Define `ColorSwatch.lerp()` function (#103701)

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

@@ -457,6 +457,44 @@ class ColorSwatch<T> extends Color {
 
   @override
   String toString() => '${objectRuntimeType(this, 'ColorSwatch')}(primary value: ${super.toString()})';
+
+  /// Linearly interpolate between two [ColorSwatch]es.
+  ///
+  /// It delegates to [Color.lerp] to interpolate the different colors of the
+  /// swatch.
+  ///
+  /// If either color is null, this function linearly interpolates from a
+  /// transparent instance of the other color.
+  ///
+  /// The `t` argument represents position on the timeline, with 0.0 meaning
+  /// that the interpolation has not started, returning `a` (or something
+  /// equivalent to `a`), 1.0 meaning that the interpolation has finished,
+  /// returning `b` (or something equivalent to `b`), and values in between
+  /// meaning that the interpolation is at the relevant point on the timeline
+  /// between `a` and `b`. The interpolation can be extrapolated beyond 0.0 and
+  /// 1.0, so negative values and values greater than 1.0 are valid (and can
+  /// easily be generated by curves such as [Curves.elasticInOut]). Each channel
+  /// will be clamped to the range 0 to 255.
+  ///
+  /// Values for `t` are usually obtained from an [Animation<double>], such as
+  /// an [AnimationController].
+  static ColorSwatch<T>? lerp<T>(ColorSwatch<T>? a, ColorSwatch<T>? b, double t) {
+    final Map<T, Color> swatch;
+    if (b == null) {
+      if (a == null) {
+        return null;
+      } else {
+        swatch = a._swatch.map((T key, Color color) => MapEntry<T, Color>(key, Color.lerp(color, null, t)!));
+      }
+    } else {
+      if (a == null) {
+        swatch = b._swatch.map((T key, Color color) => MapEntry<T, Color>(key, Color.lerp(null, color, t)!));
+      } else {
+        swatch = a._swatch.map((T key, Color color) => MapEntry<T, Color>(key, Color.lerp(color, b[key], t)!));
+      }
+    }
+    return ColorSwatch<T>(Color.lerp(a, b, t)!.value, swatch);
+  }
 }
 
 /// [DiagnosticsProperty] that has an [Color] as value.

packages/flutter/test/painting/colors_test.dart

@@ -426,6 +426,31 @@ void main() {
     expect(greens1.value, 0xFF027223);
   });
 
+  test('ColorSwatch.lerp', () {
+    const ColorSwatch<int> swatchA = ColorSwatch<int>(0x00000000, <int, Color>{1: Color(0x00000000)});
+    const ColorSwatch<int> swatchB = ColorSwatch<int>(0xFFFFFFFF, <int, Color>{1: Color(0xFFFFFFFF)});
+    expect(
+      ColorSwatch.lerp(swatchA, swatchB, 0.0),
+      const ColorSwatch<int>(0x00000000, <int, Color>{1: Color(0x00000000)}),
+    );
+    expect(
+      ColorSwatch.lerp(swatchA, swatchB, 0.5),
+      const ColorSwatch<int>(0x7F7F7F7F, <int, Color>{1: Color(0x7F7F7F7F)}),
+    );
+    expect(
+      ColorSwatch.lerp(swatchA, swatchB, 1.0),
+      const ColorSwatch<int>(0xFFFFFFFF, <int, Color>{1: Color(0xFFFFFFFF)}),
+    );
+    expect(
+      ColorSwatch.lerp(swatchA, swatchB, -0.1),
+      const ColorSwatch<int>(0x00000000, <int, Color>{1: Color(0x00000000)}),
+    );
+    expect(
+      ColorSwatch.lerp(swatchA, swatchB, 1.1),
+      const ColorSwatch<int>(0xFFFFFFFF, <int, Color>{1: Color(0xFFFFFFFF)}),
+    );
+  });
+
   test('ColorDiagnosticsProperty includes valueProperties in JSON', () {
     ColorProperty property = ColorProperty('foo', const Color.fromARGB(10, 20, 30, 40));
     final Map<String, Object> valueProperties = property.toJsonMap(const DiagnosticsSerializationDelegate())['valueProperties']! as Map<String, Object>;