Extract BoxShadow from box_painter.dart (#12207)

This is the last class other than BoxDecoration in this file, so by
extracting it we finally have all the parts of BoxDecoration
extracted.

packages/flutter/lib/painting.dart

@@ -22,6 +22,7 @@ export 'src/painting/border.dart';
 export 'src/painting/border_radius.dart';
 export 'src/painting/box_fit.dart';
 export 'src/painting/box_painter.dart';
+export 'src/painting/box_shadow.dart';
 export 'src/painting/colors.dart';
 export 'src/painting/decoration.dart';
 export 'src/painting/edge_insets.dart';

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

@@ -3,7 +3,7 @@
 // found in the LICENSE file.
 
 import 'dart:math' as math;
-import 'dart:ui' as ui show Image, lerpDouble;
+import 'dart:ui' as ui show Image;
 
 import 'package:flutter/foundation.dart';
 import 'package:flutter/services.dart';
@@ -11,126 +11,12 @@ import 'package:flutter/services.dart';
 import 'basic_types.dart';
 import 'border.dart';
 import 'border_radius.dart';
+import 'box_shadow.dart';
 import 'decoration.dart';
 import 'edge_insets.dart';
 import 'gradient.dart';
 import 'image.dart';
 
-/// A shadow cast by a box.
-///
-/// BoxShadow can cast non-rectangular shadows if the box is non-rectangular
-/// (e.g., has a border radius or a circular shape).
-///
-/// This class is similar to CSS box-shadow.
-@immutable
-class BoxShadow {
-  /// Creates a box shadow.
-  ///
-  /// By default, the shadow is solid black with zero [offset], [blurRadius],
-  /// and [spreadRadius].
-  const BoxShadow({
-    this.color: const Color(0xFF000000),
-    this.offset: Offset.zero,
-    this.blurRadius: 0.0,
-    this.spreadRadius: 0.0
-  });
-
-  /// The color of the shadow.
-  final Color color;
-
-  /// The displacement of the shadow from the box.
-  final Offset offset;
-
-  /// The standard deviation of the Gaussian to convolve with the box's shape.
-  final double blurRadius;
-
-  /// The amount the box should be inflated prior to applying the blur.
-  final double spreadRadius;
-
-  /// Converts a blur radius in pixels to sigmas.
-  ///
-  /// See the sigma argument to [MaskFilter.blur].
-  //
-  // See SkBlurMask::ConvertRadiusToSigma().
-  // <https://github.com/google/skia/blob/bb5b77db51d2e149ee66db284903572a5aac09be/src/effects/SkBlurMask.cpp#L23>
-  static double convertRadiusToSigma(double radius) {
-    return radius * 0.57735 + 0.5;
-  }
-
-  /// The [blurRadius] in sigmas instead of logical pixels.
-  ///
-  /// See the sigma argument to [MaskFilter.blur].
-  double get blurSigma => convertRadiusToSigma(blurRadius);
-
-  /// Returns a new box shadow with its offset, blurRadius, and spreadRadius scaled by the given factor.
-  BoxShadow scale(double factor) {
-    return new BoxShadow(
-      color: color,
-      offset: offset * factor,
-      blurRadius: blurRadius * factor,
-      spreadRadius: spreadRadius * factor
-    );
-  }
-
-  /// Linearly interpolate between two box shadows.
-  ///
-  /// If either box shadow is null, this function linearly interpolates from a
-  /// a box shadow that matches the other box shadow in color but has a zero
-  /// offset and a zero blurRadius.
-  static BoxShadow lerp(BoxShadow a, BoxShadow b, double t) {
-    if (a == null && b == null)
-      return null;
-    if (a == null)
-      return b.scale(t);
-    if (b == null)
-      return a.scale(1.0 - t);
-    return new BoxShadow(
-      color: Color.lerp(a.color, b.color, t),
-      offset: Offset.lerp(a.offset, b.offset, t),
-      blurRadius: ui.lerpDouble(a.blurRadius, b.blurRadius, t),
-      spreadRadius: ui.lerpDouble(a.spreadRadius, b.spreadRadius, t)
-    );
-  }
-
-  /// Linearly interpolate between two lists of box shadows.
-  ///
-  /// If the lists differ in length, excess items are lerped with null.
-  static List<BoxShadow> lerpList(List<BoxShadow> a, List<BoxShadow> b, double t) {
-    if (a == null && b == null)
-      return null;
-    a ??= <BoxShadow>[];
-    b ??= <BoxShadow>[];
-    final List<BoxShadow> result = <BoxShadow>[];
-    final int commonLength = math.min(a.length, b.length);
-    for (int i = 0; i < commonLength; ++i)
-      result.add(BoxShadow.lerp(a[i], b[i], t));
-    for (int i = commonLength; i < a.length; ++i)
-      result.add(a[i].scale(1.0 - t));
-    for (int i = commonLength; i < b.length; ++i)
-      result.add(b[i].scale(t));
-    return result;
-  }
-
-  @override
-  bool operator ==(dynamic other) {
-    if (identical(this, other))
-      return true;
-    if (runtimeType != other.runtimeType)
-      return false;
-    final BoxShadow typedOther = other;
-    return color == typedOther.color &&
-           offset == typedOther.offset &&
-           blurRadius == typedOther.blurRadius &&
-           spreadRadius == typedOther.spreadRadius;
-  }
-
-  @override
-  int get hashCode => hashValues(color, offset, blurRadius, spreadRadius);
-
-  @override
-  String toString() => 'BoxShadow($color, $offset, $blurRadius, $spreadRadius)';
-}
-
 /// An immutable description of how to paint a box.
 ///
 /// The [BoxDecoration] class provides a variety of ways to draw a box.

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

+// Copyright 2015 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 'dart:ui' as ui show lerpDouble;
+
+import 'package:flutter/foundation.dart';
+
+import 'basic_types.dart';
+
+/// A shadow cast by a box.
+///
+/// BoxShadow can cast non-rectangular shadows if the box is non-rectangular
+/// (e.g., has a border radius or a circular shape).
+///
+/// This class is similar to CSS box-shadow.
+@immutable
+class BoxShadow {
+  /// Creates a box shadow.
+  ///
+  /// By default, the shadow is solid black with zero [offset], [blurRadius],
+  /// and [spreadRadius].
+  const BoxShadow({
+    this.color: const Color(0xFF000000),
+    this.offset: Offset.zero,
+    this.blurRadius: 0.0,
+    this.spreadRadius: 0.0
+  });
+
+  /// The color of the shadow.
+  final Color color;
+
+  /// The displacement of the shadow from the box.
+  final Offset offset;
+
+  /// The standard deviation of the Gaussian to convolve with the box's shape.
+  final double blurRadius;
+
+  /// The amount the box should be inflated prior to applying the blur.
+  final double spreadRadius;
+
+  /// Converts a blur radius in pixels to sigmas.
+  ///
+  /// See the sigma argument to [MaskFilter.blur].
+  //
+  // See SkBlurMask::ConvertRadiusToSigma().
+  // <https://github.com/google/skia/blob/bb5b77db51d2e149ee66db284903572a5aac09be/src/effects/SkBlurMask.cpp#L23>
+  static double convertRadiusToSigma(double radius) {
+    return radius * 0.57735 + 0.5;
+  }
+
+  /// The [blurRadius] in sigmas instead of logical pixels.
+  ///
+  /// See the sigma argument to [MaskFilter.blur].
+  double get blurSigma => convertRadiusToSigma(blurRadius);
+
+  /// Returns a new box shadow with its offset, blurRadius, and spreadRadius scaled by the given factor.
+  BoxShadow scale(double factor) {
+    return new BoxShadow(
+      color: color,
+      offset: offset * factor,
+      blurRadius: blurRadius * factor,
+      spreadRadius: spreadRadius * factor
+    );
+  }
+
+  /// Linearly interpolate between two box shadows.
+  ///
+  /// If either box shadow is null, this function linearly interpolates from a
+  /// a box shadow that matches the other box shadow in color but has a zero
+  /// offset and a zero blurRadius.
+  static BoxShadow lerp(BoxShadow a, BoxShadow b, double t) {
+    if (a == null && b == null)
+      return null;
+    if (a == null)
+      return b.scale(t);
+    if (b == null)
+      return a.scale(1.0 - t);
+    return new BoxShadow(
+      color: Color.lerp(a.color, b.color, t),
+      offset: Offset.lerp(a.offset, b.offset, t),
+      blurRadius: ui.lerpDouble(a.blurRadius, b.blurRadius, t),
+      spreadRadius: ui.lerpDouble(a.spreadRadius, b.spreadRadius, t)
+    );
+  }
+
+  /// Linearly interpolate between two lists of box shadows.
+  ///
+  /// If the lists differ in length, excess items are lerped with null.
+  static List<BoxShadow> lerpList(List<BoxShadow> a, List<BoxShadow> b, double t) {
+    if (a == null && b == null)
+      return null;
+    a ??= <BoxShadow>[];
+    b ??= <BoxShadow>[];
+    final List<BoxShadow> result = <BoxShadow>[];
+    final int commonLength = math.min(a.length, b.length);
+    for (int i = 0; i < commonLength; ++i)
+      result.add(BoxShadow.lerp(a[i], b[i], t));
+    for (int i = commonLength; i < a.length; ++i)
+      result.add(a[i].scale(1.0 - t));
+    for (int i = commonLength; i < b.length; ++i)
+      result.add(b[i].scale(t));
+    return result;
+  }
+
+  @override
+  bool operator ==(dynamic other) {
+    if (identical(this, other))
+      return true;
+    if (runtimeType != other.runtimeType)
+      return false;
+    final BoxShadow typedOther = other;
+    return color == typedOther.color &&
+           offset == typedOther.offset &&
+           blurRadius == typedOther.blurRadius &&
+           spreadRadius == typedOther.spreadRadius;
+  }
+
+  @override
+  int get hashCode => hashValues(color, offset, blurRadius, spreadRadius);
+
+  @override
+  String toString() => 'BoxShadow($color, $offset, $blurRadius, $spreadRadius)';
+}