Change opacity without a round trip through HLC

change(.opacity, …) converted the color to HLC and back just to set
the opacity, which shifted the RGB components by up to about 1e-6
(e.g. green 0 became 1.7e-6). Opacity is the same in every color
space, so it now goes through the RGB path. The never reached
.opacity branches of the HSB and HLC helpers are removed.

Author: Jeehut

Sources/HandySwiftUI/Extensions/ColorExt.swift

@@ -504,7 +504,7 @@ extension Color {
          return self.change(attribute, to: self.hlco.chroma + addition)
 
       case .opacity:
-         return self.change(attribute, to: self.hlco.opacity + addition)
+         return self.change(attribute, to: self.rgbo.opacity + addition)
       }
    }
 
@@ -516,13 +516,14 @@ extension Color {
    /// - Returns: The resulting new `Color` with the specified change applied.
    public func change(_ attribute: ChangeableAttribute, to newValue: Double) -> Self {
       switch attribute {
-      case .red, .green, .blue:
+      case .red, .green, .blue, .opacity:
+         // Opacity is the same in every color space, so the RGB path changes it without a lossy round trip through HLC
          return self.newRgboColor(attribute, newValue)
 
       case .hueHSB, .saturation, .brightness:
          return self.newHsboColor(attribute, newValue)
 
-      case .hueHLC, .luminance, .chroma, .opacity:
+      case .hueHLC, .luminance, .chroma:
          return self.newHlcoColor(attribute, newValue)
       }
    }
@@ -540,9 +541,6 @@ extension Color {
       case .chroma:
          newHlco.chroma = newValue
 
-      case .opacity:
-         newHlco.opacity = newValue
-
       default:
          break
       }
@@ -563,9 +561,6 @@ extension Color {
       case .brightness:
          newHsbo.brightness = newValue
 
-      case .opacity:
-         newHsbo.opacity = newValue
-
       default:
          break
       }

Tests/HandySwiftUITests/ColorExtTests.swift

@@ -0,0 +1,24 @@
+import SwiftUI
+import Testing
+
+@testable import HandySwiftUI
+
+@MainActor
+struct ColorExtTests {
+   @Test(arguments: [(1.0, 0.0, 0.5), (0.2, 0.4, 0.6), (0.0, 1.0, 0.0)])
+   func changingOpacityKeepsTheRGBComponents(red: Double, green: Double, blue: Double) {
+      let color = Color(red: red, green: green, blue: blue)
+
+      // Float precision of the platform colors allows for deviations around 1e-7, the HLC round trip caused around 1e-6
+      func expectSameRGB(_ changed: (red: Double, green: Double, blue: Double, opacity: Double), opacity: Double) {
+         let tolerance = 0.000_000_5
+         #expect(abs(changed.red - color.rgbo.red) < tolerance)
+         #expect(abs(changed.green - color.rgbo.green) < tolerance)
+         #expect(abs(changed.blue - color.rgbo.blue) < tolerance)
+         #expect(abs(changed.opacity - opacity) < tolerance)
+      }
+
+      expectSameRGB(color.change(.opacity, to: 0.5).rgbo, opacity: 0.5)
+      expectSameRGB(color.change(.opacity, by: -0.25).rgbo, opacity: 0.75)
+   }
+}