app-starlock/lib/tools/CustomUnderlineTabIndicator.dart

import 'package:flutter/material.dart';

// ------------------------------------------------------
// author：AllenSu
// date  ：2021/4/18 11:23 AM
// usage ：自定义 UnderlineTabIndicator
// CSDN  ：https://blog.csdn.net/qq_42351033
// ------------------------------------------------------

class CustomUnderlineTabIndicator extends Decoration { // 控制器的宽度

  const CustomUnderlineTabIndicator({
    this.borderSide = const BorderSide(width: 2, color: Colors.white),
    this.insets = EdgeInsets.zero,
    this.strokeCap: StrokeCap.square,
    this.width: 20,
  })  : assert(borderSide != null),
        assert(insets != null);
  final BorderSide? borderSide;
  final EdgeInsetsGeometry? insets;
  final StrokeCap? strokeCap; // 控制器的边角形状
  final double? width;

  @override
  Decoration? lerpFrom(Decoration? a, double t) {
    if (a is CustomUnderlineTabIndicator) {
      return CustomUnderlineTabIndicator(
        borderSide: BorderSide.lerp(a.borderSide!, borderSide!, t),
        insets: EdgeInsetsGeometry.lerp(a.insets!, insets!, t),
      );
    }
    return lerpFrom(a, t);
  }

  @override
  Decoration? lerpTo(Decoration? b, double t) {
    if (b is CustomUnderlineTabIndicator) {
      return CustomUnderlineTabIndicator(
        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!);
  }

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

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

    // 希望的宽度
    double wantWidth = width!;
    // 取中间坐标
    double cw = (indicator.left + indicator.right) / 2;
    // 这里是核心代码
    return Rect.fromLTWH(cw - wantWidth / 2,
        indicator.bottom - borderSide!.width, wantWidth, borderSide!.width);
  }
}

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

  final CustomUnderlineTabIndicator? 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);
    final Paint paint = decoration!.borderSide!.toPaint()
      ..strokeCap = decoration!.strokeCap!; // 这里修改控制器边角的形状
    canvas.drawLine(indicator.bottomLeft, indicator.bottomRight, paint);
  }
}