初始化

lib/tools_base/indicator/custom_tab_indicator.dart

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+import 'package:flutter/foundation.dart';
+import 'package:flutter/material.dart';
+
+//TabBar 自定义指示器：底部一小条，支持纯色/渐变、自定义圆角与离底距离
+class CustomIndicator extends Decoration {
+  final double width; //指示器宽度
+  final double height; //指示器高度
+  final Color color; //纯色时的颜色
+  final bool isGradient; //是否渐变
+  final List<Color> gradientColors; //渐变色，isGradient 为 true 才生效
+  final double offsetY; //在贴底的基础上再往上移多少
+  final BorderRadius? borderRadius; //圆角，不传默认 1
+
+  const CustomIndicator({
+    this.width = 16.0,
+    this.height = 3.0,
+    this.color = const Color(0xffF68804),
+    this.gradientColors = const [Color(0x00F68804), Color(0xffF68804)],
+    this.isGradient = false,
+    this.offsetY = 0,
+    this.borderRadius,
+  });
+
+  @override
+  BoxPainter createBoxPainter([VoidCallback? onChanged]) => _IndicatorPainter(this, onChanged);
+
+  //必须实现值相等：TabBar 用 indicator != oldWidget.indicator 决定要不要重建 painter
+  //(tabs.dart didUpdateWidget / _IndicatorPainter.shouldRepaint)。调用方都是在 build 里内联
+  //new 出来的，只按引用比较的话每次 rebuild 都判不等 → 白重建画笔、白重绘一次
+  @override
+  bool operator ==(Object other) {
+    if (identical(this, other)) return true;
+    return other is CustomIndicator &&
+        other.width == width &&
+        other.height == height &&
+        other.color == color &&
+        other.isGradient == isGradient &&
+        other.offsetY == offsetY &&
+        other.borderRadius == borderRadius &&
+        listEquals(other.gradientColors, gradientColors);
+  }
+
+  @override
+  int get hashCode => Object.hash(width, height, color, isGradient, offsetY, borderRadius, Object.hashAll(gradientColors));
+}
+
+class _IndicatorPainter extends BoxPainter {
+  final CustomIndicator deco;
+
+  _IndicatorPainter(this.deco, super.onChanged);
+
+  @override
+  void paint(Canvas canvas, Offset offset, ImageConfiguration configuration) {
+    assert(configuration.size != null);
+    final size = configuration.size!;
+    //横向居中，纵向贴底再上移 offsetY
+    final topLeft = Offset(
+      offset.dx + (size.width - deco.width) / 2,
+      size.height - deco.height - deco.offsetY,
+    );
+    final rect = topLeft & Size(deco.width, deco.height);
+
+    final paint = Paint();
+    if (deco.isGradient) {
+      paint.shader = LinearGradient(colors: deco.gradientColors).createShader(rect);
+    } else {
+      paint.color = deco.color;
+    }
+
+    final borderRadius = deco.borderRadius ?? const BorderRadius.all(Radius.circular(1));
+    canvas.drawRRect(borderRadius.toRRect(rect), paint);
+  }
+}