Make many_cubes example more interesting (#4015)

# Objective

- Make the many_cubes example more interesting (and look more like many_sprites)

## Solution

- Actually display many cubes
- Move the camera around

examples/3d/many_cubes.rs

@@ -9,6 +9,8 @@ fn main() {
         .add_plugin(FrameTimeDiagnosticsPlugin::default())
         .add_plugin(LogDiagnosticsPlugin::default())
         .add_startup_system(setup)
+        .add_system(move_camera)
+        .add_system(print_mesh_count)
         .run();
 }
 
@@ -17,8 +19,8 @@ fn setup(
     mut meshes: ResMut<Assets<Mesh>>,
     mut materials: ResMut<Assets<StandardMaterial>>,
 ) {
-    const WIDTH: usize = 100;
-    const HEIGHT: usize = 100;
+    const WIDTH: usize = 200;
+    const HEIGHT: usize = 200;
     let mesh = meshes.add(Mesh::from(shape::Cube { size: 1.0 }));
     let material = materials.add(StandardMaterial {
         base_color: Color::PINK,
@@ -28,17 +30,88 @@ fn setup(
         for y in 0..HEIGHT {
             // cube
             commands.spawn_bundle(PbrBundle {
-                mesh: mesh.clone(),
-                material: material.clone(),
+                mesh: mesh.clone_weak(),
+                material: material.clone_weak(),
                 transform: Transform::from_xyz((x as f32) * 2.0, (y as f32) * 2.0, 0.0),
                 ..default()
             });
+            commands.spawn_bundle(PbrBundle {
+                mesh: mesh.clone_weak(),
+                material: material.clone_weak(),
+                transform: Transform::from_xyz(
+                    (x as f32) * 2.0,
+                    HEIGHT as f32 * 2.0,
+                    (y as f32) * 2.0,
+                ),
+                ..Default::default()
+            });
+            commands.spawn_bundle(PbrBundle {
+                mesh: mesh.clone_weak(),
+                material: material.clone_weak(),
+                transform: Transform::from_xyz((x as f32) * 2.0, 0.0, (y as f32) * 2.0),
+                ..Default::default()
+            });
+            commands.spawn_bundle(PbrBundle {
+                mesh: mesh.clone_weak(),
+                material: material.clone_weak(),
+                transform: Transform::from_xyz(0.0, (x as f32) * 2.0, (y as f32) * 2.0),
+                ..Default::default()
+            });
         }
     }
 
+    // add one cube, the only one with strong handles
+    // also serves as a reference point during rotation
+    commands.spawn_bundle(PbrBundle {
+        mesh,
+        material,
+        transform: Transform {
+            translation: Vec3::new(0.0, HEIGHT as f32 * 2.0, 0.0),
+            scale: Vec3::splat(5.0),
+            ..Default::default()
+        },
+        ..Default::default()
+    });
+
     // camera
     commands.spawn_bundle(PerspectiveCameraBundle {
-        transform: Transform::from_xyz(80.0, 80.0, 300.0),
+        transform: Transform::from_xyz(WIDTH as f32, HEIGHT as f32, WIDTH as f32),
         ..default()
     });
+
+    commands.spawn_bundle(DirectionalLightBundle {
+        ..Default::default()
+    });
+}
+
+// System for rotating the camera
+fn move_camera(time: Res<Time>, mut camera_query: Query<&mut Transform, With<Camera>>) {
+    let mut camera_transform = camera_query.single_mut();
+    camera_transform.rotate(Quat::from_rotation_z(time.delta_seconds() * 0.5));
+    camera_transform.rotate(Quat::from_rotation_x(time.delta_seconds() * 0.5));
+}
+
+// System for printing the number of meshes on every tick of the timer
+fn print_mesh_count(
+    time: Res<Time>,
+    mut timer: Local<PrintingTimer>,
+    sprites: Query<(&Handle<Mesh>, &ComputedVisibility)>,
+) {
+    timer.0.tick(time.delta());
+
+    if timer.0.just_finished() {
+        info!(
+            "Meshes: {} - Visible Meshes {}",
+            sprites.iter().len(),
+            sprites.iter().filter(|(_, cv)| cv.is_visible).count(),
+        );
+    }
+}
+
+struct PrintingTimer(Timer);
+
+impl Default for PrintingTimer {
+    fn default() -> Self {
+        Self(Timer::from_seconds(1.0, true))
+    }
 }