Show basic application integration testing (#2896)

tests/how_to_test_full_app.rs

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+//! This module serves as an example, and test-case, for how to automatically test full bevy apps.
+//!
+//! By substituting [`DefaultPlugins`] with [`MinimalPlugins`], Bevy can run completely headless.
+//!
+//! The list of minimal plugins does not include things like window or input handling.
+//! This has as downside that the resources / entities associated with those systems
+//! (for example: `Input::<KeyCode>`) need to be manually added, either directly or via e.g. [`InputPlugin`].
+//! The upside however, is that the test has complete control over these resources, meaning
+//! we can fake user input, fake the window being moved around, and more.
+
+use bevy::prelude::*;
+
+const DEFAULT_MANA: u32 = 10;
+
+#[derive(Component)]
+struct Player {
+    mana: u32,
+}
+
+fn window_title_system(mut windows: Query<&mut Window>) {
+    for (index, mut window) in windows.iter_mut().enumerate() {
+        window.title = format!("This is window {index}!");
+    }
+}
+
+fn spawn_player(mut commands: Commands) {
+    commands.spawn(Player { mana: DEFAULT_MANA });
+}
+
+fn spell_casting(mut player: Query<&mut Player>, keyboard_input: Res<Input<KeyCode>>) {
+    if keyboard_input.just_pressed(KeyCode::Space) {
+        let Ok(mut player) = player.get_single_mut() else {
+            return;
+        };
+
+        if player.mana > 0 {
+            player.mana -= 1;
+        }
+    }
+}
+
+/// This struct would normally be defined somewhere in the game code.
+/// It could be the plugin that sets up the entire game (except for the [`DefaultPlugins`]).
+/// Or the game could be split into multiple smaller plugins, allowing one to write tests
+/// for each of the games' parts in isolation (on top of some tests for the entire game).
+pub struct GamePlugin;
+
+impl Plugin for GamePlugin {
+    fn build(&self, app: &mut App) {
+        app.add_startup_system(spawn_player)
+            .add_startup_system(window_title_system)
+            .add_system(spell_casting);
+    }
+}
+
+fn create_test_app() -> App {
+    let mut app = App::new();
+
+    // Note how we use `MinimalPlugins` instead of `DefaultPlugins`.
+    // This is what allows the test to run without a window, or real user input.
+    app.add_plugins(MinimalPlugins);
+    // Inserting a KeyCode input resource allows us to inject keyboard inputs,
+    // as if the user had pressed them.
+    app.insert_resource(Input::<KeyCode>::default());
+
+    // Spawning a fake window allows testing systems that require a window.
+    app.world.spawn(Window::default());
+
+    app
+}
+
+#[test]
+fn test_player_spawn() {
+    let mut app = create_test_app();
+    app.add_plugin(GamePlugin);
+
+    // The `update` function needs to be called at least once for the startup
+    // systems to run.
+    app.update();
+
+    // Now that the startup systems have run, we can check if the player has
+    // spawned as expected.
+    let player = app.world.query::<&Player>().get_single(&app.world);
+    assert!(player.is_ok(), "There should be exactly 1 player.");
+    assert_eq!(
+        player.unwrap().mana,
+        DEFAULT_MANA,
+        "Player does not have expected starting mana."
+    );
+}
+
+#[test]
+fn test_spell_casting() {
+    let mut app = create_test_app();
+    app.add_plugin(GamePlugin);
+
+    // We simulate pressing `space` to trigger the spell casting system.
+    app.world
+        .resource_mut::<Input<KeyCode>>()
+        .press(KeyCode::Space);
+    // Allow the systems to realize space got pressed.
+    app.update();
+
+    // The spell casting should have used up a single mana.
+    let player = app.world.query::<&Player>().single(&app.world);
+    assert_eq!(player.mana, DEFAULT_MANA - 1);
+
+    // Clear the `just_pressed` status for all `KeyCode`s
+    app.world.resource_mut::<Input<KeyCode>>().clear();
+    app.update();
+
+    // No extra spells should have been cast, so no mana should have been lost.
+    let player = app.world.query::<&Player>().single(&app.world);
+    assert_eq!(player.mana, DEFAULT_MANA - 1);
+}
+
+#[test]
+fn test_faking_windows() {
+    let mut app = create_test_app();
+    app.add_plugin(GamePlugin);
+
+    // The `update` function needs to be called at least once for the startup
+    // systems to run.
+    app.update();
+
+    let window = app.world.query::<&Window>().single(&app.world);
+    assert_eq!(window.title, "This is window 0!");
+}