible,
        LoadingState::LevelReady => Visibility::Hidden,
    };

    **loading_screen = visibility;
}

mod pipelines_ready {
    use bevy::{
        prelude::*,
        render::{render_resource::*, *},
    };

    pub struct PipelinesReadyPlugin;
    impl Plugin for PipelinesReadyPlugin {
        fn build(&self, app: &mut App) {
            app.insert_resource(PipelinesReady::default());

            // In order to gain access to the pipelines status, we have to
            // go into the `RenderApp`, grab the resource from the main App
            // and then update the pipelines status from there.
            // Writing between these Apps can only be done through the
            // `ExtractSchedule`.
            app.sub_app_mut(RenderApp)
                .add_systems(ExtractSchedule, update_pipelines_ready);
        }
    }

    #[derive(Resource, Debug, Default)]
    pub struct PipelinesReady(pub bool);

    fn update_pipelines_ready(mut main_world: ResMut<MainWorld>, pipelines: Res<PipelineCache>) {
        if let Some(mut pipelines_ready) = main_world.get_resource_mut::<PipelinesReady>() {
            pipelines_ready.0 = pipelines.waiting_pipelines().count() == 0;
        }
    }
}