在2021年末，酷狗发布了最新版11.0.0版本，这是一次重大的UI重构，更新完打开着实让我耳目一新。在原有风格上，整个App变得更加清爽，流畅。其中Tabbar的风格让我非常感兴趣，如果用Flutter来实现，或许是一个很有趣的事情。

效果图

分析效果

研究酷狗Tabbar的动画可以发现，默认状态下在当前Tab的中心处展示圆点，滑动时的效果拆分成两个以下部分:

• 从单个Tab A的中心根据X轴平移到Tab B的中心位置；

• 指示器的长度从圆点变长，再缩短为圆点。其中最大长度是可变的，跟两个Tab的大小和距离都有关系；

• 指示器虽然依赖Tab的size和offset来变换，但和Tab却基本是同一时间渲染的，整个过程非常顺滑；

• 总的来说，酷狗的效果就是改变了指示器的渲染动画而已。

开发思路

从上面的分析可以明确，指示器的滑动效果一定跟每个Tab的size和offset相关。那在Flutter中，获取渲染信息我们马上能想到GlobalKey，通过GlobalKey的currentContext对象获取Rander信息，但这必须在视图渲染完成后才能获取，也就是说Tab渲染完才能开始计算并渲染指示器。很显然不符合体验要求，同时频繁使用GlobalKey也会导致性能较差。

转变思路，我们需要在Tab渲染的不断把信息传给指示器，然后更新指示器，这种方式自然想到了CustomPainter【之前写了很多Canvas的控件，都是根据传入的值进行绘制，从而实现控件的变化了layout类】。在Tab updateWidget的时候，不断把Rander的信息传给画笔Painter，然后更新绘制，理论上这样做是完全行得通的。

Flutter Tabbar 解析源码

为了验证我的思路，我开始研究官方Tabbar是如何写的：

• 进入TabBar类，直接查看build方法，可以看到为每个Tab加入了Globalkey，然后指示器用CustomPaint进行绘制；

Widget build(BuildContext context) {// ...此处省略部分代码...final List<Widget> wrappedTabs = List<Widget>.generate(widget.tabs.length, (int index) {const double verticalAdjustment = (_kTextAndIconTabHeight - _kTabHeight)/2.0;EdgeInsetsGeometry? adjustedPadding;// 这里为tab加入Globalkey，以便后续获取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);}}}// ...此处省略部分代码...// 可以看到指示器是CustomPaint对象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,),),);

• 绘制指示器用CustomPaint跟我们的预想一致，那如何把绘制的size和offset传进去呢。我们来看_TabLabelBar继承于Flex，而Flex又继承自MultiChildRenderObjectWidget，重写其createRenderObject方法；

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;@overrideRenderFlex createRenderObject(BuildContext context) {// 查看下_TabLabelBarRendererreturn _TabLabelBarRenderer(direction: direction,mainAxisAlignment: mainAxisAlignment,mainAxisSize: mainAxisSize,crossAxisAlignment: crossAxisAlignment,textDirection: getEffectiveTextDirection(context)!,verticalDirection: verticalDirection,onPerformLayout: onPerformLayout,);}@overridevoid updateRenderObject(BuildContext context, _TabLabelBarRenderer renderObject) {super.updateRenderObject(context, renderObject);renderObject.onPerformLayout = onPerformLayout;}
}

查看真实的渲染对象：_TabLabelBarRenderer，在performLayout中返回渲染的size和offset，并通过TabBar传入的_saveTabOffsets方法保存到_indicatorPainter中；_saveTabOffsets尤为重要，把Tabbar的渲染位移通知给Painter，从而让Painter可以轻松算出tab之间的宽度差

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;@overridevoid 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);}
}

• 通过Tabbar中的didChangeDependencies和didUpdateWidget生命周期，更新指示器；

@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);}
}

• 然后重点就在指示器_IndicatorPainter如何进行绘制了。

实现步骤

通过理解Flutter Tabbar的实现思路，大体跟我们预想的差不多。不过官方继承了Flex来计算Offset和size，实现起来很优雅。所以我也不班门弄斧了，直接改动官方的Tabbar就可以了。

1. 创建KuGouTabbar，复制官方代码，修改引用，删除无关的类，只保留Tabbar相关的代码。

2. 重点修改_IndicatorPainter，根据我们的需求来绘制指示器。在painter方法中，我们可以通过controller拿到当前tab的index以及animation!.value， 我们模拟下切换的过程，当tab从第0个移到第1个，动画的值从0变成1，然后动画走到0.5时，tab的index会从0突然变为1，指示器应该是先变长，然后在动画走到0.5时，再变短。因此动画0.5之前，我们用动画的value-index作为指示器缩放的倍数，指示器不断增大；动画0.5之后，用index-value作为缩放倍数，不断缩小。

final double index = controller.index.toDouble();final double value = controller.animation!.value;
/// 改动 ltr为false，表示索引还是0，动画执行未超过50%；ltr为true，表示索引变为1，动画执行超过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);/// 改动 通过ltr来决定是放大还是缩小倍数，可以得出公式：ltr ? (index - value) : (value - index)
final Rect fromRect =indicatorRect(size, from, ltr ? (index - value) : (value - index));/// 改动
final Rect toRect =indicatorRect(size, to, ltr ? (index - value) : (value - index));
_currentRect = Rect.lerp(fromRect, toRect, (value - from).abs());

而指示器接收缩放倍数的前提还需要计算指示器最大的宽度，并且上面是根据动画的0.5作为最大的宽度，也就是移动到一半的时候，指示器应该达到最大宽度。因此指示器最大的宽度是需要✖️2的。请看下面代码：

class _IndicatorPainter extends CustomPainter {......此处省略部分代码......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;}/// 接收上面代码分析中传入的倍数 scaleRect 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;}/// 改动，通过GlobalKey计算出渲染的文本的宽度tabWidth = tabKeys[tabIndex].currentContext!.size!.width;final double delta = ((tabRight - tabLeft) - tabWidth) / 2.0;tabLeft += delta;tabRight -= delta;final EdgeInsets insets = indicatorPadding.resolve(_currentTextDirection);/// 改动，算出指示器的最大宽度，记得*2double maxLen = (tabRight - tabLeft + insets.horizontal) * 2;double res =scale == 0 ? minWidth : maxLen * (scale < 0.5 ? scale : 1 - scale);/// 改动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);}
}

1. 如上，指示器的宽度我们根据controller切换时的index和动画值进行转化，实现宽度的变化。而Offset的最小值和最大值分别是切换前后两个Tab的中心点，这里应该做下相应的的限制，然后传给Rect.fromLTWH。

【由于时间和精力问题，我并没有去做这一步的实现，而且酷狗那边动画跟滑动逻辑的关系需要UI给出具体的公式，才能百分百还原。】

最后就是加多一个参数，让业务方传入指示器的最小宽度。

/// 指示器的最小宽度
final double indicatorMinWidth;

业务使用

在上面我们已经把简单的动画效果改完了，接下来就是传入圆角的indicator、最小宽度indicatorMinWidth,就可以正常使用啦。

• 圆角的指示器，我直接上源码

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;@overrideDecoration? 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);}@overrideDecoration? 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,);}@overridePath 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;@overridevoid 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);}
}

• 调用非常简单，跟原来官方代码一模一样。

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: "音乐"), Tab(text: "动态"), Tab(text: "语文")],// 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,),),
);

写在最后

模仿酷狗的Tabbar效果，就分享到这里啦，重点在于实现步骤的第2、3步，涉及到一些简单的数学知识。说说心得吧，Flutter UI层面的问题，其实技术栈已经很单一了。只要跟着官方的实现思路，能写出跟其类似的代码，把Rander层理解透彻，笔者认为已经足够了。往深了还是得往原生、混编、解决Flutter痛点问题为主。希望一起共勉！！！

实现源码

记得给个Star哦！(https://github.com/WxqKb/KuGouTabbar.git)

作者：Karl_wei
链接：https://juejin.cn/post/7057168681943433246

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