// Copyright 2018 The Chromium Authors. All rights reserved.

// Use of this source code is governed by a BSD-style license that can be

// found in the LICENSE file.

import 'package:flutter/material.dart';

import 'package:flutter/widgets.dart';

/// Used with [TabBar.indicator] to draw a horizontal line below the

/// selected tab.

///

/// The selected tab underline is inset from the tab's boundary by [insets].

/// The [borderSide] defines the line's color and weight.

///

/// The [TabBar.indicatorSize] property can be used to define the indicator's

/// bounds in terms of its (centered) widget with [TabIndicatorSize.label],

/// or the entire tab with [TabIndicatorSize.tab].

class TabSizeIndicator extends Decoration {

  final double wantWidth;//传入的指示器宽度，默认20

  /// Create an underline style selected tab indicator.

  ///

  /// The [borderSide] and [insets] arguments must not be null.

  const TabSizeIndicator({

    this.borderSide = const BorderSide(width: 2.0, color: Colors.blue),

    this.insets = EdgeInsets.zero,

    this.wantWidth = 20

  }) : assert(borderSide != null),

        assert(insets != null),

        assert(wantWidth != null);

  /// The color and weight of the horizontal line drawn below the selected tab.

  final BorderSide borderSide;//指示器高度以及颜色 ，默认高度2，颜色蓝

  /// 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

  /// [TabIndicatorSize.label], or the entire tab with [TabIndicatorSize.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 TabSizeIndicator) {

      return TabSizeIndicator(

        borderSide: BorderSide.lerp(borderSide, b.borderSide, t),

        insets: EdgeInsetsGeometry.lerp(insets, b.insets, t),

      );

    }

    return super.lerpTo(b, t);

  }

  @override

  _MyUnderlinePainter createBoxPainter([ VoidCallback onChanged ]) {

    return _MyUnderlinePainter(this,wantWidth, onChanged);

  }

}

class _MyUnderlinePainter extends BoxPainter {

  final double wantWidth;

  _MyUnderlinePainter(this.decoration, this.wantWidth,VoidCallback onChanged)

      : assert(decoration != null),

        super(onChanged);

  final TabSizeIndicator 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);

    //希望的宽度

    // double wantWidth = 20;

    //取中间坐标

    double cw = (indicator.left + indicator.right) / 2;

    return Rect.fromLTWH(cw - wantWidth / 2,

        indicator.bottom - borderSide.width, wantWidth, 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;

    canvas.drawLine(indicator.bottomLeft, indicator.bottomRight, paint);

  }

}

TabBar(

                    indicator: TabSizeIndicator(wantWidth:

                                     15,borderSide: BorderSide(width:

                                            2.0,color: Colors.red)),//TabSizeIndicator已有默认值，宽度默认20，颜色蓝色，高度2，不需要修改的可以不传参数
　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　//TabSizeIndicator();即可　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　

                              isScrollable: true,

                              controller: _tabController,

                              unselectedLabelStyle: TextStyle(

                                  fontSize: 13, fontWeight: FontWeight.bold),

                              labelStyle: TextStyle(

                                  fontWeight: FontWeight.bold, fontSize: 13),

                              tabs:_tabbarItem()

),