Implement `Canvas` support for `iced_tiny_skia`

4 files changed, 232 insertions, 0 deletions

diff --git a/core/Cargo.toml b/core/Cargo.toml
index 43865e4d..7ccb7b7a 100644
--- a/core/Cargo.toml
+++ b/core/Cargo.toml
@@ -9,6 +9,7 @@ repository = "https://github.com/iced-rs/iced"
 
 [dependencies]
 bitflags = "1.2"
+thiserror = "1"
 
 [dependencies.palette]
 version = "0.6"
diff --git a/core/src/gradient.rs b/core/src/gradient.rs
new file mode 100644
index 00000000..61e919d6
--- /dev/null
+++ b/core/src/gradient.rs
@@ -0,0 +1,117 @@
+//! For creating a Gradient.
+pub mod linear;
+
+pub use linear::Linear;
+
+use crate::{Color, Point, Size};
+
+#[derive(Debug, Clone, PartialEq)]
+/// A fill which transitions colors progressively along a direction, either linearly, radially (TBD),
+/// or conically (TBD).
+pub enum Gradient {
+    /// A linear gradient interpolates colors along a direction from its `start` to its `end`
+    /// point.
+    Linear(Linear),
+}
+
+impl Gradient {
+    /// Creates a new linear [`linear::Builder`].
+    pub fn linear(position: impl Into<Position>) -> linear::Builder {
+        linear::Builder::new(position.into())
+    }
+}
+
+#[derive(Debug, Clone, Copy, PartialEq)]
+/// A point along the gradient vector where the specified [`color`] is unmixed.
+///
+/// [`color`]: Self::color
+pub struct ColorStop {
+    /// Offset along the gradient vector.
+    pub offset: f32,
+
+    /// The color of the gradient at the specified [`offset`].
+    ///
+    /// [`offset`]: Self::offset
+    pub color: Color,
+}
+
+#[derive(Debug)]
+/// The position of the gradient within its bounds.
+pub enum Position {
+    /// The gradient will be positioned with respect to two points.
+    Absolute {
+        /// The starting point of the gradient.
+        start: Point,
+        /// The ending point of the gradient.
+        end: Point,
+    },
+    /// The gradient will be positioned relative to the provided bounds.
+    Relative {
+        /// The top left position of the bounds.
+        top_left: Point,
+        /// The width & height of the bounds.
+        size: Size,
+        /// The start [Location] of the gradient.
+        start: Location,
+        /// The end [Location] of the gradient.
+        end: Location,
+    },
+}
+
+impl From<(Point, Point)> for Position {
+    fn from((start, end): (Point, Point)) -> Self {
+        Self::Absolute { start, end }
+    }
+}
+
+#[derive(Debug, Clone, Copy)]
+/// The location of a relatively-positioned gradient.
+pub enum Location {
+    /// Top left.
+    TopLeft,
+    /// Top.
+    Top,
+    /// Top right.
+    TopRight,
+    /// Right.
+    Right,
+    /// Bottom right.
+    BottomRight,
+    /// Bottom.
+    Bottom,
+    /// Bottom left.
+    BottomLeft,
+    /// Left.
+    Left,
+}
+
+impl Location {
+    fn to_absolute(self, top_left: Point, size: Size) -> Point {
+        match self {
+            Location::TopLeft => top_left,
+            Location::Top => {
+                Point::new(top_left.x + size.width / 2.0, top_left.y)
+            }
+            Location::TopRight => {
+                Point::new(top_left.x + size.width, top_left.y)
+            }
+            Location::Right => Point::new(
+                top_left.x + size.width,
+                top_left.y + size.height / 2.0,
+            ),
+            Location::BottomRight => {
+                Point::new(top_left.x + size.width, top_left.y + size.height)
+            }
+            Location::Bottom => Point::new(
+                top_left.x + size.width / 2.0,
+                top_left.y + size.height,
+            ),
+            Location::BottomLeft => {
+                Point::new(top_left.x, top_left.y + size.height)
+            }
+            Location::Left => {
+                Point::new(top_left.x, top_left.y + size.height / 2.0)
+            }
+        }
+    }
+}
diff --git a/core/src/gradient/linear.rs b/core/src/gradient/linear.rs
new file mode 100644
index 00000000..c886db47
--- /dev/null
+++ b/core/src/gradient/linear.rs
@@ -0,0 +1,112 @@
+//! Linear gradient builder & definition.
+use crate::gradient::{ColorStop, Gradient, Position};
+use crate::{Color, Point};
+
+/// A linear gradient that can be used in the style of [`Fill`] or [`Stroke`].
+///
+/// [`Fill`]: crate::widget::canvas::Fill
+/// [`Stroke`]: crate::widget::canvas::Stroke
+#[derive(Debug, Clone, PartialEq)]
+pub struct Linear {
+    /// The point where the linear gradient begins.
+    pub start: Point,
+    /// The point where the linear gradient ends.
+    pub end: Point,
+    /// [`ColorStop`]s along the linear gradient path.
+    pub color_stops: Vec<ColorStop>,
+}
+
+/// A [`Linear`] builder.
+#[derive(Debug)]
+pub struct Builder {
+    start: Point,
+    end: Point,
+    stops: Vec<ColorStop>,
+    error: Option<BuilderError>,
+}
+
+impl Builder {
+    /// Creates a new [`Builder`].
+    pub fn new(position: Position) -> Self {
+        let (start, end) = match position {
+            Position::Absolute { start, end } => (start, end),
+            Position::Relative {
+                top_left,
+                size,
+                start,
+                end,
+            } => (
+                start.to_absolute(top_left, size),
+                end.to_absolute(top_left, size),
+            ),
+        };
+
+        Self {
+            start,
+            end,
+            stops: vec![],
+            error: None,
+        }
+    }
+
+    /// Adds a new stop, defined by an offset and a color, to the gradient.
+    ///
+    /// `offset` must be between `0.0` and `1.0` or the gradient cannot be built.
+    ///
+    /// Note: when using the [`glow`] backend, any color stop added after the 16th
+    /// will not be displayed.
+    ///
+    /// On the [`wgpu`] backend this limitation does not exist (technical limit is 524,288 stops).
+    ///
+    /// [`glow`]: https://docs.rs/iced_glow
+    /// [`wgpu`]: https://docs.rs/iced_wgpu
+    pub fn add_stop(mut self, offset: f32, color: Color) -> Self {
+        if offset.is_finite() && (0.0..=1.0).contains(&offset) {
+            match self.stops.binary_search_by(|stop| {
+                stop.offset.partial_cmp(&offset).unwrap()
+            }) {
+                Ok(_) => {
+                    self.error = Some(BuilderError::DuplicateOffset(offset))
+                }
+                Err(index) => {
+                    self.stops.insert(index, ColorStop { offset, color });
+                }
+            }
+        } else {
+            self.error = Some(BuilderError::InvalidOffset(offset))
+        };
+
+        self
+    }
+
+    /// Builds the linear [`Gradient`] of this [`Builder`].
+    ///
+    /// Returns `BuilderError` if gradient in invalid.
+    pub fn build(self) -> Result<Gradient, BuilderError> {
+        if self.stops.is_empty() {
+            Err(BuilderError::MissingColorStop)
+        } else if let Some(error) = self.error {
+            Err(error)
+        } else {
+            Ok(Gradient::Linear(Linear {
+                start: self.start,
+                end: self.end,
+                color_stops: self.stops,
+            }))
+        }
+    }
+}
+
+/// An error that happened when building a [`Linear`] gradient.
+#[derive(Debug, thiserror::Error)]
+pub enum BuilderError {
+    #[error("Gradients must contain at least one color stop.")]
+    /// Gradients must contain at least one color stop.
+    MissingColorStop,
+    #[error("Offset {0} must be a unique, finite number.")]
+    /// Offsets in a gradient must all be unique & finite.
+    DuplicateOffset(f32),
+    #[error("Offset {0} must be between 0.0..=1.0.")]
+    /// Offsets in a gradient must be between 0.0..=1.0.
+    InvalidOffset(f32),
+}
diff --git a/core/src/lib.rs b/core/src/lib.rs
index d7314851..1e4f0411 100644
--- a/core/src/lib.rs
+++ b/core/src/lib.rs
@@ -26,6 +26,7 @@
 #![allow(clippy::inherent_to_string, clippy::type_complexity)]
 pub mod alignment;
 pub mod font;
+pub mod gradient;
 pub mod keyboard;
 pub mod mouse;
 pub mod time;
@@ -46,6 +47,7 @@ pub use background::Background;
 pub use color::Color;
 pub use content_fit::ContentFit;
 pub use font::Font;
+pub use gradient::Gradient;
 pub use length::Length;
 pub use padding::Padding;
 pub use pixels::Pixels;