flutter-实现一个圆角指示器(下划线)的TabBar

查看UnderlineTabIndicator发现,底部的指示器是通过_UnderlinePainter画笔划了一条线.
下面我们直接把这个画笔改成圆角的就可以咯~

///参考UnderlineTabIndicator,仅仅修改画笔StrokeCap.square 为 StrokeCap.round
class CustomUnderlineTabIndicator extends Decoration {
  /// Create an underline style selected tab indicator.
  ///
  /// The [borderSide] and [insets] arguments must not be null.
  const CustomUnderlineTabIndicator({
    this.borderSide = const BorderSide(width: 2.0, color: Colors.white),
    this.insets = EdgeInsets.zero,
  })  : assert(borderSide != null),
        assert(insets != null);

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

  Rect _indicatorRectFor(Rect rect, TextDirection textDirection) {
    assert(rect != null);
    assert(textDirection != null);
    final Rect indicator = insets.resolve(textDirection).deflateRect(rect);
    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 _UnderlinePainter extends BoxPainter {
  _UnderlinePainter(this.decoration, VoidCallback? onChanged)
      : assert(decoration != null),
        super(onChanged);

  final CustomUnderlineTabIndicator? decoration;

  BorderSide? get borderSide => decoration?.borderSide;

  EdgeInsetsGeometry? get insets => decoration?.insets;

  @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();
    paint.strokeWidth = 5;
    paint.strokeCap = StrokeCap.round; //主要是修改此处  圆角
    canvas.drawLine(indicator.bottomLeft, indicator.bottomRight, paint);
  }
}

然后再Tabbar中指定这个CustomUnderlineTabIndicator

TabBar(
      indicatorWeight: 5,
      indicatorSize: TabBarIndicatorSize.label,
      indicator: CustomUnderlineTabIndicator(
        insets: EdgeInsets.only(top: 10.w, bottom: 2.w),
        borderSide: BorderSide(width: 5.w, color: Colors.red),
      ),
      // indicator: BoxDecoration(
      //     borderRadius: BorderRadius.circular(3.5.w),
      //     gradient: LinearGradient(
      //       begin: Alignment.centerLeft,
      //       end: Alignment.centerRight,
      //       colors: [Color(0xff54C9FF), Colors.blue],
      //     )),
      controller: _tabController,
      tabs: _tabList.map((str) {
        return Container(
          height: 50,
          color: Colors.white,
          child: Center(
            child: new Text(
              str,
              style: TextStyle(color: Colors.black, fontSize: 25.sp),
            ),
          ),
        );
      }).toList(),
    )

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