qinglong_app/lib/base/ui/abs_underline_tabindicator.dart

import 'package:flutter/material.dart';


///tabbar样式下方横条固定宽度
class AbsUnderlineTabIndicator extends Decoration {
  const AbsUnderlineTabIndicator({
    this.wantWidth,
    this.borderSide = const BorderSide(width: 2.0, color: Colors.white),
    this.insets = EdgeInsets.zero,
  })  : assert(borderSide != null),
        assert(insets != null);

  final BorderSide? borderSide;
  final double? wantWidth;

  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
  _UnderlinePainter createBoxPainter([VoidCallback? onChanged]) {
    return _UnderlinePainter(this, onChanged);
  }
}

class _UnderlinePainter extends BoxPainter {
  _UnderlinePainter(this.decoration, VoidCallback? onChanged) : super(onChanged);

  final AbsUnderlineTabIndicator decoration;

  BorderSide? get borderSide => decoration.borderSide;

  EdgeInsetsGeometry? get insets => decoration.insets;

  Rect _indicatorRectFor(Rect rect, TextDirection textDirection) {
    final Rect indicator = insets!.resolve(textDirection).deflateRect(rect);
    //取中间坐标
    double cw = (indicator.left + indicator.right) / 2;
    return Rect.fromLTWH(cw - decoration.wantWidth! / 2, indicator.bottom - borderSide!.width, decoration.wantWidth!, borderSide!.width);
  }

  @override
  void paint(Canvas canvas, Offset offset, ImageConfiguration configuration) {
    assert(configuration.size != null);
    final Rect rect = offset & configuration.size!;
    final TextDirection textDirection = configuration.textDirection!;
    final Rect indicator = _indicatorRectFor(rect, textDirection).deflate(borderSide!.width / 2.0);
    final Paint paint = borderSide!.toPaint()..strokeCap = StrokeCap.round;
    canvas.drawLine(indicator.bottomLeft, indicator.bottomRight, paint);
  }
}