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// Copyright 2014 The Flutter 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 Shadow, lerpDouble;
import 'package:flutter/foundation.dart';
import 'basic_types.dart';
import 'debug.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.
/// See also:
/// * [Canvas.drawShadow], which is a more efficient way to draw shadows.
/// * [PhysicalModel], a widget for showing shadows.
/// * [kElevationToShadow], for some predefined shadows used in Material
/// Design.
/// * [Shadow], which is the parent class that lacks [spreadRadius].
class BoxShadow extends ui.Shadow {
/// Creates a box shadow.
/// By default, the shadow is solid black with zero [offset], [blurRadius],
/// and [spreadRadius].
const BoxShadow({
Color color = const Color(0xFF000000),
Offset offset =,
double blurRadius = 0.0,
this.spreadRadius = 0.0,
}) : super(color: color, offset: offset, blurRadius: blurRadius);
/// The amount the box should be inflated prior to applying the blur.
final double spreadRadius;
/// Create the [Paint] object that corresponds to this shadow description.
/// The [offset] and [spreadRadius] are not represented in the [Paint] object.
/// To honor those as well, the shape should be inflated by [spreadRadius] pixels
/// in every direction and then translated by [offset] before being filled using
/// this [Paint].
Paint toPaint() {
final Paint result = Paint()
..color = color
..maskFilter = MaskFilter.blur(BlurStyle.normal, blurSigma);
assert(() {
if (debugDisableShadows)
result.maskFilter = null;
return true;
return result;
/// Returns a new box shadow with its offset, blurRadius, and spreadRadius scaled by the given factor.
BoxShadow scale(double factor) {
return 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.
/// {@macro dart.ui.shadow.lerp}
static BoxShadow lerp(BoxShadow a, BoxShadow b, double t) {
assert(t != null);
if (a == null && b == null)
return null;
if (a == null)
return b.scale(t);
if (b == null)
return a.scale(1.0 - t);
return 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.
/// {@macro dart.ui.shadow.lerp}
static List<BoxShadow> lerpList(List<BoxShadow> a, List<BoxShadow> b, double t) {
assert(t != null);
if (a == null && b == null)
return null;
a ??= <BoxShadow>[];
b ??= <BoxShadow>[];
final int commonLength = math.min(a.length, b.length);
return <BoxShadow>[
for (int i = 0; i < commonLength; i += 1) BoxShadow.lerp(a[i], b[i], t),
for (int i = commonLength; i < a.length; i += 1) a[i].scale(1.0 - t),
for (int i = commonLength; i < b.length; i += 1) b[i].scale(t),
bool operator ==(Object other) {
if (identical(this, other))
return true;
if (other.runtimeType != runtimeType)
return false;
return other is BoxShadow
&& other.color == color
&& other.offset == offset
&& other.blurRadius == blurRadius
&& other.spreadRadius == spreadRadius;
int get hashCode => hashValues(color, offset, blurRadius, spreadRadius);
String toString() => 'BoxShadow($color, $offset, ${debugFormatDouble(blurRadius)}, ${debugFormatDouble(spreadRadius)})';