Flutter-如何制作具有固定高度的自定义圆形指示器？

Question

我想制作一个圆形的指示器，如下图所示。 Tabview Image

我为此做了很多尝试，但无法使其外观像设计一样。

而且还告诉我如何用颤动制作分段可扩展的listview？ Design for listview

Answer 1

您可以使用CustomPainer

  final Paint paint = Paint();
paint.color =  Color(0xff1967d2);
paint.style = PaintingStyle.fill;
canvas.drawRRect(
    RRect.fromRectAndCorners(rect,
        topRight: Radius.circular(8), topLeft: Radius.circular(8)),
    paint);

这里是完整的示例

  import 'package:flutter/material.dart';

final Color darkBlue = Color.fromARGB(255, 18, 32, 47);

void main() {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      theme: ThemeData.dark().copyWith(scaffoldBackgroundColor: darkBlue),
      debugShowCheckedModeBanner: false,
      home: DefaultTabController(
      length: 4,
      child: Scaffold(
        appBar: AppBar(
          centerTitle: true,
          title: Text("Demo page", style: TextStyle(
            color: Colors.black87
          ),),
          backgroundColor: Colors.white,
          bottom: TabBar(
            labelStyle: TextStyle(
              fontWeight: FontWeight.w700
            ),
            indicatorSize: TabBarIndicatorSize.label,
            labelColor: Color(0xff1967d2),
            unselectedLabelColor: Color(0xff5f6368),
            isScrollable: true,
            indicator: MD2Indicator(
              indicatorSize: MD2IndicatorSize.full,
              indicatorHeight: 8.0,
              indicatorColor: Colors.green,
            ),
            tabs: <Widget>[
              Tab(
                text: "Home",
              ),
              Tab(
                text: "Personal info",
              ),
              Tab(
                text: "Data & personalization",
              ),
              Tab(
                text: "Security",
              )
            ],
          ),
        ),
      )
      ,
    ),
    );
  }
}



enum MD2IndicatorSize {
  tiny,
  normal,
  full,
}

class MD2Indicator extends Decoration {
  final double indicatorHeight;
  final Color indicatorColor;
  final MD2IndicatorSize indicatorSize;

  const MD2Indicator(
      {@required this.indicatorHeight,
      @required this.indicatorColor,
      @required this.indicatorSize});

  @override
  _MD2Painter createBoxPainter([VoidCallback onChanged]) {
    return new _MD2Painter(this, onChanged);
  }
}

class _MD2Painter extends BoxPainter {
  final MD2Indicator decoration;

  _MD2Painter(this.decoration, VoidCallback onChanged)
      : assert(decoration != null),
        super(onChanged);

  @override
  void paint(Canvas canvas, Offset offset, ImageConfiguration configuration) {
    assert(configuration != null);
    assert(configuration.size != null);

    Rect rect;
    if (decoration.indicatorSize == MD2IndicatorSize.full) {
      rect = Offset(offset.dx,
              (configuration.size.height - decoration.indicatorHeight ?? 3)) &
          Size(configuration.size.width, decoration.indicatorHeight ?? 3);
    } else if (decoration.indicatorSize == MD2IndicatorSize.normal) {
      rect = Offset(offset.dx + 6,
              (configuration.size.height - decoration.indicatorHeight ?? 3)) &
          Size(configuration.size.width - 12, decoration.indicatorHeight ?? 3);
    } else if (decoration.indicatorSize == MD2IndicatorSize.tiny) {
      rect = Offset(offset.dx + configuration.size.width / 2 - 8,
              (configuration.size.height - decoration.indicatorHeight ?? 3)) &
          Size(16, decoration.indicatorHeight ?? 3);
    }

    final Paint paint = Paint();
    paint.color = decoration.indicatorColor ?? Color(0xff1967d2);
    paint.style = PaintingStyle.fill;
    canvas.drawRRect(
        RRect.fromRectAndCorners(rect,
            topRight: Radius.circular(8), topLeft: Radius.circular(8)),
        paint);
  }
}