Flutter TabBar圆角下划线指示器

RoundUnderlineTabIndicator：可指定宽度，与label等宽与否的圆角指示器

简单粗暴，直接上代码

指定宽度的使用：

 TabBar(
    controller: _controller,
    tabs: _tabs.map((e) => Tab(text: e)).toList(),
    isScrollable: true,
    indicator: RoundUnderlineTabIndicator(
        borderSide: BorderSide(width: 3, color: xxxx),
        width: 43.0,/// 下划线宽度
        widthEqualTitle: false),///下划线与label等宽
  ),

不指定宽度与lable等宽的使用：

 TabBar(
    controller: _controller,
    tabs: _tabs.map((e) => Tab(text: e)).toList(),
    isScrollable: true,
    indicatorSize: TabBarIndicatorSize.label,
    indicator: RoundUnderlineTabIndicator(
        borderSide: BorderSide(width: 3, color: xxxx),
    ),
  ),

源码（修改自带的UnderlineTabIndicator）：

import 'package:flutter/material.dart';
import 'package:flutter/widgets.dart';

class RoundUnderlineTabIndicator extends Decoration {
  
  const RoundUnderlineTabIndicator({
    this.borderSide = const BorderSide(width: 2.0, color: Colors.white),
    this.insets = EdgeInsets.zero,
    this.width = 0.0,
    this.widthEqualTitle = true,
  })  : assert(borderSide != null),
        assert(insets != null);

  final BorderSide borderSide;
  final EdgeInsetsGeometry insets;
  final double width;
  final bool widthEqualTitle;

  @override
  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);
  }

  @override
  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);
  }

  @override
  _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);

    if (widthEqualTitle) {
      return Rect.fromLTWH(
        indicator.left,
        indicator.bottom - borderSide.width,
        indicator.width,
        borderSide.width,
      );
    } else {
      double w = (indicator.left + indicator.right) / 2;
      return Rect.fromLTWH(w - width / 2, indicator.bottom - borderSide.width, width, borderSide.width);
    }
  }

  @override
  Path getClipPath(Rect rect, TextDirection textDirection) {
    return Path()..addRect(_indicatorRectFor(rect, textDirection));
  }
}

class _UnderlinePainter extends BoxPainter {
  _UnderlinePainter(this.decoration, VoidCallback onChanged)
      : assert(decoration != null),
        super(onChanged);

  final RoundUnderlineTabIndicator decoration;

  @override
  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.round;
    canvas.drawLine(indicator.bottomLeft, indicator.bottomRight, paint);
  }
}

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