在flutter中TabBar + TabBarView可以实现类似Android中的indicator + viewpage的效果.

一.TabBar参数

一般可以为indicator设置UnderlineTabIndicator对象. 而indicatorColor和indicatorWeight 内部其实也是构建了 UnderlineTabIndicator.

具体源码为:(tabs.dart/_TabBarState)

  Decoration get _indicator {
    if (widget.indicator != null)
      return widget.indicator;
    final ThemeData themeData = Theme.of(context);
    if (themeData.tabBarTheme.indicator != null)
      return themeData.tabBarTheme.indicator;

    Color color = widget.indicatorColor ?? themeData.indicatorColor;
    if (color.value == Material.of(context).color.value)
      color = Colors.white;

    return UnderlineTabIndicator(
      insets: widget.indicatorPadding,
      borderSide: BorderSide(
        width: widget.indicatorWeight,
        color: color,
      ),
    );
  }

而indicatorSize则传递给了_IndicatorPainter. 其根据taborlabel计算绘制区域大小.

  Rect indicatorRect(Size tabBarSize, int tabIndex) {
    assert(_currentTabOffsets != null);
    assert(_currentTextDirection != null);
    assert(_currentTabOffsets.isNotEmpty);
    assert(tabIndex >= 0);
    assert(tabIndex <= maxTabIndex);
    double tabLeft, tabRight;
    switch (_currentTextDirection) {
      case TextDirection.rtl:
        tabLeft = _currentTabOffsets[tabIndex + 1];
        tabRight = _currentTabOffsets[tabIndex];
        break;
      case TextDirection.ltr:
        tabLeft = _currentTabOffsets[tabIndex];
        tabRight = _currentTabOffsets[tabIndex + 1];
        break;
    }

    if (indicatorSize == TabBarIndicatorSize.label) {
      final double tabWidth = tabKeys[tabIndex].currentContext.size.width;
      final double delta = ((tabRight - tabLeft) - tabWidth) / 2.0;
      tabLeft += delta;
      tabRight -= delta;
    }

    return Rect.fromLTWH(tabLeft, 0.0, tabRight - tabLeft, tabBarSize.height);
  }

二. 自定义indicator

比如我们想支持指定宽度, 上面的那些参数无法达成, 那就需要重写一个自定义Decoration. 直接copyUnderlineTabIndicator代码

  Rect _indicatorRectFor(Rect rect, TextDirection textDirection) {
    assert(rect != null);
    assert(textDirection != null);
    final Rect indicator = insets.resolve(textDirection).deflateRect(rect);
    return Rect.fromLTWH(
      indicator.left,
      indicator.bottom - borderSide.width,
      indicator.width,
      borderSide.width,
    );
  }

修改为

  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);
  }