examples on how to tests systems (#1714)

well... those are examples on how to tests systems despawning entities, modifying components, accessing resources, spawning entities

examples/README.md

@@ -47,6 +47,7 @@ git checkout v0.4.0
   - [Reflection](#reflection)
   - [Scene](#scene)
   - [Shaders](#shaders)
+  - [Tests](#tests)
   - [Tools](#tools)
   - [UI (User Interface)](#ui-user-interface)
   - [Window](#window)
@@ -130,8 +131,8 @@ Example | File | Description
 
 Example | File | Description
 --- | --- | ---
-`log_diagnostics` | [`diagnostics/log_diagnostics.rs`](./diagnostics/log_diagnostics.rs) | Add a plugin that logs diagnostics to the console
 `custom_diagnostic` | [`diagnostics/custom_diagnostic.rs`](./diagnostics/custom_diagnostic.rs) | Shows how to create a custom diagnostic
+`log_diagnostics` | [`diagnostics/log_diagnostics.rs`](./diagnostics/log_diagnostics.rs) | Add a plugin that logs diagnostics to the console
 
 ## ECS (Entity Component System)
 
@@ -197,6 +198,12 @@ Example | File | Description
 `shader_custom_material` | [`shader/shader_custom_material.rs`](./shader/shader_custom_material.rs) | Illustrates creating a custom material and a shader that uses it
 `shader_defs` | [`shader/shader_defs.rs`](./shader/shader_defs.rs) | Demonstrates creating a custom material that uses "shaders defs" (a tool to selectively toggle parts of a shader)
 
+## Tests
+
+Example | File | Description
+--- | --- | ---
+`how_to_test_systems` | [`../tests/how_to_test_systems.rs`](../tests/how_to_test_systems.rs) | How to test systems with commands, queries or resources
+
 ## Tools
 
 Example | File | Description

tests/how_to_test_systems.rs

@@ -0,0 +1,97 @@
+use bevy::prelude::*;
+
+#[derive(Default)]
+struct Enemy {
+    hit_points: u32,
+}
+
+fn despawn_dead_enemies(mut commands: Commands, enemies: Query<(Entity, &Enemy)>) {
+    for (entity, enemy) in enemies.iter() {
+        if enemy.hit_points == 0 {
+            commands.entity(entity).despawn_recursive();
+        }
+    }
+}
+
+fn hurt_enemies(mut enemies: Query<&mut Enemy>) {
+    for mut enemy in enemies.iter_mut() {
+        enemy.hit_points -= 1;
+    }
+}
+
+fn spawn_enemy(mut commands: Commands, keyboard_input: Res<Input<KeyCode>>) {
+    if keyboard_input.just_pressed(KeyCode::Space) {
+        commands.spawn().insert(Enemy { hit_points: 5 });
+    }
+}
+
+#[test]
+fn did_hurt_enemy() {
+    // Setup world
+    let mut world = World::default();
+
+    // Setup stage with our two systems
+    let mut update_stage = SystemStage::parallel();
+    update_stage.add_system(hurt_enemies.system().before("death"));
+    update_stage.add_system(despawn_dead_enemies.system().label("death"));
+
+    // Setup test entities
+    let enemy_id = world.spawn().insert(Enemy { hit_points: 5 }).id();
+
+    // Run systems
+    update_stage.run(&mut world);
+
+    // Check resulting changes
+    assert!(world.get::<Enemy>(enemy_id).is_some());
+    assert_eq!(world.get::<Enemy>(enemy_id).unwrap().hit_points, 4);
+}
+
+#[test]
+fn did_despawn_enemy() {
+    // Setup world
+    let mut world = World::default();
+
+    // Setup stage with our two systems
+    let mut update_stage = SystemStage::parallel();
+    update_stage.add_system(hurt_enemies.system().before("death"));
+    update_stage.add_system(despawn_dead_enemies.system().label("death"));
+
+    // Setup test entities
+    let enemy_id = world.spawn().insert(Enemy { hit_points: 1 }).id();
+
+    // Run systems
+    update_stage.run(&mut world);
+
+    // Check resulting changes
+    assert!(world.get::<Enemy>(enemy_id).is_none());
+}
+
+#[test]
+fn spawn_enemy_using_input_resource() {
+    // Setup world
+    let mut world = World::default();
+
+    // Setup stage with a system
+    let mut update_stage = SystemStage::parallel();
+    update_stage.add_system(spawn_enemy.system());
+
+    // Setup test resource
+    let mut input = Input::<KeyCode>::default();
+    input.press(KeyCode::Space);
+    world.insert_resource(input);
+
+    // Run systems
+    update_stage.run(&mut world);
+
+    // Check resulting changes, one entity has been spawned with `Enemy` component
+    assert_eq!(world.query::<&Enemy>().iter(&world).len(), 1);
+
+    // Clear the `just_pressed` status for all `KeyCode`s
+    world.get_resource_mut::<Input<KeyCode>>().unwrap().clear();
+
+    // Run systems
+    update_stage.run(&mut world);
+
+    // Check resulting changes, no new entity has been spawned
+    assert_eq!(world.query::<&Enemy>().iter(&world).len(), 1);
+}