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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 'package:flutter/widgets.dart';
import 'colors.dart';
/// Used with [TabBar.indicator] to draw a horizontal line below the
/// selected tab.
/// The selected tab underline is inset from the tab's boundary by [insets].
/// The [borderSide] defines the line's color and weight.
/// The [TabBar.indicatorSize] property can be used to define the indicator's
/// bounds in terms of its (centered) widget with [TabBarIndicatorSize.label],
/// or the entire tab with [].
class UnderlineTabIndicator extends Decoration {
/// Create an underline style selected tab indicator.
/// The [borderSide] and [insets] arguments must not be null.
const UnderlineTabIndicator({
this.borderSide = const BorderSide(width: 2.0, color: Colors.white),
this.insets =,
}) : assert(borderSide != null),
assert(insets != null);
/// The color and weight of the horizontal line drawn below the selected tab.
final BorderSide borderSide;
/// Locates the selected tab's underline relative to the tab's boundary.
/// The [TabBar.indicatorSize] property can be used to define the tab
/// indicator's bounds in terms of its (centered) tab widget with
/// [TabBarIndicatorSize.label], or the entire tab with
/// [].
final EdgeInsetsGeometry insets;
Decoration? lerpFrom(Decoration? a, double t) {
if (a is UnderlineTabIndicator) {
return UnderlineTabIndicator(
borderSide: BorderSide.lerp(a.borderSide, borderSide, t),
insets: EdgeInsetsGeometry.lerp(a.insets, insets, t)!,
return super.lerpFrom(a, t);
Decoration? lerpTo(Decoration? b, double t) {
if (b is UnderlineTabIndicator) {
return UnderlineTabIndicator(
borderSide: BorderSide.lerp(borderSide, b.borderSide, t),
insets: EdgeInsetsGeometry.lerp(insets, b.insets, t)!,
return super.lerpTo(b, t);
_UnderlinePainter createBoxPainter([ VoidCallback? onChanged ]) {
return _UnderlinePainter(this, onChanged);
Rect _indicatorRectFor(Rect rect, TextDirection textDirection) {
assert(rect != null);
assert(textDirection != null);
final Rect indicator = insets.resolve(textDirection).deflateRect(rect);
return Rect.fromLTWH(
indicator.bottom - borderSide.width,
Path getClipPath(Rect rect, TextDirection textDirection) {
return Path()..addRect(_indicatorRectFor(rect, textDirection));
class _UnderlinePainter extends BoxPainter {
_UnderlinePainter(this.decoration, VoidCallback? onChanged)
: assert(decoration != null),
final UnderlineTabIndicator decoration;
void paint(Canvas canvas, Offset offset, ImageConfiguration configuration) {
assert(configuration != null);
assert(configuration.size != null);
final Rect rect = offset & configuration.size!;
final TextDirection textDirection = configuration.textDirection!;
final Rect indicator = decoration._indicatorRectFor(rect, textDirection).deflate(decoration.borderSide.width / 2.0);
final Paint paint = decoration.borderSide.toPaint()..strokeCap = StrokeCap.square;
canvas.drawLine(indicator.bottomLeft, indicator.bottomRight, paint);