Flutter實現酷狗流暢Tabbar效果

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「這是我參與2022首次更文挑戰的第1天,活動詳情檢視:2022首次更文挑戰」。

在2021年末,酷狗釋出了最新版11.0.0版本,這是一次重大的UI重構,更新完開啟著實讓我耳目一新。在原有風格上,整個App變得更加清爽,流暢。其中Tabbar的風格讓我非常感興趣,如果用Flutter來實現,或許是一個很有趣的事情。

效果圖

酷狗的.gif

我的.gif

分析效果

研究酷狗Tabbar的動畫可以發現,預設狀態下在當前Tab的中心處展示圓點,滑動時的效果拆分成兩個以下部分: - 從單個Tab A的中心根據X軸平移到Tab B的中心位置; - 指示器的長度從圓點變長,再縮短為圓點。其中最大長度是可變的,跟兩個Tab的大小和距離都有關係; - 指示器雖然依賴Tab的size和offset來變換,但和Tab卻基本是同一時間渲染的,整個過程非常順滑; - 總的來說,酷狗的效果就是改變了指示器的渲染動畫而已。

開發思路

從上面的分析可以明確,指示器的滑動效果一定跟每個Tab的size和offset相關。那在Flutter中,獲取渲染資訊我們馬上能想到GlobalKey,通過GlobalKeycurrentContext物件獲取Rander資訊,但這必須在檢視渲染完成後才能獲取,也就是說Tab渲染完才能開始計算並渲染指示器。很顯然不符合體驗要求,同時頻繁使用GlobalKey也會導致效能較差。

轉變思路,我們需要在Tab渲染的不斷把資訊傳給指示器,然後更新指示器,這種方式自然想到了CustomPainter之前寫了很多Canvas的控制元件,都是根據傳入的值進行繪製,從而實現控制元件的變化了layout類】。在Tab updateWidget的時候,不斷把Rander的資訊傳給畫筆Painter,然後更新繪製,理論上這樣做是完全行得通的。

Flutter Tabbar 解析原始碼

為了驗證我的思路,我開始研究官方Tabbar是如何寫的: - 進入TabBar類,直接檢視build方法,可以看到為每個Tab加入了Globalkey,然後指示器用CustomPaint進行繪製; ``` dart Widget build(BuildContext context) {

// ...此處省略部分程式碼...

final List wrappedTabs = List.generate(widget.tabs.length, (int index) { const double verticalAdjustment = (_kTextAndIconTabHeight - _kTabHeight)/2.0; EdgeInsetsGeometry? adjustedPadding; // 這裡為tab加入Globalkey,以便後續獲取Tab的渲染資訊 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); } } }

// ...此處省略部分程式碼...

// 可以看到指示器是CustomPaint物件
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,
    ),
  ),
);

- 繪製指示器用CustomPaint跟我們的預想一致,那如何把繪製的size和offset傳進去呢。我們來看_TabLabelBar繼承於Flex,而Flex又繼承自MultiChildRenderObjectWidget,重寫其createRenderObject方法; dart class _TabLabelBar extends Flex { _TabLabelBar({ Key? key, List children = const [], 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) { // 檢視下_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; } } 檢視真實的渲染物件:_TabLabelBarRenderer,在performLayout中返回渲染的size和offset,並通過TabBar傳入的_saveTabOffsets方法儲存到_indicatorPainter中;_saveTabOffsets尤為重要,把Tabbar的渲染位移通知給Painter,從而讓Painter可以輕鬆算出tab之間的寬度差 dart class _TabLabelBarRenderer extends RenderFlex { _TabLabelBarRenderer({ List? 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 xOffsets = []; 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); } } - 通過Tabbar中的didChangeDependenciesdidUpdateWidget生命週期,更新指示器; dart @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.generate(delta, (int n) => GlobalKey())); } else if (widget.tabs.length < oldWidget.tabs.length) { _tabKeys.removeRange(widget.tabs.length, oldWidget.tabs.length); } } - 然後重點就在指示器_IndicatorPainter```如何進行繪製了。

實現步驟

通過理解Flutter Tabbar的實現思路,大體跟我們預想的差不多。不過官方繼承了Flex來計算Offset和size,實現起來很優雅。所以我也不班門弄斧了,直接改動官方的Tabbar就可以了。

  1. 建立KuGouTabbar,複製官方程式碼,修改引用,刪除無關的類,只保留Tabbar相關的程式碼。 image.png
  2. 重點修改_IndicatorPainter,根據我們的需求來繪製指示器。在painter方法中,我們可以通過controller拿到當前tab的index以及animation!.value, 我們模擬下切換的過程,當tab從第0個移到第1個,動畫的值從0變成1,然後動畫走到0.5時,tab的index會從0突然變為1,指示器應該是先變長,然後在動畫走到0.5時,再變短。因此動畫0.5之前,我們用動畫的value-index作為指示器縮放的倍數,指示器不斷增大;動畫0.5之後,用index-value作為縮放倍數,不斷縮小。

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

final double value = controller.animation!.value; /// 改動 ltr為false,表示索引還是0,動畫執行未超過50%;ltr為true,表示索引變為1,動畫執行超過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 通過ltr來決定是放大還是縮小倍數,可以得出公式:ltr ? (index - value) : (value - index) final Rect fromRect = indicatorRect(size, from, ltr ? (index - value) : (value - index));

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

``` 而指示器接收縮放倍數的前提還需要計算指示器最大的寬度,並且上面是根據動畫的0.5作為最大的寬度,也就是移動到一半的時候,指示器應該達到最大寬度。因此指示器最大的寬度是需要✖️2的。請看下面程式碼:

``` dart class _IndicatorPainter extends CustomPainter { ......此處省略部分程式碼......

void saveTabOffsets(List? 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; }

/// 接收上面程式碼分析中傳入的倍數 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,通過GlobalKey計算出渲染的文字的寬度
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,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
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,指示器的寬度我們根據controller切換時的index和動畫值進行轉化,實現寬度的變化。而Offset的最小值和最大值分別是切換前後兩個Tab的中心點,這裡應該做下相應的的限制,然後傳給Rect.fromLTWH。 【由於時間和精力問題,我並沒有去做這一步的實現,而且酷狗那邊動畫跟滑動邏輯的關係需要UI給出具體的公式,才能百分百還原。】

最後就是加多一個引數,讓業務方傳入指示器的最小寬度。 dart /// 指示器的最小寬度 final double indicatorMinWidth;

業務使用

在上面我們已經把簡單的動畫效果改完了,接下來就是傳入圓角的indicator、最小寬度indicatorMinWidth,就可以正常使用啦。 - 圓角的指示器,我直接上原始碼 ``` dart 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); } } - 呼叫非常簡單,跟原來官方程式碼一模一樣。 dart 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"), Tab(text: "動態"), Tab(text: "語文")], // 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, ), ), );
```

寫在最後

模仿酷狗的Tabbar效果,就分享到這裡啦,重點在於實現步驟的第2、3步,涉及到一些簡單的數學知識。說說心得吧,Flutter UI層面的問題,其實技術棧已經很單一了。只要跟著官方的實現思路,能寫出跟其類似的程式碼,把Rander層理解透徹,筆者認為已經足夠了。往深了還是得往原生、混編、解決Flutter痛點問題為主。 希望一起共勉!!!

實現原始碼

記得給個Star哦!