Add inset(by:) modifier

Examples/Sources/PathsExample/PathsApp.swift

@@ -64,6 +64,8 @@ struct PathsApp: App {
             HStack {
                 ZStack {
                     RoundedRectangle(cornerRadius: 12)
+                        .inset(by: 2)
+                        .stroke(.red, style: .init(width: 4))
                         .fill(.gray)
 
                     HStack {

Sources/SwiftCrossUI/Views/Shapes/Capsule.swift

@@ -1,11 +1,15 @@
 /// A rounded rectangle whose corner radius is equal to half the length of its
 /// shortest side.
-public struct Capsule: Shape {
+public struct Capsule: InsettableShape {
     /// Creates a ``Capsule`` instance.
     public nonisolated init() {}
 
     public nonisolated func path(in bounds: Path.Rect) -> Path {
         let radius = min(bounds.width, bounds.height) / 2.0
         return RoundedRectangle(cornerRadius: radius).path(in: bounds)
     }
+
+    public nonisolated func inset(by amount: Double) -> some InsettableShape {
+        InsettableShapeImpl(inset: amount, base: self)
+    }
 }

Sources/SwiftCrossUI/Views/Shapes/Circle.swift

@@ -2,7 +2,7 @@
 ///
 /// Circles have equal widths and heights; the `Circle` shape will take on the
 /// minimum of its proposed width and height.
-public struct Circle: Shape {
+public struct Circle: InsettableShape {
     /// The ideal diameter of a ``Circle``.
     nonisolated static let idealDiameter = 10.0
 
@@ -24,4 +24,8 @@ public struct Circle: Shape {
 
         return ViewSize(diameter, diameter)
     }
+
+    public nonisolated func inset(by amount: Double) -> some InsettableShape {
+        InsettableShapeImpl(inset: amount, base: self)
+    }
 }

Sources/SwiftCrossUI/Views/Shapes/Ellipse.swift

@@ -1,5 +1,5 @@
 /// An ellipse.
-public struct Ellipse: Shape {
+public struct Ellipse: InsettableShape {
     /// Creates an ``Ellipse`` instance.
     public nonisolated init() {}
 
@@ -18,4 +18,8 @@ public struct Ellipse: Shape {
                 )
             )
     }
+
+    public nonisolated func inset(by amount: Double) -> some InsettableShape {
+        InsettableShapeImpl(inset: amount, base: self)
+    }
 }

Sources/SwiftCrossUI/Views/Shapes/InsettableShape.swift

@@ -0,0 +1,46 @@
+/// A shape type that is able to inset itself to produce another shape.
+public protocol InsettableShape: Shape {
+    /// The type of the inset shape.
+    associatedtype InsetShape: InsettableShape
+
+    /// Returns `self` inset by `amount`.
+    nonisolated func inset(by amount: Double) -> InsetShape
+}
+
+/// The `InsetShape` implementation used by ``Rectangle``, ``Ellipse``, ``Circle``, and ``Capsule``.
+///
+/// This implementation only works for convex shapes where insetting the shape is equivalent to
+/// making the shape smaller.
+struct InsettableShapeImpl<Base: Shape>: InsettableShape {
+    var inset: Double
+    var base: Base
+
+    nonisolated func path(in bounds: Path.Rect) -> Path {
+        base.path(
+            in: .init(
+                x: bounds.x + inset,
+                y: bounds.y + inset,
+                width: bounds.width - 2 * inset,
+                height: bounds.height - 2 * inset
+            )
+        )
+    }
+
+    nonisolated func size(fitting proposal: ProposedViewSize) -> ViewSize {
+        let innerProposal = ProposedViewSize(
+            proposal.width.map { max(0, $0 - 2 * inset) },
+            proposal.height.map { max(0, $0 - 2 * inset) }
+        )
+
+        let innerSize = base.size(fitting: innerProposal)
+
+        return ViewSize(
+            innerSize.width + 2 * inset,
+            innerSize.height + 2 * inset
+        )
+    }
+
+    func inset(by amount: Double) -> InsettableShapeImpl<Base> {
+        .init(inset: inset + amount, base: base)
+    }
+}

Sources/SwiftCrossUI/Views/Shapes/Rectangle.swift

@@ -1,9 +1,13 @@
 /// A rectangle.
-public struct Rectangle: Shape {
+public struct Rectangle: InsettableShape {
     /// Creates a ``Rectangle`` instance.
     public nonisolated init() {}
 
     public nonisolated func path(in bounds: Path.Rect) -> Path {
         Path().addRectangle(bounds)
     }
+
+    public nonisolated func inset(by amount: Double) -> some InsettableShape {
+        InsettableShapeImpl(inset: amount, base: self)
+    }
 }

Sources/SwiftCrossUI/Views/Shapes/RoundedRectangle.swift

@@ -453,3 +453,42 @@ extension RoundedRectangle: Shape {
             .addLine(to: SIMD2(x: bounds.center.x, y: bounds.y))
     }
 }
+
+// MARK: InsettableShape
+extension RoundedRectangle: InsettableShape {
+    public func inset(by amount: Double) -> some InsettableShape {
+        InsetShapeImpl(initialCornerRadius: cornerRadius, insetAmount: amount)
+    }
+
+    struct InsetShapeImpl {
+        var initialCornerRadius: Double
+        var insetAmount: Double
+    }
+}
+
+extension RoundedRectangle.InsetShapeImpl: InsettableShape {
+    private var actualCornerRadius: Double { max(0, initialCornerRadius - insetAmount) }
+
+    func path(in bounds: Path.Rect) -> Path {
+        RoundedRectangle(cornerRadius: actualCornerRadius)
+            .path(
+                in: .init(
+                    x: bounds.x + insetAmount,
+                    y: bounds.y + insetAmount,
+                    width: bounds.width - 2 * insetAmount,
+                    height: bounds.height - 2 * insetAmount
+                )
+            )
+    }
+
+    func size(fitting proposal: ProposedViewSize) -> ViewSize {
+        let proposedWidth = proposal.width ?? 10
+        let proposedHeight = proposal.height ?? 10
+
+        return ViewSize(max(proposedWidth, insetAmount * 2), max(proposedHeight, insetAmount * 2))
+    }
+
+    func inset(by amount: Double) -> Self {
+        Self(initialCornerRadius: initialCornerRadius, insetAmount: insetAmount + amount)
+    }
+}