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How I can make the cursor of tab bar with an arrow like this?

您可以使用自定义画家和选项卡指示器来实现您的愿望指示器。

import 'package:flutter/material.dart';

class Delete extends StatefulWidget {
  Delete({Key key}) : super(key: key);

  @override
  _DeleteState createState() => _DeleteState();
}

class _DeleteState extends State<Delete> with SingleTickerProviderStateMixin {
  @override
  Widget build(BuildContext context) {
    return DefaultTabController(
      length: 3,
      child: Scaffold(
        appBar: AppBar(
          bottom: TabBar(
            indicatorSize: TabBarIndicatorSize.tab,
            indicator: CircleTabIndicator(color: Colors.orange),
            tabs: <Widget>[
              Tab(
                child: Text(
                  'fruits',
                ),
              ),
              Tab(
                child: Text(
                  'vegetables',
                ),
              ),
              Tab(
                child: Text(
                  'berries',
                ),
              ),
            ],
          ),
        ),
        body: TabBarView(
          children: <Widget>[
            Center(child: Text('Tab 1')),
            Center(child: Text('Tab 2')),
            Center(child: Text('Tab 3')),
          ],
        ),
      ),
    );
  }
}

class CircleTabIndicator extends Decoration {
  final BoxPainter _painter;

  CircleTabIndicator({@required Color color})
      : _painter = _CirclePainter(color);

  @override
  BoxPainter createBoxPainter([onChanged]) => _painter;
}

class _CirclePainter extends BoxPainter {
  final Paint _paint;

  _CirclePainter(Color color)
      : _paint = Paint()
          ..color = color
          ..isAntiAlias = true;

  @override
  void paint(Canvas canvas, Offset offset, ImageConfiguration cfg) {
    Path _trianglePath = Path();

    _trianglePath.moveTo(cfg.size.width / 2 - 10, cfg.size.height);
    _trianglePath.lineTo(cfg.size.width / 2 + 10, cfg.size.height);
    _trianglePath.lineTo(cfg.size.width / 2, cfg.size.height - 10);
    _trianglePath.lineTo(cfg.size.width / 2 - 10, cfg.size.height);
    _trianglePath.close();
    canvas.drawPath(_trianglePath, _paint);
  }
}