add basic example of a custom update loop (#799)

Cargo.toml

@@ -148,6 +148,10 @@ path = "examples/3d/texture.rs"
 name = "z_sort_debug"
 path = "examples/3d/z_sort_debug.rs"
 
+[[example]]
+name = "custom_loop"
+path = "examples/app/custom_loop.rs"
+
 [[example]]
 name = "empty_defaults"
 path = "examples/app/empty_defaults.rs"

examples/app/custom_loop.rs

@@ -0,0 +1,31 @@
+use bevy::prelude::*;
+use std::{io, io::BufRead};
+
+struct Input(String);
+
+/// This example demonstrates you can create a custom runner (to update an app manually). It reads
+/// lines from stdin and prints them from within the ecs.
+fn my_runner(mut app: App) {
+    app.initialize();
+
+    println!("Type stuff into the console");
+    for line in io::stdin().lock().lines() {
+        {
+            let mut input = app.resources.get_mut::<Input>().unwrap();
+            input.0 = line.unwrap();
+        }
+        app.update();
+    }
+}
+
+fn print_system(input: Res<Input>) {
+    println!("You typed: {}", input.0);
+}
+
+fn main() {
+    App::build()
+        .add_resource(Input(String::new()))
+        .set_runner(my_runner)
+        .add_system(print_system.system())
+        .run();
+}