Add SweepGradient (#17368)

This PR adds a SweepGradient class, extending Gradient to expose the engine's ui.Gradient.sweep shader.

Similar to LinearGradient and RadialGradient - SweepGradients can be used in a BoxDecoration or passed to a Paint's shader.

AUTHORS

@@ -23,3 +23,4 @@ Ali Bitek <alibitek@protonmail.ch>
 Tetsuhiro Ueda <najeira@gmail.com>
 Dan Field <dfield@gmail.com>
 Noah Groß <gross@ngsger.de>
+Victor Choueiri <victor@ctrlanddev.com>

packages/flutter/lib/src/widgets/basic.dart

@@ -233,9 +233,9 @@ class ShaderMask extends SingleChildRenderObjectWidget {
   /// The shader callback is called with the current size of the child so that
   /// it can customize the shader to the size and location of the child.
   ///
-  /// Typically this will use a [LinearGradient] or [RadialGradient] to create
-  /// the [dart:ui.Shader], though the [dart:ui.ImageShader] class could also be
-  /// used.
+  /// Typically this will use a [LinearGradient], [RadialGradient], or
+  /// [SweepGradient] to create the [dart:ui.Shader], though the 
+  /// [dart:ui.ImageShader] class could also be used.
   final ShaderCallback shaderCallback;
 
   /// The [BlendMode] to use when applying the shader to the child.

packages/flutter/test/painting/gradient_test.dart

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
+import 'dart:math' as math;
 
 import 'package:flutter_test/flutter_test.dart';
 import 'package:flutter/painting.dart';
@@ -193,6 +194,45 @@ void main() {
     );
   });
 
+  test('SweepGradient with AlignmentDirectional', () {
+    expect(
+      () {
+        return const SweepGradient(
+          center: AlignmentDirectional.topStart,
+          colors: const <Color>[ const Color(0xFFFFFFFF), const Color(0xFFFFFFFF) ]
+        ).createShader(new Rect.fromLTWH(0.0, 0.0, 100.0, 100.0));
+      },
+      throwsAssertionError,
+    );
+    expect(
+      () {
+        return const SweepGradient(
+          center: AlignmentDirectional.topStart,
+          colors: const <Color>[ const Color(0xFFFFFFFF), const Color(0xFFFFFFFF) ]
+        ).createShader(new Rect.fromLTWH(0.0, 0.0, 100.0, 100.0), textDirection: TextDirection.rtl);
+      },
+      returnsNormally,
+    );
+    expect(
+      () {
+        return const SweepGradient(
+          center: AlignmentDirectional.topStart,
+          colors: const <Color>[ const Color(0xFFFFFFFF), const Color(0xFFFFFFFF) ]
+        ).createShader(new Rect.fromLTWH(0.0, 0.0, 100.0, 100.0), textDirection: TextDirection.ltr);
+      },
+      returnsNormally,
+    );
+    expect(
+      () {
+        return const SweepGradient(
+          center: Alignment.topLeft,
+          colors: const <Color>[ const Color(0xFFFFFFFF), const Color(0xFFFFFFFF) ]
+        ).createShader(new Rect.fromLTWH(0.0, 0.0, 100.0, 100.0));
+      },
+      returnsNormally,
+    );
+  });
+
   test('RadialGradient lerp test', () {
     const RadialGradient testGradient1 = const RadialGradient(
       center: Alignment.topLeft,
@@ -263,6 +303,106 @@ void main() {
     ));
   });
 
+  test('SweepGradient lerp test', () {
+    const SweepGradient testGradient1 = const SweepGradient(
+      center: Alignment.topLeft,
+      startAngle: 0.0,
+      endAngle: math.pi / 2,
+      colors: const <Color>[
+        const Color(0x33333333),
+        const Color(0x66666666),
+      ],
+    );
+    const SweepGradient testGradient2 = const SweepGradient(
+      center: Alignment.topRight,
+      startAngle: math.pi / 2,
+      endAngle: math.pi,
+      colors: const <Color>[
+        const Color(0x44444444),
+        const Color(0x88888888),
+      ],
+    );
+
+    final SweepGradient actual = SweepGradient.lerp(testGradient1, testGradient2, 0.5);
+    expect(actual, const SweepGradient(
+      center: const Alignment(0.0, -1.0),
+      startAngle: math.pi / 4,
+      endAngle: math.pi * 3/4,
+      colors: const <Color>[
+        const Color(0x3B3B3B3B),
+        const Color(0x77777777),
+      ],
+    ));
+  });
+
+  test('SweepGradient lerp test with stops', () {
+    const SweepGradient testGradient1 = const SweepGradient(
+      center: Alignment.topLeft,
+      startAngle: 0.0,
+      endAngle: math.pi / 2,
+      colors: const <Color>[
+        const Color(0x33333333),
+        const Color(0x66666666),
+      ],
+      stops: const <double>[
+        0.0,
+        0.5,
+      ],
+    );
+    const SweepGradient testGradient2 = const SweepGradient(
+      center: Alignment.topRight,
+      startAngle: math.pi / 2,
+      endAngle:  math.pi,
+      colors: const <Color>[
+        const Color(0x44444444),
+        const Color(0x88888888),
+      ],
+      stops: const <double>[
+        0.5,
+        1.0,
+      ],
+    );
+
+    final SweepGradient actual = SweepGradient.lerp(testGradient1, testGradient2, 0.5);
+    expect(actual, const SweepGradient(
+      center: const Alignment(0.0, -1.0),
+      startAngle: math.pi / 4,
+      endAngle: math.pi * 3/4,
+      colors: const <Color>[
+        const Color(0x3B3B3B3B),
+        const Color(0x77777777),
+      ],
+      stops: const <double>[
+        0.25,
+        0.75,
+      ],
+    ));
+  });
+
+  test('SweepGradient scale test)', () {
+    const SweepGradient testGradient = const SweepGradient(
+      center: Alignment.topLeft,
+      startAngle: 0.0,
+      endAngle: math.pi / 2,
+      colors: const <Color>[
+        const Color(0xff333333),
+        const Color(0xff666666),
+      ],
+    );
+    
+    final SweepGradient actual = testGradient.scale(0.5);
+    
+    expect(actual, const SweepGradient(
+      center: Alignment.topLeft,
+      startAngle: 0.0,
+      endAngle: math.pi / 2,
+      colors: const <Color>[
+        const Color(0x80333333),
+        const Color(0x80666666),
+      ],
+    ));
+  });
+
   test('Gradient lerp test (with RadialGradient)', () {
     const RadialGradient testGradient1 = const RadialGradient(
       center: Alignment.topLeft,
@@ -357,4 +497,4 @@ void main() {
     expect(() { test2a.createShader(rect); }, throwsArgumentError);
     expect(() { test2b.createShader(rect); }, throwsArgumentError);
   });
-}
\ No newline at end of file
+}