Utility methods for Val (#6134)

# Objective

Adds a better interface for performing mathematical operations with UI unit `Val`. Fixes #6080.

## Solution

- Added `try_add` and `try_sub` methods to Val.
- Removed the `Add` and `AddAssign` impls for `Val` that introduced unintuitive and bug-prone behaviour.
- As a consequence of the prior,  ~~changed the `Add` and `Sub` impls for the `Size` struct to take a `(Val, Val)` instead of `Vec2`~~ deleted the `Add` and `Sub` impls for the `Size` struct
- Added a `From<(Val, Val)>` impl for the `Size` struct
- Added `evaluate(size: f32)` method that converts from `Val::Percent` to `Val::Px`.
- Added `try_add_with_size` and `try_sub_with_size` methods to `Val`, which evaluate `Val::Percent` values into `Val::Px` values before adding.

---

## Migration Guide

Instead of using the + and - operators, perform calculations on `Val`s using the new `try_add` and `try_sub` methods. Multiplication and division remained unchanged. Also, when adding or subtracting from `Size`, ~~use a `Val` tuple instead of `Vec2`~~ perform the addition on `width` and `height` separately.


Co-authored-by: Dawid Piotrowski <41804418+Pietrek14@users.noreply.github.com>

crates/bevy_ui/Cargo.toml

@@ -34,3 +34,4 @@ taffy = "0.1.0"
 serde = { version = "1", features = ["derive"] }
 smallvec = { version = "1.6", features = ["union", "const_generics"] }
 bytemuck = { version = "1.5", features = ["derive"] }
+thiserror = "1.0.37"

crates/bevy_ui/src/geometry.rs

@@ -1,7 +1,6 @@
 use crate::Val;
-use bevy_math::Vec2;
 use bevy_reflect::Reflect;
-use std::ops::{Add, AddAssign, Div, DivAssign, Mul, MulAssign, Sub, SubAssign};
+use std::ops::{Div, DivAssign, Mul, MulAssign};
 
 /// A type which is commonly used to define positions, margins, paddings and borders.
 ///
@@ -356,42 +355,15 @@ impl Size {
     };
 }
 
-impl Add<Vec2> for Size {
+impl From<(Val, Val)> for Size {
-    type Output = Size;
+    fn from(vals: (Val, Val)) -> Self {
-
-    fn add(self, rhs: Vec2) -> Self::Output {
         Self {
-            width: self.width + rhs.x,
+            width: vals.0,
-            height: self.height + rhs.y,
+            height: vals.1,
         }
     }
 }
 
-impl AddAssign<Vec2> for Size {
-    fn add_assign(&mut self, rhs: Vec2) {
-        self.width += rhs.x;
-        self.height += rhs.y;
-    }
-}
-
-impl Sub<Vec2> for Size {
-    type Output = Size;
-
-    fn sub(self, rhs: Vec2) -> Self::Output {
-        Self {
-            width: self.width - rhs.x,
-            height: self.height - rhs.y,
-        }
-    }
-}
-
-impl SubAssign<Vec2> for Size {
-    fn sub_assign(&mut self, rhs: Vec2) {
-        self.width -= rhs.x;
-        self.height -= rhs.y;
-    }
-}
-
 impl Mul<f32> for Size {
     type Output = Size;
 
@@ -433,27 +405,16 @@ mod tests {
     use super::*;
 
     #[test]
-    fn test_size_add() {
+    fn test_size_from() {
-        assert_eq!(
+        let size: Size = (Val::Px(20.), Val::Px(30.)).into();
-            Size::new(Val::Px(10.), Val::Px(10.)) + Vec2::new(10., 10.),
-            Size::new(Val::Px(20.), Val::Px(20.))
-        );
 
-        let mut size = Size::new(Val::Px(10.), Val::Px(10.));
+        assert_eq!(
-        size += Vec2::new(10., 10.);
+            size,
-        assert_eq!(size, Size::new(Val::Px(20.), Val::Px(20.)));
+            Size {
+                width: Val::Px(20.),
+                height: Val::Px(30.),
             }
-
-    #[test]
-    fn test_size_sub() {
-        assert_eq!(
-            Size::new(Val::Px(20.), Val::Px(20.)) - Vec2::new(10., 10.),
-            Size::new(Val::Px(10.), Val::Px(10.))
         );
-
-        let mut size = Size::new(Val::Px(20.), Val::Px(20.));
-        size -= Vec2::new(10., 10.);
-        assert_eq!(size, Size::new(Val::Px(10.), Val::Px(10.)));
     }
 
     #[test]

crates/bevy_ui/src/ui_node.rs

@@ -9,7 +9,8 @@ use bevy_render::{
     texture::{Image, DEFAULT_IMAGE_HANDLE},
 };
 use serde::{Deserialize, Serialize};
-use std::ops::{Add, AddAssign, Div, DivAssign, Mul, MulAssign, Sub, SubAssign};
+use std::ops::{Div, DivAssign, Mul, MulAssign};
+use thiserror::Error;
 
 /// Describes the size of a UI node
 #[derive(Component, Debug, Clone, Default, Reflect)]
@@ -43,50 +44,6 @@ pub enum Val {
     Percent(f32),
 }
 
-impl Add<f32> for Val {
-    type Output = Val;
-
-    fn add(self, rhs: f32) -> Self::Output {
-        match self {
-            Val::Undefined => Val::Undefined,
-            Val::Auto => Val::Auto,
-            Val::Px(value) => Val::Px(value + rhs),
-            Val::Percent(value) => Val::Percent(value + rhs),
-        }
-    }
-}
-
-impl AddAssign<f32> for Val {
-    fn add_assign(&mut self, rhs: f32) {
-        match self {
-            Val::Undefined | Val::Auto => {}
-            Val::Px(value) | Val::Percent(value) => *value += rhs,
-        }
-    }
-}
-
-impl Sub<f32> for Val {
-    type Output = Val;
-
-    fn sub(self, rhs: f32) -> Self::Output {
-        match self {
-            Val::Undefined => Val::Undefined,
-            Val::Auto => Val::Auto,
-            Val::Px(value) => Val::Px(value - rhs),
-            Val::Percent(value) => Val::Percent(value - rhs),
-        }
-    }
-}
-
-impl SubAssign<f32> for Val {
-    fn sub_assign(&mut self, rhs: f32) {
-        match self {
-            Val::Undefined | Val::Auto => {}
-            Val::Px(value) | Val::Percent(value) => *value -= rhs,
-        }
-    }
-}
-
 impl Mul<f32> for Val {
     type Output = Val;
 
@@ -131,6 +88,105 @@ impl DivAssign<f32> for Val {
     }
 }
 
+#[derive(Debug, Eq, PartialEq, Clone, Copy, Error)]
+pub enum ValArithmeticError {
+    #[error("the variants of the Vals don't match")]
+    NonIdenticalVariants,
+    #[error("the given variant of Val is not evaluateable (non-numeric)")]
+    NonEvaluateable,
+}
+
+impl Val {
+    /// Tries to add the values of two [`Val`]s.
+    /// Returns [`ValArithmeticError::NonIdenticalVariants`] if two [`Val`]s are of different variants.
+    /// When adding non-numeric [`Val`]s, it returns the value unchanged.
+    pub fn try_add(&self, rhs: Val) -> Result<Val, ValArithmeticError> {
+        match (self, rhs) {
+            (Val::Undefined, Val::Undefined) | (Val::Auto, Val::Auto) => Ok(*self),
+            (Val::Px(value), Val::Px(rhs_value)) => Ok(Val::Px(value + rhs_value)),
+            (Val::Percent(value), Val::Percent(rhs_value)) => Ok(Val::Percent(value + rhs_value)),
+            _ => Err(ValArithmeticError::NonIdenticalVariants),
+        }
+    }
+
+    /// Adds `rhs` to `self` and assigns the result to `self` (see [`Val::try_add`])
+    pub fn try_add_assign(&mut self, rhs: Val) -> Result<(), ValArithmeticError> {
+        *self = self.try_add(rhs)?;
+        Ok(())
+    }
+
+    /// Tries to subtract the values of two [`Val`]s.
+    /// Returns [`ValArithmeticError::NonIdenticalVariants`] if two [`Val`]s are of different variants.
+    /// When adding non-numeric [`Val`]s, it returns the value unchanged.
+    pub fn try_sub(&self, rhs: Val) -> Result<Val, ValArithmeticError> {
+        match (self, rhs) {
+            (Val::Undefined, Val::Undefined) | (Val::Auto, Val::Auto) => Ok(*self),
+            (Val::Px(value), Val::Px(rhs_value)) => Ok(Val::Px(value - rhs_value)),
+            (Val::Percent(value), Val::Percent(rhs_value)) => Ok(Val::Percent(value - rhs_value)),
+            _ => Err(ValArithmeticError::NonIdenticalVariants),
+        }
+    }
+
+    /// Subtracts `rhs` from `self` and assigns the result to `self` (see [`Val::try_sub`])
+    pub fn try_sub_assign(&mut self, rhs: Val) -> Result<(), ValArithmeticError> {
+        *self = self.try_sub(rhs)?;
+        Ok(())
+    }
+
+    /// A convenience function for simple evaluation of [`Val::Percent`] variant into a concrete [`Val::Px`] value.
+    /// Returns a [`ValArithmeticError::NonEvaluateable`] if the [`Val`] is impossible to evaluate into [`Val::Px`].
+    /// Otherwise it returns an [`f32`] containing the evaluated value in pixels.
+    ///
+    /// **Note:** If a [`Val::Px`] is evaluated, it's innver value returned unchanged.
+    pub fn evaluate(&self, size: f32) -> Result<f32, ValArithmeticError> {
+        match self {
+            Val::Percent(value) => Ok(size * value / 100.0),
+            Val::Px(value) => Ok(*value),
+            _ => Err(ValArithmeticError::NonEvaluateable),
+        }
+    }
+
+    /// Similar to [`Val::try_add`], but performs [`Val::evaluate`] on both values before adding.
+    /// Returns an [`f32`] value in pixels.
+    pub fn try_add_with_size(&self, rhs: Val, size: f32) -> Result<f32, ValArithmeticError> {
+        let lhs = self.evaluate(size)?;
+        let rhs = rhs.evaluate(size)?;
+
+        Ok(lhs + rhs)
+    }
+
+    /// Similar to [`Val::try_add_assign`], but performs [`Val::evaluate`] on both values before adding.
+    /// The value gets converted to [`Val::Px`].
+    pub fn try_add_assign_with_size(
+        &mut self,
+        rhs: Val,
+        size: f32,
+    ) -> Result<(), ValArithmeticError> {
+        *self = Val::Px(self.evaluate(size)? + rhs.evaluate(size)?);
+        Ok(())
+    }
+
+    /// Similar to [`Val::try_sub`], but performs [`Val::evaluate`] on both values before subtracting.
+    /// Returns an [`f32`] value in pixels.
+    pub fn try_sub_with_size(&self, rhs: Val, size: f32) -> Result<f32, ValArithmeticError> {
+        let lhs = self.evaluate(size)?;
+        let rhs = rhs.evaluate(size)?;
+
+        Ok(lhs - rhs)
+    }
+
+    /// Similar to [`Val::try_sub_assign`], but performs [`Val::evaluate`] on both values before adding.
+    /// The value gets converted to [`Val::Px`].
+    pub fn try_sub_assign_with_size(
+        &mut self,
+        rhs: Val,
+        size: f32,
+    ) -> Result<(), ValArithmeticError> {
+        *self = Val::Px(self.try_add_with_size(rhs, size)?);
+        Ok(())
+    }
+}
+
 /// Describes the style of a UI node
 ///
 /// It uses the [Flexbox](https://cssreference.io/flexbox/) system.
@@ -422,3 +478,168 @@ pub struct CalculatedClip {
     /// The rect of the clip
     pub clip: Rect,
 }
+
+#[cfg(test)]
+mod tests {
+    use crate::ValArithmeticError;
+
+    use super::Val;
+
+    #[test]
+    fn val_try_add() {
+        let undefined_sum = Val::Undefined.try_add(Val::Undefined).unwrap();
+        let auto_sum = Val::Auto.try_add(Val::Auto).unwrap();
+        let px_sum = Val::Px(20.).try_add(Val::Px(22.)).unwrap();
+        let percent_sum = Val::Percent(50.).try_add(Val::Percent(50.)).unwrap();
+
+        assert_eq!(undefined_sum, Val::Undefined);
+        assert_eq!(auto_sum, Val::Auto);
+        assert_eq!(px_sum, Val::Px(42.));
+        assert_eq!(percent_sum, Val::Percent(100.));
+    }
+
+    #[test]
+    fn val_try_add_to_self() {
+        let mut val = Val::Px(5.);
+
+        val.try_add_assign(Val::Px(3.)).unwrap();
+
+        assert_eq!(val, Val::Px(8.));
+    }
+
+    #[test]
+    fn val_try_sub() {
+        let undefined_sum = Val::Undefined.try_sub(Val::Undefined).unwrap();
+        let auto_sum = Val::Auto.try_sub(Val::Auto).unwrap();
+        let px_sum = Val::Px(72.).try_sub(Val::Px(30.)).unwrap();
+        let percent_sum = Val::Percent(100.).try_sub(Val::Percent(50.)).unwrap();
+
+        assert_eq!(undefined_sum, Val::Undefined);
+        assert_eq!(auto_sum, Val::Auto);
+        assert_eq!(px_sum, Val::Px(42.));
+        assert_eq!(percent_sum, Val::Percent(50.));
+    }
+
+    #[test]
+    fn different_variant_val_try_add() {
+        let different_variant_sum_1 = Val::Undefined.try_add(Val::Auto);
+        let different_variant_sum_2 = Val::Px(50.).try_add(Val::Percent(50.));
+        let different_variant_sum_3 = Val::Percent(50.).try_add(Val::Undefined);
+
+        assert_eq!(
+            different_variant_sum_1,
+            Err(ValArithmeticError::NonIdenticalVariants)
+        );
+        assert_eq!(
+            different_variant_sum_2,
+            Err(ValArithmeticError::NonIdenticalVariants)
+        );
+        assert_eq!(
+            different_variant_sum_3,
+            Err(ValArithmeticError::NonIdenticalVariants)
+        );
+    }
+
+    #[test]
+    fn different_variant_val_try_sub() {
+        let different_variant_diff_1 = Val::Undefined.try_sub(Val::Auto);
+        let different_variant_diff_2 = Val::Px(50.).try_sub(Val::Percent(50.));
+        let different_variant_diff_3 = Val::Percent(50.).try_sub(Val::Undefined);
+
+        assert_eq!(
+            different_variant_diff_1,
+            Err(ValArithmeticError::NonIdenticalVariants)
+        );
+        assert_eq!(
+            different_variant_diff_2,
+            Err(ValArithmeticError::NonIdenticalVariants)
+        );
+        assert_eq!(
+            different_variant_diff_3,
+            Err(ValArithmeticError::NonIdenticalVariants)
+        );
+    }
+
+    #[test]
+    fn val_evaluate() {
+        let size = 250.;
+        let result = Val::Percent(80.).evaluate(size).unwrap();
+
+        assert_eq!(result, size * 0.8);
+    }
+
+    #[test]
+    fn val_evaluate_px() {
+        let size = 250.;
+        let result = Val::Px(10.).evaluate(size).unwrap();
+
+        assert_eq!(result, 10.);
+    }
+
+    #[test]
+    fn val_invalid_evaluation() {
+        let size = 250.;
+        let evaluate_undefined = Val::Undefined.evaluate(size);
+        let evaluate_auto = Val::Auto.evaluate(size);
+
+        assert_eq!(evaluate_undefined, Err(ValArithmeticError::NonEvaluateable));
+        assert_eq!(evaluate_auto, Err(ValArithmeticError::NonEvaluateable));
+    }
+
+    #[test]
+    fn val_try_add_with_size() {
+        let size = 250.;
+
+        let px_sum = Val::Px(21.).try_add_with_size(Val::Px(21.), size).unwrap();
+        let percent_sum = Val::Percent(20.)
+            .try_add_with_size(Val::Percent(30.), size)
+            .unwrap();
+        let mixed_sum = Val::Px(20.)
+            .try_add_with_size(Val::Percent(30.), size)
+            .unwrap();
+
+        assert_eq!(px_sum, 42.);
+        assert_eq!(percent_sum, 0.5 * size);
+        assert_eq!(mixed_sum, 20. + 0.3 * size);
+    }
+
+    #[test]
+    fn val_try_sub_with_size() {
+        let size = 250.;
+
+        let px_sum = Val::Px(60.).try_sub_with_size(Val::Px(18.), size).unwrap();
+        let percent_sum = Val::Percent(80.)
+            .try_sub_with_size(Val::Percent(30.), size)
+            .unwrap();
+        let mixed_sum = Val::Percent(50.)
+            .try_sub_with_size(Val::Px(30.), size)
+            .unwrap();
+
+        assert_eq!(px_sum, 42.);
+        assert_eq!(percent_sum, 0.5 * size);
+        assert_eq!(mixed_sum, 0.5 * size - 30.);
+    }
+
+    #[test]
+    fn val_try_add_non_numeric_with_size() {
+        let size = 250.;
+
+        let undefined_sum = Val::Undefined.try_add_with_size(Val::Undefined, size);
+        let percent_sum = Val::Auto.try_add_with_size(Val::Auto, size);
+
+        assert_eq!(undefined_sum, Err(ValArithmeticError::NonEvaluateable));
+        assert_eq!(percent_sum, Err(ValArithmeticError::NonEvaluateable));
+    }
+
+    #[test]
+    fn val_arithmetic_error_messages() {
+        assert_eq!(
+            format!("{}", ValArithmeticError::NonIdenticalVariants),
+            "the variants of the Vals don't match"
+        );
+        assert_eq!(
+            format!("{}", ValArithmeticError::NonEvaluateable),
+            "the given variant of Val is not evaluateable (non-numeric)"
+        );
+    }
+}