bevy 0.4.0

use bevy::prelude::*;

/// This example illustrates how to use States to control transitioning from a Menu state to an InGame state.
fn main() {
    App::build()
        .add_plugins(DefaultPlugins)
        .init_resource::<ButtonMaterials>()
        .add_resource(State::new(AppState::Menu))
        .add_stage_after(stage::UPDATE, STAGE, StateStage::<AppState>::default())
        .on_state_enter(STAGE, AppState::Menu, setup_menu.system())
        .on_state_update(STAGE, AppState::Menu, menu.system())
        .on_state_exit(STAGE, AppState::Menu, cleanup_menu.system())
        .on_state_enter(STAGE, AppState::InGame, setup_game.system())
        .on_state_update(STAGE, AppState::InGame, movement.system())
        .on_state_update(STAGE, AppState::InGame, change_color.system())
        .run();
}

const STAGE: &str = "app_state";

#[derive(Clone)]
enum AppState {
    Menu,
    InGame,
}

struct MenuData {
    button_entity: Entity,
}

fn setup_menu(
    commands: &mut Commands,
    asset_server: Res<AssetServer>,
    button_materials: Res<ButtonMaterials>,
) {
    commands
        // ui camera
        .spawn(CameraUiBundle::default())
        .spawn(ButtonBundle {
            style: Style {
                size: Size::new(Val::Px(150.0), Val::Px(65.0)),
                // center button
                margin: Rect::all(Val::Auto),
                // horizontally center child text
                justify_content: JustifyContent::Center,
                // vertically center child text
                align_items: AlignItems::Center,
                ..Default::default()
            },
            material: button_materials.normal.clone(),
            ..Default::default()
        })
        .with_children(|parent| {
            parent.spawn(TextBundle {
                text: Text {
                    value: "Play".to_string(),
                    font: asset_server.load("fonts/FiraSans-Bold.ttf"),
                    style: TextStyle {
                        font_size: 40.0,
                        color: Color::rgb(0.9, 0.9, 0.9),
                        ..Default::default()
                    },
                },
                ..Default::default()
            });
        });
    commands.insert_resource(MenuData {
        button_entity: commands.current_entity().unwrap(),
    });
}

fn menu(
    mut state: ResMut<State<AppState>>,
    button_materials: Res<ButtonMaterials>,
    mut interaction_query: Query<
        (&Interaction, &mut Handle<ColorMaterial>),
        (Mutated<Interaction>, With<Button>),
    >,
) {
    for (interaction, mut material) in interaction_query.iter_mut() {
        match *interaction {
            Interaction::Clicked => {
                *material = button_materials.pressed.clone();
                state.set_next(AppState::InGame).unwrap();
            }
            Interaction::Hovered => {
                *material = button_materials.hovered.clone();
            }
            Interaction::None => {
                *material = button_materials.normal.clone();
            }
        }
    }
}

fn cleanup_menu(commands: &mut Commands, menu_data: Res<MenuData>) {
    commands.despawn_recursive(menu_data.button_entity);
}

fn setup_game(
    commands: &mut Commands,
    asset_server: Res<AssetServer>,
    mut materials: ResMut<Assets<ColorMaterial>>,
) {
    let texture_handle = asset_server.load("branding/icon.png");
    commands
        .spawn(Camera2dBundle::default())
        .spawn(SpriteBundle {
            material: materials.add(texture_handle.into()),
            ..Default::default()
        });
}

const SPEED: f32 = 100.0;
fn movement(
    time: Res<Time>,
    input: Res<Input<KeyCode>>,
    mut query: Query<&mut Transform, With<Sprite>>,
) {
    for mut transform in query.iter_mut() {
        let mut direction = Vec3::default();
        if input.pressed(KeyCode::Left) {
            direction.x += 1.0;
        }
        if input.pressed(KeyCode::Right) {
            direction.x -= 1.0;
        }
        if input.pressed(KeyCode::Up) {
            direction.y += 1.0;
        }
        if input.pressed(KeyCode::Down) {
            direction.y -= 1.0;
        }

        if direction != Vec3::default() {
            transform.translation += direction.normalize() * SPEED * time.delta_seconds();
        }
    }
}

fn change_color(
    time: Res<Time>,
    mut assets: ResMut<Assets<ColorMaterial>>,
    query: Query<&Handle<ColorMaterial>, With<Sprite>>,
) {
    for handle in query.iter() {
        let material = assets.get_mut(handle).unwrap();
        material
            .color
            .set_b((time.seconds_since_startup() * 5.0).sin() as f32 + 2.0);
    }
}

struct ButtonMaterials {
    normal: Handle<ColorMaterial>,
    hovered: Handle<ColorMaterial>,
    pressed: Handle<ColorMaterial>,
}

impl FromResources for ButtonMaterials {
    fn from_resources(resources: &Resources) -> Self {
        let mut materials = resources.get_mut::<Assets<ColorMaterial>>().unwrap();
        ButtonMaterials {
            normal: materials.add(Color::rgb(0.15, 0.15, 0.15).into()),
            hovered: materials.add(Color::rgb(0.25, 0.25, 0.25).into()),
            pressed: materials.add(Color::rgb(0.35, 0.75, 0.35).into()),
        }
    }
}