android技术流：用Flutter来实现酷狗流畅Tabbar

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;

  @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;
  }
}

• 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;

  @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);
  }
}

• 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 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*2
    double 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);
   }
}

3.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;

  @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);
  }
}

• 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！！！

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