bevy/crates/bevy_ui/src/ui_node.rs

use crate::{Size, UiRect};
use bevy_asset::Handle;
use bevy_ecs::{prelude::Component, reflect::ReflectComponent};
use bevy_math::{Rect, Vec2};
use bevy_reflect::prelude::*;
use bevy_render::{
    color::Color,
    texture::{Image, DEFAULT_IMAGE_HANDLE},
};
use serde::{Deserialize, Serialize};
use std::ops::{Div, DivAssign, Mul, MulAssign};
use thiserror::Error;

/// Describes the size of a UI node
#[derive(Component, Debug, Clone, Reflect)]
#[reflect(Component, Default)]
pub struct Node {
    /// The size of the node as width and height in pixels
    /// automatically calculated by [`super::flex::flex_node_system`]
    pub(crate) calculated_size: Vec2,
}

impl Node {
    /// The calculated node size as width and height in pixels
    /// automatically calculated by [`super::flex::flex_node_system`]
    pub fn size(&self) -> Vec2 {
        self.calculated_size
    }
}

impl Node {
    pub const DEFAULT: Self = Self {
        calculated_size: Vec2::ZERO,
    };
}

impl Default for Node {
    fn default() -> Self {
        Self::DEFAULT
    }
}

/// An enum that describes possible types of value in flexbox layout options
#[derive(Copy, Clone, PartialEq, Debug, Serialize, Deserialize, Reflect)]
#[reflect(PartialEq, Serialize, Deserialize)]
pub enum Val {
    /// No value defined
    Undefined,
    /// Automatically determine this value
    Auto,
    /// Set this value in pixels
    Px(f32),
    /// Set this value in percent
    Percent(f32),
}

impl Val {
    pub const DEFAULT: Self = Self::Undefined;
}

impl Default for Val {
    fn default() -> Self {
        Self::DEFAULT
    }
}

impl Mul<f32> for Val {
    type Output = Val;

    fn mul(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 MulAssign<f32> for Val {
    fn mul_assign(&mut self, rhs: f32) {
        match self {
            Val::Undefined | Val::Auto => {}
            Val::Px(value) | Val::Percent(value) => *value *= rhs,
        }
    }
}

impl Div<f32> for Val {
    type Output = Val;

    fn div(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 DivAssign<f32> for Val {
    fn div_assign(&mut self, rhs: f32) {
        match self {
            Val::Undefined | Val::Auto => {}
            Val::Px(value) | Val::Percent(value) => *value /= rhs,
        }
    }
}

#[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 inner 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.
#[derive(Component, Clone, PartialEq, Debug, Reflect)]
#[reflect(Component, Default, PartialEq)]
pub struct Style {
    /// Whether to arrange this node and its children with flexbox layout
    ///
    /// If this is set to [`Display::None`], this node will be collapsed.
    pub display: Display,
    /// Whether to arrange this node relative to other nodes, or positioned absolutely
    pub position_type: PositionType,
    /// Which direction the content of this node should go
    pub direction: Direction,
    /// Whether to use column or row layout
    pub flex_direction: FlexDirection,
    /// How to wrap nodes
    pub flex_wrap: FlexWrap,
    /// How items are aligned according to the cross axis
    pub align_items: AlignItems,
    /// How this item is aligned according to the cross axis.
    /// Overrides [`AlignItems`].
    pub align_self: AlignSelf,
    /// How to align each line, only applies if flex_wrap is set to
    /// [`FlexWrap::Wrap`] and there are multiple lines of items
    pub align_content: AlignContent,
    /// How items align according to the main axis
    pub justify_content: JustifyContent,
    /// The position of the node as described by its Rect
    pub position: UiRect,
    /// The margin of the node
    pub margin: UiRect,
    /// The padding of the node
    pub padding: UiRect,
    /// The border of the node
    pub border: UiRect,
    /// Defines how much a flexbox item should grow if there's space available
    pub flex_grow: f32,
    /// How to shrink if there's not enough space available
    pub flex_shrink: f32,
    /// The initial length of the main axis, before other properties are applied.
    ///
    /// If both are set, `flex_basis` overrides `size` on the main axis but it obeys the bounds defined by `min_size` and `max_size`.
    pub flex_basis: Val,
    /// The ideal size of the flexbox
    ///
    /// `size.width` is used when it is within the bounds defined by `min_size.width` and `max_size.width`.
    /// `size.height` is used when it is within the bounds defined by `min_size.height` and `max_size.height`.
    pub size: Size,
    /// The minimum size of the flexbox
    ///
    /// `min_size.width` is used if it is greater than either `size.width` or `max_size.width`, or both.
    /// `min_size.height` is used if it is greater than either `size.height` or `max_size.height`, or both.
    pub min_size: Size,
    /// The maximum size of the flexbox
    ///
    /// `max_size.width` is used if it is within the bounds defined by `min_size.width` and `size.width`.
    /// `max_size.height` is used if it is within the bounds defined by `min_size.height` and `size.height.
    pub max_size: Size,
    /// The aspect ratio of the flexbox
    pub aspect_ratio: Option<f32>,
    /// How to handle overflow
    pub overflow: Overflow,
    /// The size of the gutters between the rows and columns of the flexbox layout
    ///
    /// Values of `Size::UNDEFINED` and `Size::AUTO` are treated as zero.
    pub gap: Size,
}

impl Style {
    pub const DEFAULT: Self = Self {
        display: Display::DEFAULT,
        position_type: PositionType::DEFAULT,
        direction: Direction::DEFAULT,
        flex_direction: FlexDirection::DEFAULT,
        flex_wrap: FlexWrap::DEFAULT,
        align_items: AlignItems::DEFAULT,
        align_self: AlignSelf::DEFAULT,
        align_content: AlignContent::DEFAULT,
        justify_content: JustifyContent::DEFAULT,
        position: UiRect::DEFAULT,
        margin: UiRect::DEFAULT,
        padding: UiRect::DEFAULT,
        border: UiRect::DEFAULT,
        flex_grow: 0.0,
        flex_shrink: 1.0,
        flex_basis: Val::Auto,
        size: Size::AUTO,
        min_size: Size::AUTO,
        max_size: Size::AUTO,
        aspect_ratio: None,
        overflow: Overflow::DEFAULT,
        gap: Size::UNDEFINED,
    };
}

impl Default for Style {
    fn default() -> Self {
        Self::DEFAULT
    }
}

/// How items are aligned according to the cross axis
#[derive(Copy, Clone, PartialEq, Eq, Debug, Serialize, Deserialize, Reflect)]
#[reflect(PartialEq, Serialize, Deserialize)]
pub enum AlignItems {
    /// Items are aligned at the start
    FlexStart,
    /// Items are aligned at the end
    FlexEnd,
    /// Items are aligned at the center
    Center,
    /// Items are aligned at the baseline
    Baseline,
    /// Items are stretched across the whole cross axis
    Stretch,
}

impl AlignItems {
    pub const DEFAULT: Self = Self::Stretch;
}

impl Default for AlignItems {
    fn default() -> Self {
        Self::DEFAULT
    }
}

/// How this item is aligned according to the cross axis.
/// Overrides [`AlignItems`].
#[derive(Copy, Clone, PartialEq, Eq, Debug, Serialize, Deserialize, Reflect)]
#[reflect(PartialEq, Serialize, Deserialize)]
pub enum AlignSelf {
    /// Use the parent node's [`AlignItems`] value to determine how this item should be aligned
    Auto,
    /// This item will be aligned at the start
    FlexStart,
    /// This item will be aligned at the end
    FlexEnd,
    /// This item will be aligned at the center
    Center,
    /// This item will be aligned at the baseline
    Baseline,
    /// This item will be stretched across the whole cross axis
    Stretch,
}

impl AlignSelf {
    pub const DEFAULT: Self = Self::Auto;
}

impl Default for AlignSelf {
    fn default() -> Self {
        Self::DEFAULT
    }
}

/// Defines how each line is aligned within the flexbox.
///
/// It only applies if [`FlexWrap::Wrap`] is present and if there are multiple lines of items.
#[derive(Copy, Clone, PartialEq, Eq, Debug, Serialize, Deserialize, Reflect)]
#[reflect(PartialEq, Serialize, Deserialize)]
pub enum AlignContent {
    /// Each line moves towards the start of the cross axis
    FlexStart,
    /// Each line moves towards the end of the cross axis
    FlexEnd,
    /// Each line moves towards the center of the cross axis
    Center,
    /// Each line will stretch to fill the remaining space
    Stretch,
    /// Each line fills the space it needs, putting the remaining space, if any
    /// inbetween the lines
    SpaceBetween,
    /// Each line fills the space it needs, putting the remaining space, if any
    /// around the lines
    SpaceAround,
}

impl AlignContent {
    pub const DEFAULT: Self = Self::Stretch;
}

impl Default for AlignContent {
    fn default() -> Self {
        Self::DEFAULT
    }
}

/// Defines the text direction
///
/// For example English is written LTR (left-to-right) while Arabic is written RTL (right-to-left).
#[derive(Copy, Clone, PartialEq, Eq, Debug, Serialize, Deserialize, Reflect)]
#[reflect(PartialEq, Serialize, Deserialize)]
pub enum Direction {
    /// Inherit from parent node
    Inherit,
    /// Text is written left to right
    LeftToRight,
    /// Text is written right to left
    RightToLeft,
}

impl Direction {
    pub const DEFAULT: Self = Self::Inherit;
}

impl Default for Direction {
    fn default() -> Self {
        Self::DEFAULT
    }
}

/// Whether to use a Flexbox layout model.
///
/// Part of the [`Style`] component.
#[derive(Copy, Clone, PartialEq, Eq, Debug, Serialize, Deserialize, Reflect)]
#[reflect(PartialEq, Serialize, Deserialize)]
pub enum Display {
    /// Use Flexbox layout model to determine the position of this [`Node`].
    Flex,
    /// Use no layout, don't render this node and its children.
    ///
    /// If you want to hide a node and its children,
    /// but keep its layout in place, set its [`Visibility`](bevy_render::view::Visibility) component instead.
    None,
}

impl Display {
    pub const DEFAULT: Self = Self::Flex;
}

impl Default for Display {
    fn default() -> Self {
        Self::DEFAULT
    }
}

/// Defines how flexbox items are ordered within a flexbox
#[derive(Copy, Clone, PartialEq, Eq, Debug, Serialize, Deserialize, Reflect)]
#[reflect(PartialEq, Serialize, Deserialize)]
pub enum FlexDirection {
    /// Same way as text direction along the main axis
    Row,
    /// Flex from top to bottom
    Column,
    /// Opposite way as text direction along the main axis
    RowReverse,
    /// Flex from bottom to top
    ColumnReverse,
}

impl FlexDirection {
    pub const DEFAULT: Self = Self::Row;
}

impl Default for FlexDirection {
    fn default() -> Self {
        Self::DEFAULT
    }
}

/// Defines how items are aligned according to the main axis
#[derive(Copy, Clone, PartialEq, Eq, Debug, Serialize, Deserialize, Reflect)]
#[reflect(PartialEq, Serialize, Deserialize)]
pub enum JustifyContent {
    /// Pushed towards the start
    FlexStart,
    /// Pushed towards the end
    FlexEnd,
    /// Centered along the main axis
    Center,
    /// Remaining space is distributed between the items
    SpaceBetween,
    /// Remaining space is distributed around the items
    SpaceAround,
    /// Like [`JustifyContent::SpaceAround`] but with even spacing between items
    SpaceEvenly,
}

impl JustifyContent {
    pub const DEFAULT: Self = Self::FlexStart;
}

impl Default for JustifyContent {
    fn default() -> Self {
        Self::DEFAULT
    }
}

/// Whether to show or hide overflowing items
#[derive(Copy, Clone, PartialEq, Eq, Debug, Reflect, Serialize, Deserialize)]
#[reflect(PartialEq, Serialize, Deserialize)]
pub enum Overflow {
    /// Show overflowing items
    Visible,
    /// Hide overflowing items
    Hidden,
}

impl Overflow {
    pub const DEFAULT: Self = Self::Visible;
}

impl Default for Overflow {
    fn default() -> Self {
        Self::DEFAULT
    }
}

/// The strategy used to position this node
#[derive(Copy, Clone, PartialEq, Eq, Debug, Serialize, Deserialize, Reflect)]
#[reflect(PartialEq, Serialize, Deserialize)]
pub enum PositionType {
    /// Relative to all other nodes with the [`PositionType::Relative`] value
    Relative,
    /// Independent of all other nodes
    ///
    /// As usual, the `Style.position` field of this node is specified relative to its parent node
    Absolute,
}

impl PositionType {
    const DEFAULT: Self = Self::Relative;
}

impl Default for PositionType {
    fn default() -> Self {
        Self::DEFAULT
    }
}

/// Defines if flexbox items appear on a single line or on multiple lines
#[derive(Copy, Clone, PartialEq, Eq, Debug, Serialize, Deserialize, Reflect)]
#[reflect(PartialEq, Serialize, Deserialize)]
pub enum FlexWrap {
    /// Single line, will overflow if needed
    NoWrap,
    /// Multiple lines, if needed
    Wrap,
    /// Same as [`FlexWrap::Wrap`] but new lines will appear before the previous one
    WrapReverse,
}

impl FlexWrap {
    const DEFAULT: Self = Self::NoWrap;
}

impl Default for FlexWrap {
    fn default() -> Self {
        Self::DEFAULT
    }
}

/// The calculated size of the node
#[derive(Component, Copy, Clone, Debug, Reflect)]
#[reflect(Component)]
pub struct CalculatedSize {
    /// The size of the node in logical pixels
    pub size: Vec2,
    /// Whether to attempt to preserve the aspect ratio when determining the layout for this item
    pub preserve_aspect_ratio: bool,
}

impl CalculatedSize {
    const DEFAULT: Self = Self {
        size: Vec2::ZERO,
        preserve_aspect_ratio: false,
    };
}

impl Default for CalculatedSize {
    fn default() -> Self {
        Self::DEFAULT
    }
}

/// The background color of the node
///
/// This serves as the "fill" color.
/// When combined with [`UiImage`], tints the provided texture.
#[derive(Component, Copy, Clone, Debug, Reflect)]
#[reflect(Component, Default)]
pub struct BackgroundColor(pub Color);

impl BackgroundColor {
    pub const DEFAULT: Self = Self(Color::WHITE);
}

impl Default for BackgroundColor {
    fn default() -> Self {
        Self::DEFAULT
    }
}

impl From<Color> for BackgroundColor {
    fn from(color: Color) -> Self {
        Self(color)
    }
}

/// The 2D texture displayed for this UI node
#[derive(Component, Clone, Debug, Reflect)]
#[reflect(Component, Default)]
pub struct UiImage {
    /// Handle to the texture
    pub texture: Handle<Image>,
    /// Whether the image should be flipped along its x-axis
    pub flip_x: bool,
    /// Whether the image should be flipped along its y-axis
    pub flip_y: bool,
}

impl Default for UiImage {
    fn default() -> UiImage {
        UiImage {
            texture: DEFAULT_IMAGE_HANDLE.typed(),
            flip_x: false,
            flip_y: false,
        }
    }
}

impl UiImage {
    pub fn new(texture: Handle<Image>) -> Self {
        Self {
            texture,
            ..Default::default()
        }
    }
}

impl From<Handle<Image>> for UiImage {
    fn from(texture: Handle<Image>) -> Self {
        Self::new(texture)
    }
}

/// The calculated clip of the node
#[derive(Component, Default, Copy, Clone, Debug, Reflect)]
#[reflect(Component)]
pub struct CalculatedClip {
    /// The rect of the clip
    pub clip: Rect,
}

/// Indicates that this [`Node`] entity's front-to-back ordering is not controlled solely
/// by its location in the UI hierarchy. A node with a higher z-index will appear on top
/// of other nodes with a lower z-index.
///
/// UI nodes that have the same z-index will appear according to the order in which they
/// appear in the UI hierarchy. In such a case, the last node to be added to its parent
/// will appear in front of this parent's other children.
///
/// Internally, nodes with a global z-index share the stacking context of root UI nodes
/// (nodes that have no parent). Because of this, there is no difference between using
/// [`ZIndex::Local(n)`] and [`ZIndex::Global(n)`] for root nodes.
///
/// Nodes without this component will be treated as if they had a value of [`ZIndex::Local(0)`].
#[derive(Component, Copy, Clone, Debug, Reflect)]
pub enum ZIndex {
    /// Indicates the order in which this node should be rendered relative to its siblings.
    Local(i32),
    /// Indicates the order in which this node should be rendered relative to root nodes and
    /// all other nodes that have a global z-index.
    Global(i32),
}

impl Default for ZIndex {
    fn default() -> Self {
        Self::Local(0)
    }
}

#[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)"
        );
    }
}