Newtype Anchor (#18439)

# Objective

The `Anchor` component doesn't need to be a enum. The variants are just
mapped to `Vec2`s so it could be changed to a newtype with associated
const values, saving the space needed for the discriminator by the enum.

Also there was no benefit I think in hiding the underlying `Vec2`
representation of `Anchor`s.

Suggested by @atlv24.

Fixes #18459
Fixes #18460

## Solution

Change `Anchor` to a struct newtyping a `Vec2`, and its variants into
associated constants.

## Migration Guide

The anchor component has been changed from an enum to a struct newtyping
a `Vec2`. The `Custom` variant has been removed, instead to construct a
custom `Anchor` use its tuple constructor:
```rust
Sprite {
     anchor: Anchor(Vec2::new(0.25, 0.4)),
     ..default()
}
```
The other enum variants have been replaced with corresponding constants:
* `Anchor::BottomLeft` to `Anchor::BOTTOM_LEFT`
* `Anchor::Center` to `Anchor::CENTER`
* `Anchor::TopRight` to `Anchor::TOP_RIGHT`
* .. and so on for the remaining variants

crates/bevy_sprite/src/lib.rs

@@ -331,7 +331,7 @@ mod test {
             .world_mut()
             .spawn(Sprite {
                 rect: Some(Rect::new(0., 0., 0.5, 1.)),
-                anchor: Anchor::TopRight,
+                anchor: Anchor::TOP_RIGHT,
                 image: image_handle,
                 ..default()
             })

crates/bevy_sprite/src/sprite.rs

@@ -1,5 +1,6 @@
 use bevy_asset::{Assets, Handle};
 use bevy_color::Color;
+use bevy_derive::{Deref, DerefMut};
 use bevy_ecs::{component::Component, reflect::ReflectComponent};
 use bevy_image::{Image, TextureAtlas, TextureAtlasLayout};
 use bevy_math::{Rect, UVec2, Vec2};
@@ -240,42 +241,38 @@ pub enum ScalingMode {
     FitEnd,
 }
 
-/// How a sprite is positioned relative to its [`Transform`].
-/// It defaults to `Anchor::Center`.
-#[derive(Component, Debug, Clone, Copy, PartialEq, Default, Reflect)]
+/// Normalized (relative to its size) offset of a 2d renderable entity from its [`Transform`].
+#[derive(Component, Debug, Clone, Copy, PartialEq, Deref, DerefMut, Reflect)]
 #[reflect(Component, Default, Debug, PartialEq, Clone)]
 #[doc(alias = "pivot")]
-pub enum Anchor {
-    #[default]
-    Center,
-    BottomLeft,
-    BottomCenter,
-    BottomRight,
-    CenterLeft,
-    CenterRight,
-    TopLeft,
-    TopCenter,
-    TopRight,
-    /// Custom anchor point. Top left is `(-0.5, 0.5)`, center is `(0.0, 0.0)`. The value will
-    /// be scaled with the sprite size.
-    Custom(Vec2),
-}
+pub struct Anchor(pub Vec2);
 
 impl Anchor {
+    pub const BOTTOM_LEFT: Self = Self(Vec2::new(-0.5, -0.5));
+    pub const BOTTOM_CENTER: Self = Self(Vec2::new(0.0, -0.5));
+    pub const BOTTOM_RIGHT: Self = Self(Vec2::new(0.5, -0.5));
+    pub const CENTER_LEFT: Self = Self(Vec2::new(-0.5, 0.0));
+    pub const CENTER: Self = Self(Vec2::ZERO);
+    pub const CENTER_RIGHT: Self = Self(Vec2::new(0.5, 0.0));
+    pub const TOP_LEFT: Self = Self(Vec2::new(-0.5, 0.5));
+    pub const TOP_CENTER: Self = Self(Vec2::new(0.0, 0.5));
+    pub const TOP_RIGHT: Self = Self(Vec2::new(0.5, 0.5));
+
     pub fn as_vec(&self) -> Vec2 {
-        match self {
-            Anchor::Center => Vec2::ZERO,
-            Anchor::BottomLeft => Vec2::new(-0.5, -0.5),
-            Anchor::BottomCenter => Vec2::new(0.0, -0.5),
-            Anchor::BottomRight => Vec2::new(0.5, -0.5),
-            Anchor::CenterLeft => Vec2::new(-0.5, 0.0),
-            Anchor::CenterRight => Vec2::new(0.5, 0.0),
-            Anchor::TopLeft => Vec2::new(-0.5, 0.5),
-            Anchor::TopCenter => Vec2::new(0.0, 0.5),
-            Anchor::TopRight => Vec2::new(0.5, 0.5),
-            Anchor::Custom(point) => *point,
+        self.0
     }
 }
+
+impl Default for Anchor {
+    fn default() -> Self {
+        Self::CENTER
+    }
+}
+
+impl From<Vec2> for Anchor {
+    fn from(value: Vec2) -> Self {
+        Self(value)
+    }
 }
 
 #[cfg(test)]
@@ -358,7 +355,7 @@ mod tests {
 
         let sprite = Sprite {
             image,
-            anchor: Anchor::BottomLeft,
+            anchor: Anchor::BOTTOM_LEFT,
             ..Default::default()
         };
 
@@ -380,7 +377,7 @@ mod tests {
 
         let sprite = Sprite {
             image,
-            anchor: Anchor::TopRight,
+            anchor: Anchor::TOP_RIGHT,
             ..Default::default()
         };
 
@@ -402,7 +399,7 @@ mod tests {
 
         let sprite = Sprite {
             image,
-            anchor: Anchor::BottomLeft,
+            anchor: Anchor::BOTTOM_LEFT,
             flip_x: true,
             ..Default::default()
         };
@@ -425,7 +422,7 @@ mod tests {
 
         let sprite = Sprite {
             image,
-            anchor: Anchor::TopRight,
+            anchor: Anchor::TOP_RIGHT,
             flip_y: true,
             ..Default::default()
         };
@@ -449,7 +446,7 @@ mod tests {
         let sprite = Sprite {
             image,
             rect: Some(Rect::new(1.5, 3.0, 3.0, 9.5)),
-            anchor: Anchor::BottomLeft,
+            anchor: Anchor::BOTTOM_LEFT,
             ..Default::default()
         };
 
@@ -473,7 +470,7 @@ mod tests {
 
         let sprite = Sprite {
             image,
-            anchor: Anchor::BottomLeft,
+            anchor: Anchor::BOTTOM_LEFT,
             texture_atlas: Some(TextureAtlas {
                 layout: texture_atlas,
                 index: 0,
@@ -501,7 +498,7 @@ mod tests {
 
         let sprite = Sprite {
             image,
-            anchor: Anchor::BottomLeft,
+            anchor: Anchor::BOTTOM_LEFT,
             texture_atlas: Some(TextureAtlas {
                 layout: texture_atlas,
                 index: 0,

crates/bevy_text/src/text2d.rs

@@ -213,7 +213,7 @@ pub fn extract_text2d_sprite(
                 image_handle_id: atlas_info.texture.id(),
                 flip_x: false,
                 flip_y: false,
-                anchor: Anchor::Center.as_vec(),
+                anchor: Anchor::CENTER.as_vec(),
                 original_entity,
                 scaling_mode: None,
             });

examples/2d/sprite_scale.rs

@@ -132,7 +132,7 @@ fn setup_sprites(mut commands: Commands, asset_server: Res<AssetServer>) {
                 TextLayout::new_with_justify(JustifyText::Center),
                 TextFont::from_font_size(15.),
                 Transform::from_xyz(0., -0.5 * rect.size.y - 10., 0.),
-                bevy::sprite::Anchor::TopCenter,
+                bevy::sprite::Anchor::TOP_CENTER,
             ));
         });
     }
@@ -278,7 +278,7 @@ fn setup_texture_atlas(
                 TextLayout::new_with_justify(JustifyText::Center),
                 TextFont::from_font_size(15.),
                 Transform::from_xyz(0., -0.5 * sprite_sheet.size.y - 10., 0.),
-                bevy::sprite::Anchor::TopCenter,
+                bevy::sprite::Anchor::TOP_CENTER,
             ));
         });
     }

examples/2d/sprite_slice.rs

@@ -96,7 +96,7 @@ fn spawn_sprites(
                 text_style,
                 TextLayout::new_with_justify(JustifyText::Center),
                 Transform::from_xyz(0., -0.5 * size.y - 10., 0.0),
-                bevy::sprite::Anchor::TopCenter,
+                bevy::sprite::Anchor::TOP_CENTER,
             )],
         ));
         position.x += 0.5 * size.x + gap;

examples/2d/text2d.rs

@@ -129,10 +129,10 @@ fn setup(mut commands: Commands, asset_server: Res<AssetServer>) {
         ))
         .with_children(|commands| {
             for (text_anchor, color) in [
-                (Anchor::TopLeft, Color::Srgba(LIGHT_SALMON)),
-                (Anchor::TopRight, Color::Srgba(LIGHT_GREEN)),
-                (Anchor::BottomRight, Color::Srgba(LIGHT_BLUE)),
-                (Anchor::BottomLeft, Color::Srgba(LIGHT_YELLOW)),
+                (Anchor::TOP_LEFT, Color::Srgba(LIGHT_SALMON)),
+                (Anchor::TOP_RIGHT, Color::Srgba(LIGHT_GREEN)),
+                (Anchor::BOTTOM_RIGHT, Color::Srgba(LIGHT_BLUE)),
+                (Anchor::BOTTOM_LEFT, Color::Srgba(LIGHT_YELLOW)),
             ] {
                 commands
                     .spawn((

examples/picking/sprite_picking.rs

@@ -38,16 +38,15 @@ fn setup(mut commands: Commands, asset_server: Res<AssetServer>) {
         .spawn((Transform::default(), Visibility::default()))
         .with_children(|commands| {
             for (anchor_index, anchor) in [
-                Anchor::TopLeft,
-                Anchor::TopCenter,
-                Anchor::TopRight,
-                Anchor::CenterLeft,
-                Anchor::Center,
-                Anchor::CenterRight,
-                Anchor::BottomLeft,
-                Anchor::BottomCenter,
-                Anchor::BottomRight,
-                Anchor::Custom(Vec2::new(0.5, 0.5)),
+                Anchor::TOP_LEFT,
+                Anchor::TOP_CENTER,
+                Anchor::TOP_RIGHT,
+                Anchor::CENTER_LEFT,
+                Anchor::CENTER,
+                Anchor::CENTER_RIGHT,
+                Anchor::BOTTOM_LEFT,
+                Anchor::BOTTOM_CENTER,
+                Anchor::BOTTOM_RIGHT,
             ]
             .iter()
             .enumerate()

examples/stress_tests/many_glyphs.rs

@@ -101,7 +101,7 @@ fn setup(mut commands: Commands, args: Res<Args>) {
             Text2d::new(text_string),
             text_font.clone(),
             TextColor(RED.into()),
-            bevy::sprite::Anchor::Center,
+            bevy::sprite::Anchor::CENTER,
             TextBounds::new_horizontal(1000.),
             text_block,
         ));

examples/testbed/2d.rs

@@ -215,10 +215,10 @@ mod text {
         ));
 
         for anchor in [
-            Anchor::TopLeft,
-            Anchor::TopRight,
-            Anchor::BottomRight,
-            Anchor::BottomLeft,
+            Anchor::TOP_LEFT,
+            Anchor::TOP_RIGHT,
+            Anchor::BOTTOM_RIGHT,
+            Anchor::BOTTOM_LEFT,
         ] {
             let mut text = commands.spawn((
                 Text2d::new("L R\n"),
@@ -229,7 +229,7 @@ mod text {
             ));
             text.with_children(|parent| {
                 parent.spawn((
-                    TextSpan::new(format!("{anchor:?}\n")),
+                    TextSpan::new(format!("{}, {}\n", anchor.x, anchor.y)),
                     TextFont::from_font_size(14.0),
                     TextColor(palettes::tailwind::BLUE_400.into()),
                 ));

examples/tools/gamepad_viewer.rs

@@ -287,7 +287,7 @@ fn setup_sticks(
                 (
                     Text2d::default(),
                     Transform::from_xyz(0., STICK_BOUNDS_SIZE + 2., 4.),
-                    Anchor::BottomCenter,
+                    Anchor::BOTTOM_CENTER,
                     TextWithAxes { x_axis, y_axis },
                     children![
                         (TextSpan(format!("{:.3}", 0.)), style.clone()),