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/camera.rs

@@ -4,6 +4,53 @@ use bevy::input::mouse::AccumulatedMouseMotion;
 use bevy::prelude::*;
 use bevy::window::{CursorGrabMode, CursorOptions};
 
+pub fn cursor_grab(
+    mut mouse_button_input: ResMut<ButtonInput<MouseButton>>,
+    key_input: Res<ButtonInput<KeyCode>>,
+    window: Single<(&mut Window, &mut CursorOptions)>,
+    crosshair: Single<&mut Visibility, With<Crosshair>>,
+    mut request_center_cursor: Local<bool>,
+) {
+    let (mut window, mut cursor) = window.into_inner();
+    let mut crosshair_visibility = crosshair.into_inner();
+
+    let is_grabbed = cursor.grab_mode == CursorGrabMode::Locked;
+    let request_grab = mouse_button_input.any_just_pressed([MouseButton::Left, MouseButton::Right]);
+    let request_ungrab = !window.focused || key_input.just_pressed(KeyCode::Escape);
+
+    if !is_grabbed && request_grab && !request_ungrab {
+        *crosshair_visibility = Visibility::Inherited;
+        cursor.grab_mode = CursorGrabMode::Locked;
+        cursor.visible = false;
+
+        // HACK: It appears setting the cursor position in the same frame we're
+        //       locking it is not possible, so we're delaying it by a frame.
+        *request_center_cursor = true;
+
+        // To prevent other systems (such as `place_break_blocks`)
+        // from seeing the mouse button inputs, clear the state here.
+        mouse_button_input.clear();
+    }
+
+    if is_grabbed && *request_center_cursor {
+        // Set the cursor position to the middle of the window,
+        // so when it is ungrabbed again it'll reappear there.
+        let center = window.resolution.size() / 2.;
+        window.set_cursor_position(Some(center));
+        *request_center_cursor = false; // Only do this once.
+    }
+
+    if is_grabbed && request_ungrab && !request_grab {
+        *crosshair_visibility = Visibility::Hidden;
+        cursor.grab_mode = CursorGrabMode::None;
+        cursor.visible = true;
+    }
+}
+
+pub fn is_cursor_grabbed(cursor: Single<&CursorOptions>) -> bool {
+    cursor.grab_mode == CursorGrabMode::Locked
+}
+
 #[derive(Component, Debug)]
 pub struct CameraFreeLook {
     /// The mouse sensitivity, in radians per pixel.
@@ -32,49 +79,27 @@ impl Default for CameraFreeLook {
     }
 }
 
-pub fn camera_free_look(
+pub fn camera_look(
     accumulated_mouse_motion: Res<AccumulatedMouseMotion>,
-    mut mouse_button_input: ResMut<ButtonInput<MouseButton>>,
-    key_input: Res<ButtonInput<KeyCode>>,
-    window: Single<(&Window, &mut CursorOptions)>,
     camera: Single<(&mut Transform, &mut CameraFreeLook)>,
-    crosshair: Single<&mut Visibility, With<Crosshair>>,
 ) {
-    let (window, mut cursor) = window.into_inner();
     let (mut transform, mut look) = camera.into_inner();
-    let mut crosshair_visibility = crosshair.into_inner();
 
-    if !window.focused || key_input.just_pressed(KeyCode::Escape) {
-        *crosshair_visibility = Visibility::Hidden;
-        cursor.grab_mode = CursorGrabMode::None;
-        cursor.visible = true;
-    }
-    if mouse_button_input.any_just_pressed([MouseButton::Left, MouseButton::Right]) {
-        *crosshair_visibility = Visibility::Inherited;
-        cursor.grab_mode = CursorGrabMode::Locked;
-        cursor.visible = false;
-        // To prevent other systems (such as placement) from seeing
-        // the mouse buttons being pressed, clear the state here.
-        mouse_button_input.clear();
+    // Ensure the yaw and pitch are initialized once
+    // from the camera transform's current rotation.
+    if !look.initialized {
+        (look.yaw, look.pitch, _) = transform.rotation.to_euler(EulerRot::YXZ);
+        look.initialized = true;
     }
 
-    if cursor.grab_mode == CursorGrabMode::Locked {
-        // Ensure the yaw and pitch are initialized once
-        // from the camera transform's current rotation.
-        if !look.initialized {
-            (look.yaw, look.pitch, _) = transform.rotation.to_euler(EulerRot::YXZ);
-            look.initialized = true;
-        }
-
-        // Update the current camera state's internal yaw and pitch.
-        let motion = accumulated_mouse_motion.delta * look.sensitivity;
-        let (min, max) = look.pitch_limit.clone().into_inner();
-        look.yaw = (look.yaw - motion.x).rem_euclid(TAU); // keep within 0°..360°
-        look.pitch = (look.pitch - motion.y).clamp(min, max);
+    // Update the current camera state's internal yaw and pitch.
+    let motion = accumulated_mouse_motion.delta * look.sensitivity;
+    let (min, max) = look.pitch_limit.clone().into_inner();
+    look.yaw = (look.yaw - motion.x).rem_euclid(TAU); // keep within 0°..360°
+    look.pitch = (look.pitch - motion.y).clamp(min, max);
 
-        // Override the camera transform's rotation.
-        transform.rotation = Quat::from_euler(EulerRot::ZYX, 0.0, look.yaw, look.pitch);
-    }
+    // Override the camera transform's rotation.
+    transform.rotation = Quat::from_euler(EulerRot::ZYX, 0.0, look.yaw, look.pitch);
 }
 
 // TODO: Make it possible to attach this to any entity, such as the player,
@@ -116,14 +141,10 @@ impl Default for CameraNoClip {
 
 pub fn noclip_controller(
     time: Res<Time<Real>>,
-    cursor: Single<&mut CursorOptions>,
     key_input: Res<ButtonInput<KeyCode>>,
-    camera: Single<(&mut Transform, &mut CameraNoClip)>,
+    camera: Single<(&mut Transform, &CameraNoClip)>,
 ) {
     let (mut transform, noclip) = camera.into_inner();
-    if cursor.grab_mode != CursorGrabMode::Locked {
-        return;
-    }
 
     #[rustfmt::skip]
     let movement = {

src/main.rs

@@ -1,27 +1,30 @@
 use bevy::prelude::*;
 
 mod block;
-mod free_camera;
+mod camera;
 mod placement;
 
 use block::*;
-use free_camera::*;
+use camera::*;
 use placement::*;
 
 fn main() {
+    #[rustfmt::skip]
     App::new()
         .add_plugins(DefaultPlugins)
-        .add_systems(
-            Startup,
-            (setup_blocks, setup_crosshair, setup_scene).chain(),
-        )
-        .add_systems(Update, (camera_free_look, noclip_controller).chain())
-        .add_systems(
-            PostUpdate,
-            // This system requires the camera's `GlobalTransform`, so for
-            // a most up-to-date value, run it right after it's been updated.
-            place_break_blocks.after(TransformSystems::Propagate),
-        )
+
+        .add_systems(Startup, setup_crosshair)
+        .add_systems(Startup, setup_blocks)
+        .add_systems(Startup, setup_scene.after(setup_blocks))
+
+        .add_systems(Update, cursor_grab)
+        .add_systems(Update, camera_look.after(cursor_grab).run_if(is_cursor_grabbed))
+        .add_systems(Update, noclip_controller.after(camera_look).run_if(is_cursor_grabbed))
+
+        // This system requires the camera's `GlobalTransform`, so for
+        // a most up-to-date value, run it right after it's been updated.
+        .add_systems(PostUpdate, place_break_blocks.after(TransformSystems::Propagate))
+
         .run();
 }
 

src/placement.rs

@@ -12,10 +12,6 @@ pub fn place_break_blocks(
     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();