Refactor cursor grabbing and more

- Rename `free_camera` to `camera`
- Move cursor grabbing logic to its own system
- (Hopefully) improve readability
- Move cursor to window center on grab
- Reorganize system scheduling
- Use `.after` and `.run_if`

src/block.rs

@@ -8,7 +8,6 @@ pub struct Block;
 pub struct Blocks<'w, 's> {
     commands: Commands<'w, 's>,
     block_resources: Res<'w, BlockResources>,
-    blocks: Query<'w, 's, &'static Transform, With<Block>>,
 }
 
 impl Blocks<'_, '_> {
@@ -20,13 +19,6 @@ impl Blocks<'_, '_> {
             Transform::from_translation(pos.as_vec3() + Vec3::ONE / 2.),
         ));
     }
-
-    /// Gets the position of a block entity, or `None`
-    /// if the given entity is not alive or not a block.
-    pub fn position(&self, entity: Entity) -> Option<IVec3> {
-        let transform = self.blocks.get(entity).ok();
-        transform.map(|t| t.translation.floor().as_ivec3())
-    }
 }
 
 #[derive(Resource)]

src/placement.rs

@@ -10,7 +10,12 @@ pub fn place_break_blocks(
     mouse_button_input: Res<ButtonInput<MouseButton>>,
     window: Single<(&Window, &CursorOptions)>,
     camera: Single<(&GlobalTransform, &Camera)>,
+    block_lookup: Query<&Transform, With<Block>>,
 ) {
+    if !mouse_button_input.any_just_pressed([MouseButton::Left, MouseButton::Right]) {
+        return; // only run this system when left or right mouse button is pressed
+    }
+
     let (window, cursor) = window.into_inner();
     let (cam_transform, camera) = camera.into_inner();
 
@@ -26,7 +31,7 @@ pub fn place_break_blocks(
     let Some((block, hit)) = ray_cast.cast_ray(ray, settings).first() else {
         return; // ray didn't hit anything
     };
-    let Some(block_pos) = blocks.position(*block) else {
+    let Ok(block_transform) = block_lookup.get(*block) else {
         return; // entity hit is not a block
     };
 
@@ -35,10 +40,9 @@ pub fn place_break_blocks(
         commands.entity(*block).despawn();
     } else if mouse_button_input.just_pressed(MouseButton::Right) {
         // Create a new block next to the one that was just clicked.
-
+        let pos = block_transform.translation.floor().as_ivec3();
         // FIXME: This only works for axis-aligned normals.
         let offset = hit.normal.normalize().round().as_ivec3();
-
-        blocks.spawn(block_pos + offset);
+        blocks.spawn(pos + offset);
     }
 }