ansu_ui/lib/bar/as_tab_indicator.dart

import 'package:flutter/material.dart';

///渐变indicator
///
///fork from TabIndicator
///
///仅仅在发现页面中使用
class ASTabIndicator extends Decoration {
  const ASTabIndicator({
    this.borderSide = const BorderSide(width: 4.0, color: Colors.white),
    this.insets = EdgeInsets.zero,
  })  : assert(borderSide != null),
        assert(insets != null);

  final BorderSide borderSide;
  final EdgeInsetsGeometry insets;

  @override
  Decoration lerpFrom(Decoration a, double t) {
    if (a is ASTabIndicator) {
      return ASTabIndicator(
        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 ASTabIndicator) {
      return ASTabIndicator(
        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)
      : assert(decoration != null),
        super(onChanged);

  final ASTabIndicator decoration;

  BorderSide get borderSide => decoration.borderSide;
  EdgeInsetsGeometry get insets => decoration.insets;

  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
  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 =
        _indicatorRectFor(rect, textDirection).deflate(borderSide.width / 2.0);
    final Paint paint = borderSide.toPaint()
      ..strokeCap = StrokeCap.square
      ..shader = LinearGradient(colors: [
        Color(0xFFE50112),
        Color(0xFFFFB1B1),
      ]).createShader(rect);
    canvas.drawLine(indicator.bottomLeft, indicator.bottomRight, paint);
  }
}