Widget build(BuildContext context) {
  
  // ...此處省略部分代碼...
  
  final List<Widget> wrappedTabs = List<Widget>.generate(widget.tabs.length, (int index) {
    const double verticalAdjustment = (_kTextAndIconTabHeight - _kTabHeight)/2.0;
    EdgeInsetsGeometry? adjustedPadding;
    // 這里為tab加入Globalkey，以便后續(xù)獲取Tab的渲染信息
    if (widget.tabs[index] is PreferredSizeWidget) {
      final PreferredSizeWidget tab = widget.tabs[index] as PreferredSizeWidget;
      if (widget.tabHasTextAndIcon && tab.preferredSize.height == _kTabHeight) {
        if (widget.labelPadding != null || tabBarTheme.labelPadding != null) {
          adjustedPadding = (widget.labelPadding ?? tabBarTheme.labelPadding!).add(const EdgeInsets.symmetric(vertical: verticalAdjustment));
        }
        else {
          adjustedPadding = const EdgeInsets.symmetric(vertical: verticalAdjustment, horizontal: 16.0);
        }
      }
    }
    
    // ...此處省略部分代碼...
    
    // 可以看到指示器是CustomPaint對(duì)象
    Widget tabBar = CustomPaint(
        painter: _indicatorPainter,
        child: _TabStyle(
            animation: kAlwaysDismissedAnimation,
            selected: false,
            labelColor: widget.labelColor,
            unselectedLabelColor: widget.unselectedLabelColor,
            labelStyle: widget.labelStyle,
            unselectedLabelStyle: widget.unselectedLabelStyle,
            child: _TabLabelBar(
              onPerformLayout: _saveTabOffsets,
              children: wrappedTabs,
        ),
      ),
    );

class _TabLabelBar extends Flex {
  _TabLabelBar({
    Key? key,
    List<Widget> children = const <Widget>[],
    required this.onPerformLayout,
  }) : super(
    key: key,
    children: children,
    direction: Axis.horizontal,
    mainAxisSize: MainAxisSize.max,
    mainAxisAlignment: MainAxisAlignment.start,
    crossAxisAlignment: CrossAxisAlignment.center,
    verticalDirection: VerticalDirection.down,
  );

  final _LayoutCallback onPerformLayout;

  @override
  RenderFlex createRenderObject(BuildContext context) {
    // 查看下_TabLabelBarRenderer
    return _TabLabelBarRenderer(
      direction: direction,
      mainAxisAlignment: mainAxisAlignment,
      mainAxisSize: mainAxisSize,
      crossAxisAlignment: crossAxisAlignment,
      textDirection: getEffectiveTextDirection(context)!,
      verticalDirection: verticalDirection,
      onPerformLayout: onPerformLayout,
    );
  }

  @override
  void updateRenderObject(BuildContext context, _TabLabelBarRenderer renderObject) {
    super.updateRenderObject(context, renderObject);
    renderObject.onPerformLayout = onPerformLayout;
  }
}

class _TabLabelBarRenderer extends RenderFlex {
  _TabLabelBarRenderer({
    List<RenderBox>? children,
    required Axis direction,
    required MainAxisSize mainAxisSize,
    required MainAxisAlignment mainAxisAlignment,
    required CrossAxisAlignment crossAxisAlignment,
    required TextDirection textDirection,
    required VerticalDirection verticalDirection,
    required this.onPerformLayout,
  }) : assert(onPerformLayout != null),
       assert(textDirection != null),
       super(
         children: children,
         direction: direction,
         mainAxisSize: mainAxisSize,
         mainAxisAlignment: mainAxisAlignment,
         crossAxisAlignment: crossAxisAlignment,
         textDirection: textDirection,
         verticalDirection: verticalDirection,
       );

  _LayoutCallback onPerformLayout;

  @override
  void performLayout() {
    super.performLayout();
    // xOffsets will contain childCount+1 values, giving the offsets of the
    // leading edge of the first tab as the first value, of the leading edge of
    // the each subsequent tab as each subsequent value, and of the trailing
    // edge of the last tab as the last value.
    RenderBox? child = firstChild;
    final List<double> xOffsets = <double>[];
    while (child != null) {
      final FlexParentData childParentData = child.parentData! as FlexParentData;
      xOffsets.add(childParentData.offset.dx);
      assert(child.parentData == childParentData);
      child = childParentData.nextSibling;
    }
    assert(textDirection != null);
    switch (textDirection!) {
      case TextDirection.rtl:
        xOffsets.insert(0, size.width);
        break;
      case TextDirection.ltr:
        xOffsets.add(size.width);
        break;
    }
    onPerformLayout(xOffsets, textDirection!, size.width);
  }
}

@override
void didChangeDependencies() {
  super.didChangeDependencies();
  assert(debugCheckHasMaterial(context));
  final TabBarTheme tabBarTheme = TabBarTheme.of(context);
  _updateTabController();
  _initIndicatorPainter(adjustedPadding, tabBarTheme);
}

@override
void didUpdateWidget(KuGouTabBar oldWidget) {
  super.didUpdateWidget(oldWidget);
  final TabBarTheme tabBarTheme = TabBarTheme.of(context);
  if (widget.controller != oldWidget.controller) {
    _updateTabController();
    _initIndicatorPainter(adjustedPadding, tabBarTheme);
  } else if (widget.indicatorColor != oldWidget.indicatorColor ||
      widget.indicatorWeight != oldWidget.indicatorWeight ||
      widget.indicatorSize != oldWidget.indicatorSize ||
      widget.indicator != oldWidget.indicator) {
    _initIndicatorPainter(adjustedPadding, tabBarTheme);
  }

  if (widget.tabs.length > oldWidget.tabs.length) {
    final int delta = widget.tabs.length - oldWidget.tabs.length;
    _tabKeys.addAll(List<GlobalKey>.generate(delta, (int n) => GlobalKey()));
  } else if (widget.tabs.length < oldWidget.tabs.length) {
    _tabKeys.removeRange(widget.tabs.length, oldWidget.tabs.length);
  }
}

final double index = controller.index.toDouble();

final double value = controller.animation!.value;
/// 改動(dòng) ltr為false，表示索引還是0，動(dòng)畫(huà)執(zhí)行未超過(guò)50%；ltr為true，表示索引變?yōu)?，動(dòng)畫(huà)執(zhí)行超過(guò)50%
final bool ltr = index > value;
final int from = (ltr ? value.floor() : value.ceil()).clamp(0, maxTabIndex);
final int to = (ltr ? from + 1 : from - 1).clamp(0, maxTabIndex);

/// 改動(dòng) 通過(guò)ltr來(lái)決定是放大還是縮小倍數(shù)，可以得出公式：ltr ? (index - value) : (value - index)
final Rect fromRect =
    indicatorRect(size, from, ltr ? (index - value) : (value - index));

/// 改動(dòng)
final Rect toRect =
    indicatorRect(size, to, ltr ? (index - value) : (value - index));
_currentRect = Rect.lerp(fromRect, toRect, (value - from).abs());

class _IndicatorPainter extends CustomPainter {
  ......此處省略部分代碼......

  void saveTabOffsets(List<double>? tabOffsets, TextDirection? textDirection) {
    _currentTabOffsets = tabOffsets;
    _currentTextDirection = textDirection;
  }

  // _currentTabOffsets[index] is the offset of the start edge of the tab at index, and
  // _currentTabOffsets[_currentTabOffsets.length] is the end edge of the last tab.
  int get maxTabIndex => _currentTabOffsets!.length - 2;

  double centerOf(int tabIndex) {
    assert(_currentTabOffsets != null);
    assert(_currentTabOffsets!.isNotEmpty);
    assert(tabIndex >= 0);
    assert(tabIndex <= maxTabIndex);
    return (_currentTabOffsets![tabIndex] + _currentTabOffsets![tabIndex + 1]) /
        2.0;
  }

  /// 接收上面代碼分析中傳入的倍數(shù) scale
  Rect indicatorRect(Size tabBarSize, int tabIndex, double scale) {
    assert(_currentTabOffsets != null);
    assert(_currentTextDirection != null);
    assert(_currentTabOffsets!.isNotEmpty);
    assert(tabIndex >= 0);
    assert(tabIndex <= maxTabIndex);
    double tabLeft, tabRight, tabWidth = 0;
    switch (_currentTextDirection!) {
      case TextDirection.rtl:
        tabLeft = _currentTabOffsets![tabIndex + 1];
        tabRight = _currentTabOffsets![tabIndex];
        break;
      case TextDirection.ltr:
        tabLeft = _currentTabOffsets![tabIndex];
        tabRight = _currentTabOffsets![tabIndex + 1];
        break;
    }

    /// 改動(dòng)，通過(guò)GlobalKey計(jì)算出渲染的文本的寬度
    tabWidth = tabKeys[tabIndex].currentContext!.size!.width;
    final double delta = ((tabRight - tabLeft) - tabWidth) / 2.0;
    tabLeft += delta;
    tabRight -= delta;

    final EdgeInsets insets = indicatorPadding.resolve(_currentTextDirection);

    /// 改動(dòng)，算出指示器的最大寬度，記得*2
    double maxLen = (tabRight - tabLeft + insets.horizontal) * 2;

    double res =
        scale == 0 ? minWidth : maxLen * (scale < 0.5 ? scale : 1 - scale);

    /// 改動(dòng)
    final Rect rect = Rect.fromLTWH(tabLeft + tabWidth / 2 - minWidth / 2, 0.0, res > minWidth ? res : minWidth, tabBarSize.height);

    if (!(rect.size >= insets.collapsedSize)) {
      throw FlutterError(
        'indicatorPadding insets should be less than Tab Size\n'
        'Rect Size : ${rect.size}, Insets: ${insets.toString()}',
      );
    }
    return insets.deflateRect(rect);
   }
}

/// 指示器的最小寬度
final double indicatorMinWidth;

import 'package:flutter/material.dart';

class RRecTabIndicator extends Decoration {
  const RRecTabIndicator(
      {this.borderSide = const BorderSide(width: 2.0, color: Colors.white),
        this.insets = EdgeInsets.zero,
        this.radius = 0,
        this.color = Colors.white});

  final double radius;
  final Color color;
  final BorderSide borderSide;
  final EdgeInsetsGeometry insets;

  @override
  Decoration? lerpFrom(Decoration? a, double t) {
    if (a is RRecTabIndicator) {
      return RRecTabIndicator(
        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 RRecTabIndicator) {
      return RRecTabIndicator(
        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) {
    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)
      : super(onChanged);

  final RRecTabIndicator decoration;

  @override
  void paint(Canvas canvas, Offset offset, ImageConfiguration configuration) {
    final Rect rect = offset & configuration.size!;
    final TextDirection textDirection = configuration.textDirection!;
    final Rect indicator = decoration._indicatorRectFor(rect, textDirection);
    final Paint paint = decoration.borderSide.toPaint()
      ..strokeCap = StrokeCap.square
      ..color = decoration.color;
    final RRect rRect =
    RRect.fromRectAndRadius(indicator, Radius.circular(decoration.radius));
    canvas.drawRRect(rRect, paint);
  }
}

Scaffold(
  appBar: AppBar(
    // Here we take the value from the MyHomePage object that was created by
    // the App.build method, and use it to set our appbar title.
    title: Text(widget.title),
    bottom: KuGouTabBar(
      tabs: const [Tab(text: "音樂(lè)"), Tab(text: "動(dòng)態(tài)"), Tab(text: "語(yǔ)文")],
      // labelPadding: EdgeInsets.symmetric(horizontal: 8),
      controller: _tabController,
      // indicatorSize: TabBarIndicatorSize.label,
      // isScrollable: true,
      padding: EdgeInsets.zero,
      indicator: const RRecTabIndicator(
          radius: 4, insets: EdgeInsets.only(bottom: 5)),
      indicatorMinWidth: 6,
    ),
  ),
);