Elaborate HSVColor interface

Fixes #507

packages/flutter/lib/src/painting/colors.dart

@@ -2,45 +2,45 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-import 'dart:ui' show Color;
+import 'dart:ui' show Color, lerpDouble;
 
 class HSVColor {
-  const HSVColor.fromAHSV(this.a, this.h, this.s, this.v);
+  const HSVColor.fromAHSV(this.alpha, this.hue, this.saturation, this.value);
 
   /// Alpha, from 0.0 to 1.0.
-  final double a;
+  final double alpha;
 
   /// Hue, from 0.0 to 360.0.
-  final double h;
+  final double hue;
 
   /// Saturation, from 0.0 to 1.0.
-  final double s;
+  final double saturation;
 
   /// Value, from 0.0 to 1.0.
-  final double v;
+  final double value;
 
-  HSVColor withAlpha(double a) {
-    return new HSVColor.fromAHSV(a, h, s, v);
+  HSVColor withAlpha(double alpha) {
+    return new HSVColor.fromAHSV(alpha, hue, saturation, value);
   }
 
-  HSVColor withHue(double h) {
-    return new HSVColor.fromAHSV(a, h, s, v);
+  HSVColor withHue(double hue) {
+    return new HSVColor.fromAHSV(alpha, hue, saturation, value);
   }
 
-  HSVColor withSaturation(double s) {
-    return new HSVColor.fromAHSV(a, h, s, v);
+  HSVColor withSaturation(double saturation) {
+    return new HSVColor.fromAHSV(alpha, hue, saturation, value);
   }
 
-  HSVColor withValue(double v) {
-    return new HSVColor.fromAHSV(a, h, s, v);
+  HSVColor withValue(double value) {
+    return new HSVColor.fromAHSV(alpha, hue, saturation, value);
   }
 
   /// Returns this color in RGB.
   Color toColor() {
-    final double h = this.h % 360;
-    final double c = s * v;
+    final double h = hue % 360;
+    final double c = saturation * value;
     final double x = c * (1 - (((h / 60.0) % 2) - 1).abs());
-    final double m = v - c;
+    final double m = value - c;
 
     double r;
     double g;
@@ -71,10 +71,56 @@ class HSVColor {
       b = x;
     }
     return new Color.fromARGB(
-      ( a      * 0xFF).round(),
+      (alpha   * 0xFF).round(),
       ((r + m) * 0xFF).round(),
       ((g + m) * 0xFF).round(),
       ((b + m) * 0xFF).round()
     );
   }
+
+  HSVColor _scaleAlpha(double factor) {
+    return withAlpha(alpha * factor);
+  }
+
+  /// Linearly interpolate between two HSVColors.
+  ///
+  /// If either color is null, this function linearly interpolates from a
+  /// transparent instance of the other color.
+  static HSVColor lerp(HSVColor a, HSVColor b, double t) {
+    if (a == null && b == null)
+      return null;
+    if (a == null)
+      return b._scaleAlpha(t);
+    if (b == null)
+      return a._scaleAlpha(1.0 - t);
+    return new HSVColor.fromAHSV(
+      lerpDouble(a.alpha, b.alpha, t),
+      lerpDouble(a.hue, b.hue, t),
+      lerpDouble(a.saturation, b.saturation, t),
+      lerpDouble(a.value, b.value, t)
+    );
+  }
+
+  bool operator ==(dynamic other) {
+    if (identical(this, other))
+      return true;
+    if (other is! HSVColor)
+      return false;
+    final HSVColor typedOther = other;
+    return typedOther.alpha == alpha
+        && typedOther.hue == hue
+        && typedOther.saturation == saturation
+        && typedOther.value == value;
+  }
+
+  int get hashCode {
+    int value = 373;
+    value = 37 * value + alpha.hashCode;
+    value = 37 * value + hue.hashCode;
+    value = 37 * value + saturation.hashCode;
+    value = 37 * value + value.hashCode;
+    return value;
+  }
+
+  String toString() => "HSVColor($alpha, $hue, $saturation, $value)";
 }

packages/flutter/lib/src/rendering/view.dart

@@ -130,7 +130,7 @@ class RenderView extends RenderObject with RenderObjectWithChildMixin<RenderBox>
       scene.dispose();
       assert(() {
         if (debugEnableRepaintRainbox)
-          debugCurrentRepaintColor = debugCurrentRepaintColor.withHue(debugCurrentRepaintColor.h + debugRepaintRainboxHueIncrement);
+          debugCurrentRepaintColor = debugCurrentRepaintColor.withHue(debugCurrentRepaintColor.hue + debugRepaintRainboxHueIncrement);
         return true;
       });
     } finally {