virtual_time/
virtual_time.rs

1//! Shows how `Time<Virtual>` can be used to pause, resume, slow down
2//! and speed up a game.
3
4use std::time::Duration;
5
6use bevy::{
7    color::palettes::css::*, input::common_conditions::input_just_pressed, prelude::*,
8    time::common_conditions::on_real_timer,
9};
10
11fn main() {
12    App::new()
13        .add_plugins(DefaultPlugins)
14        .add_systems(Startup, setup)
15        .add_systems(
16            Update,
17            (
18                move_virtual_time_sprites,
19                move_real_time_sprites,
20                toggle_pause.run_if(input_just_pressed(KeyCode::Space)),
21                change_time_speed::<1>.run_if(input_just_pressed(KeyCode::ArrowUp)),
22                change_time_speed::<-1>.run_if(input_just_pressed(KeyCode::ArrowDown)),
23                (update_virtual_time_info_text, update_real_time_info_text)
24                    // update the texts on a timer to make them more readable
25                    // `on_timer` run condition uses `Virtual` time meaning it's scaled
26                    // and would result in the UI updating at different intervals based
27                    // on `Time<Virtual>::relative_speed` and `Time<Virtual>::is_paused()`
28                    .run_if(on_real_timer(Duration::from_millis(250))),
29            ),
30        )
31        .run();
32}
33
34/// `Real` time related marker
35#[derive(Component)]
36struct RealTime;
37
38/// `Virtual` time related marker
39#[derive(Component)]
40struct VirtualTime;
41
42/// Setup the example
43fn setup(mut commands: Commands, asset_server: Res<AssetServer>, mut time: ResMut<Time<Virtual>>) {
44    // start with double `Virtual` time resulting in one of the sprites moving at twice the speed
45    // of the other sprite which moves based on `Real` (unscaled) time
46    time.set_relative_speed(2.);
47
48    commands.spawn(Camera2d);
49
50    let virtual_color = GOLD.into();
51    let sprite_scale = Vec2::splat(0.5).extend(1.);
52    let texture_handle = asset_server.load("branding/icon.png");
53
54    // the sprite moving based on real time
55    commands.spawn((
56        Sprite::from_image(texture_handle.clone()),
57        Transform::from_scale(sprite_scale),
58        RealTime,
59    ));
60
61    // the sprite moving based on virtual time
62    commands.spawn((
63        Sprite {
64            image: texture_handle,
65            color: virtual_color,
66            ..Default::default()
67        },
68        Transform {
69            scale: sprite_scale,
70            translation: Vec3::new(0., -160., 0.),
71            ..default()
72        },
73        VirtualTime,
74    ));
75
76    // info UI
77    let font_size = 33.;
78
79    commands.spawn((
80        Node {
81            display: Display::Flex,
82            justify_content: JustifyContent::SpaceBetween,
83            width: percent(100),
84            position_type: PositionType::Absolute,
85            top: px(0),
86            padding: UiRect::all(px(20)),
87            ..default()
88        },
89        children![
90            (
91                Text::default(),
92                TextFont {
93                    font_size,
94                    ..default()
95                },
96                RealTime,
97            ),
98            (
99                Text::new("CONTROLS\nUn/Pause: Space\nSpeed+: Up\nSpeed-: Down"),
100                TextFont {
101                    font_size,
102                    ..default()
103                },
104                TextColor(Color::srgb(0.85, 0.85, 0.85)),
105                TextLayout::new_with_justify(Justify::Center),
106            ),
107            (
108                Text::default(),
109                TextFont {
110                    font_size,
111                    ..default()
112                },
113                TextColor(virtual_color),
114                TextLayout::new_with_justify(Justify::Right),
115                VirtualTime,
116            ),
117        ],
118    ));
119}
120
121/// Move sprites using `Real` (unscaled) time
122fn move_real_time_sprites(
123    mut sprite_query: Query<&mut Transform, (With<Sprite>, With<RealTime>)>,
124    // `Real` time which is not scaled or paused
125    time: Res<Time<Real>>,
126) {
127    for mut transform in sprite_query.iter_mut() {
128        // move roughly half the screen in a `Real` second
129        // when the time is scaled the speed is going to change
130        // and the sprite will stay still the time is paused
131        transform.translation.x = get_sprite_translation_x(time.elapsed_secs());
132    }
133}
134
135/// Move sprites using `Virtual` (scaled) time
136fn move_virtual_time_sprites(
137    mut sprite_query: Query<&mut Transform, (With<Sprite>, With<VirtualTime>)>,
138    // the default `Time` is either `Time<Virtual>` in regular systems
139    // or `Time<Fixed>` in fixed timestep systems so `Time::delta()`,
140    // `Time::elapsed()` will return the appropriate values either way
141    time: Res<Time>,
142) {
143    for mut transform in sprite_query.iter_mut() {
144        // move roughly half the screen in a `Virtual` second
145        // when time is scaled using `Time<Virtual>::set_relative_speed` it's going
146        // to move at a different pace and the sprite will stay still when time is
147        // `Time<Virtual>::is_paused()`
148        transform.translation.x = get_sprite_translation_x(time.elapsed_secs());
149    }
150}
151
152fn get_sprite_translation_x(elapsed: f32) -> f32 {
153    ops::sin(elapsed) * 500.
154}
155
156/// Update the speed of `Time<Virtual>.` by `DELTA`
157fn change_time_speed<const DELTA: i8>(mut time: ResMut<Time<Virtual>>) {
158    let time_speed = (time.relative_speed() + DELTA as f32)
159        .round()
160        .clamp(0.25, 5.);
161
162    // set the speed of the virtual time to speed it up or slow it down
163    time.set_relative_speed(time_speed);
164}
165
166/// pause or resume `Relative` time
167fn toggle_pause(mut time: ResMut<Time<Virtual>>) {
168    if time.is_paused() {
169        time.unpause();
170    } else {
171        time.pause();
172    }
173}
174
175/// Update the `Real` time info text
176fn update_real_time_info_text(time: Res<Time<Real>>, mut query: Query<&mut Text, With<RealTime>>) {
177    for mut text in &mut query {
178        **text = format!(
179            "REAL TIME\nElapsed: {:.1}\nDelta: {:.5}\n",
180            time.elapsed_secs(),
181            time.delta_secs(),
182        );
183    }
184}
185
186/// Update the `Virtual` time info text
187fn update_virtual_time_info_text(
188    time: Res<Time<Virtual>>,
189    mut query: Query<&mut Text, With<VirtualTime>>,
190) {
191    for mut text in &mut query {
192        **text = format!(
193            "VIRTUAL TIME\nElapsed: {:.1}\nDelta: {:.5}\nSpeed: {:.2}",
194            time.elapsed_secs(),
195            time.delta_secs(),
196            time.relative_speed()
197        );
198    }
199}
virtual_time/virtual_time.rs

virtual_time/
virtual_time.rs
