add tabbar indicator

example/lib/example_scaffold.dart

@@ -8,11 +8,30 @@ class ExampleScaffold extends StatefulWidget {
   _ExampleScaffoldState createState() => _ExampleScaffoldState();
 }
 
-class _ExampleScaffoldState extends State<ExampleScaffold> {
+class _ExampleScaffoldState extends State<ExampleScaffold>
+    with TickerProviderStateMixin {
+  TabController _tabController;
+  List<String> tabs;
+
+  @override
+  void initState() {
+    super.initState();
+    tabs = List.generate(10, (index) => 'Tab $index');
+    _tabController = TabController(
+      length: tabs.length,
+      vsync: this,
+    );
+  }
+
   @override
   Widget build(BuildContext context) {
     return ASScaffold(
       title: '标题',
+      appBarBottom: ASTabBar(
+        items: tabs,
+        isScrollable: true,
+        controller: _tabController,
+      ),
     );
   }
 }

lib/ansu_ui.dart

@@ -3,6 +3,7 @@ library ansu_ui;
 export 'buttons/as_button.dart';
 export 'scaffold/as_scaffold.dart';
 export 'styles/as_colors.dart';
+export 'tabbar/as_tabbar.dart';
 
 //`BOTTOM` is external lib
 export 'package:flutter_screenutil/flutter_screenutil.dart';

lib/tabbar/as_tab_indicator.dart

@@ -0,0 +1,84 @@
+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);
+  }
+}

lib/tabbar/as_tabbar.dart

@@ -0,0 +1,46 @@
+import 'package:ansu_ui/styles/as_colors.dart';
+import 'package:ansu_ui/tabbar/as_tab_indicator.dart';
+import 'package:flutter/material.dart';
+
+class ASTabBar extends StatefulWidget implements PreferredSizeWidget {
+  final List<String> items;
+  final TabController controller;
+
+  ///可滚动
+  final bool isScrollable;
+  ASTabBar(
+      {Key key,
+      @required this.items,
+      @required this.controller,
+      this.isScrollable = false})
+      : super(key: key);
+
+  @override
+  _ASTabBarState createState() => _ASTabBarState();
+
+  @override
+  Size get preferredSize => Size.fromHeight(46);
+}
+
+class _ASTabBarState extends State<ASTabBar> {
+  @override
+  Widget build(BuildContext context) {
+    return TabBar(
+      isScrollable: widget.isScrollable,
+      controller: widget.controller,
+      tabs: widget.items.map((e) => Tab(text: e)).toList(),
+      labelStyle: TextStyle(
+        fontSize: 14,
+        fontWeight: FontWeight.bold,
+      ),
+      unselectedLabelStyle: TextStyle(
+        fontWeight: FontWeight.normal,
+      ),
+      labelColor: kTextColor,
+      unselectedLabelColor: kTextSubColor,
+      indicatorSize: TabBarIndicatorSize.label,
+      indicatorPadding: EdgeInsets.zero,
+      indicator: ASTabIndicator(),
+    );
+  }
+}