Add new example to explore time

Cargo.toml

@@ -1231,6 +1231,16 @@ description = "Illustrates creating custom system parameters with `SystemParam`"
 category = "ECS (Entity Component System)"
 wasm = false
 
+[[example]]
+name = "time"
+path = "examples/ecs/time.rs"
+
+[package.metadata.example.time]
+name = "Time handling"
+description = "Explains how Time is handled in ECS"
+category = "ECS (Entity Component System)"
+wasm = false
+
 [[example]]
 name = "timers"
 path = "examples/ecs/timers.rs"

examples/README.md

@@ -227,6 +227,7 @@ Example | Description
 [System Closure](../examples/ecs/system_closure.rs) | Show how to use closures as systems, and how to configure `Local` variables by capturing external state
 [System Parameter](../examples/ecs/system_param.rs) | Illustrates creating custom system parameters with `SystemParam`
 [System Piping](../examples/ecs/system_piping.rs) | Pipe the output of one system into a second, allowing you to handle any errors gracefully
+[Time handling](../examples/ecs/time.rs) | Explains how Time is handled in ECS
 [Timers](../examples/ecs/timers.rs) | Illustrates ticking `Timer` resources inside systems and handling their state
 
 ## Games

examples/ecs/time.rs

@@ -0,0 +1,115 @@
+use bevy::prelude::*;
+
+use std::io::{self, BufRead};
+use std::time::Duration;
+
+fn banner() {
+    println!("This example is meant to intuitively demonstrate how Time works in Bevy.");
+    println!();
+    println!("Time will be printed in three different schedules in the app:");
+    println!("- PreUpdate: real time is printed");
+    println!("- FixedUpdate: fixed time step time is printed, may be run zero or multiple times");
+    println!("- Update: virtual game time is printed");
+    println!();
+    println!("Max delta time is set to 5 seconds. Fixed timestep is set to 1 second.");
+    println!();
+}
+
+fn help() {
+    println!("The app reads commands line-by-line from standard input.");
+    println!();
+    println!("Commands:");
+    println!("  empty line: Run app.update() once on the Bevy App");
+    println!("  q: Quit the app.");
+    println!("  f: Set speed to fast, 2x");
+    println!("  n: Set speed to normal, 1x");
+    println!("  n: Set speed to slow, 0.5x");
+    println!("  p: Pause");
+    println!("  u: Unpause");
+}
+
+fn runner(mut app: App) {
+    banner();
+    help();
+    let stdin = io::stdin();
+    for line in stdin.lock().lines() {
+        if let Err(err) = line {
+            println!("read err: {:#}", err);
+            break;
+        }
+        match line.unwrap().as_str() {
+            "" => {
+                app.update();
+            }
+            "f" => {
+                println!("FAST: setting relative speed to 2x");
+                app.world
+                    .resource_mut::<Time<Virtual>>()
+                    .set_relative_speed(2.0);
+            }
+            "n" => {
+                println!("NORMAL: setting relative speed to 1x");
+                app.world
+                    .resource_mut::<Time<Virtual>>()
+                    .set_relative_speed(1.0);
+            }
+            "s" => {
+                println!("SLOW: setting relative speed to 0.5x");
+                app.world
+                    .resource_mut::<Time<Virtual>>()
+                    .set_relative_speed(0.5);
+            }
+            "p" => {
+                println!("PAUSE: pausing virtual clock");
+                app.world.resource_mut::<Time<Virtual>>().pause();
+            }
+            "u" => {
+                println!("UNPAUSE: resuming virtual clock");
+                app.world.resource_mut::<Time<Virtual>>().unpause();
+            }
+            "q" => {
+                println!("QUITTING!");
+                break;
+            }
+            _ => {
+                help();
+            }
+        }
+    }
+}
+
+fn print_real_time(time: Res<Time<Real>>) {
+    println!(
+        "PreUpdate: this is real time clock, delta is {:?} and elapsed is {:?}",
+        time.delta(),
+        time.elapsed()
+    );
+}
+
+fn print_fixed_time(time: Res<Time>) {
+    println!(
+        "FixedUpdate: this is generic time clock inside fixed, delta is {:?} and elapsed is {:?}",
+        time.delta(),
+        time.elapsed()
+    );
+}
+
+fn print_time(time: Res<Time>) {
+    println!(
+        "Update: this is generic time clock, delta is {:?} and elapsed is {:?}",
+        time.delta(),
+        time.elapsed()
+    );
+}
+
+fn main() {
+    App::new()
+        .add_plugins(MinimalPlugins)
+        .insert_resource(Time::<Virtual>::from_max_delta(Duration::from_secs(5)))
+        .insert_resource(Time::<Fixed>::from_duration(Duration::from_secs(1)))
+        .add_systems(PreUpdate, print_real_time)
+        .add_systems(FixedUpdate, print_fixed_time)
+        .add_systems(Update, print_time)
+        .set_runner(runner)
+        .run();
+}