emm .划个水

TabBar未提供底部线条宽度的修改，平常肯定不会用默认的啦
copy默认的UnderlineTabIndicator的所有代码,进行一点修改
如下：

import 'package:flutter/material.dart';

class WDUnderlineTabIndicator extends Decoration {
  /// Create an underline style selected tab indicator.
  ///
  /// The [borderSide] and [insets] arguments must not be null.
  WDUnderlineTabIndicator({
    this.borderSide = const BorderSide(width: 2.0, color: Colors.white),
    this.insets = EdgeInsets.zero,
    this.customWidth
  }) : assert(borderSide != null),
        assert(insets != null);

  ///自定义宽度
  double? customWidth;

  /// 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
  /// [TabBarIndicatorSize.tab].
  final EdgeInsetsGeometry insets;

  @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
  BoxPainter createBoxPainter([ VoidCallback? onChanged ]) {
    return _WDUnderlinePainter(this, onChanged);
  }

  Rect _indicatorRectFor(Rect rect, TextDirection textDirection) {
    assert(rect != null);
    assert(textDirection != null);
    final Rect indicator = insets.resolve(textDirection).deflateRect(rect);

    if (customWidth != null) {
      ///修改自定义宽度
      double w = (indicator.left + indicator.right) / 2;
      return Rect.fromLTWH(w - customWidth! / 2,
          indicator.bottom - borderSide.width, customWidth!, borderSide.width);
    }else{
      return Rect.fromLTWH(
        indicator.left,
        indicator.bottom - borderSide.width,
        indicator.width,
        borderSide.width,
      );
    }
  }

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

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

  final WDUnderlineTabIndicator 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.square;
    canvas.drawLine(indicator.bottomLeft, indicator.bottomRight, paint);
  }
}

测试一下效果：

final List tabs = ["首页", "设备", "我的"];

TabBar(
                unselectedLabelColor: Colors.grey,
                unselectedLabelStyle: TextStyle(
                    fontSize: 16, fontWeight: FontWeight.normal),
                labelColor: Colors.red,
                labelStyle: TextStyle(
                    fontSize: 24, fontWeight: FontWeight.bold),
                controller: TabController(length: tabs.length, vsync: this),
                isScrollable: true,
                indicator: WDUnderlineTabIndicator(
                  customWidth: 30,
                  borderSide:
                  BorderSide(width: 2, color: Color(0xFF544693)),
                ),
                enableFeedback: false,
                padding: EdgeInsets.only(right: 416),
                tabs: tabs.map((e) => Tab(text: e,)).toList())

image.png