Flutter设置TabBar indicator宽度（爆改UnderlineTabIndicator ）-蒲公英云

在默认的TabBar中，indicator的宽度是不可以修改的，每个长度都是平分一个Item的，如下图：

在这里插入图片描述

这个是未修改的图，下面是修改的图，总的宽度为20

在这里插入图片描述

总的在源码里翻来翻去，发现并没有可以设置Indicator的字段，这时候，尝试着通过源码来修改indicator的宽度，同时也是借助了这篇文章，不过，给的有些随意，故在这记录之。

源码探索：

点进TabBar的源码

const TabBar({ 
    Key key,
    @required this.tabs,
    this.controller,
    this.isScrollable = false,
    this.indicatorColor,
    this.indicatorWeight = 2.0,
    this.indicatorPadding = EdgeInsets.zero,
    this.indicator,  //这是我们要修改的indicator
    this.indicatorSize,
    this.labelColor,
    this.labelStyle,
    this.labelPadding,
    this.unselectedLabelColor,
    this.unselectedLabelStyle,
    this.dragStartBehavior = DragStartBehavior.start,
    this.onTap,
  }) : assert(tabs != null),
       assert(isScrollable != null),
       assert(dragStartBehavior != null),
       assert(indicator != null || (indicatorWeight != null && indicatorWeight > 0.0)),
       assert(indicator != null || (indicatorPadding != null)),
       super(key: key);

再点进去indicator的源码，发现了有一个默认的UnderlineTabIndicator。

在这里插入图片描述

这时候我们再点进去默认的UnderlineTabIndicator，看下是否有进行修改宽度的地方。

刚好，我们看到了一个print输出，输出的是一个Rect，这个可能恰好是我们说要修改的。

在这里插入图片描述

修改：

这时候我们将整个UnderlineTabIndicatorcopy过来，重命名为MyUnderlineTabIndicator，为了避免与官方相冲突。再对上面截图的地方进行修改，下面是总的源码
my_underline_indicator.dart

// Copyright 2018 The Chromium Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.
import 'package:flutter/material.dart';
import 'package:flutter/widgets.dart';
/// Used with [TabBar.indicator] to draw a horizontal line below the
/// selected tab.
///
/// The selected tab underline is inset from the tab's boundary by [insets].
/// The [borderSide] defines the line's color and weight.
///
/// The [TabBar.indicatorSize] property can be used to define the indicator's
/// bounds in terms of its (centered) widget with [TabIndicatorSize.label],
/// or the entire tab with [TabIndicatorSize.tab].
class MyUnderlineTabIndicator extends Decoration { 
  /// Create an underline style selected tab indicator.
  ///
  /// The [borderSide] and [insets] arguments must not be null.
  const MyUnderlineTabIndicator({ 
    this.borderSide = const BorderSide(width: 2.0, color: Colors.white),
    this.insets = EdgeInsets.zero,
  }) : assert(borderSide != null),
        assert(insets != null);
  /// The color and weight of the horizontal line drawn below the selected tab.
  final BorderSide borderSide;
  /// Locates the selected tab's underline relative to the tab's boundary.
  ///
  /// The [TabBar.indicatorSize] property can be used to define the
  /// tab indicator's bounds in terms of its (centered) tab widget with
  /// [TabIndicatorSize.label], or the entire tab with [TabIndicatorSize.tab].
  final EdgeInsetsGeometry insets;
  @override
  Decoration lerpFrom(Decoration a, double t) { 
    if (a is UnderlineTabIndicator) { 
      return UnderlineTabIndicator(
        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 MyUnderlineTabIndicator) { 
      return MyUnderlineTabIndicator(
        borderSide: BorderSide.lerp(borderSide, b.borderSide, t),
        insets: EdgeInsetsGeometry.lerp(insets, b.insets, t),
      );
    }
    return super.lerpTo(b, t);
  }
  @override
  _MyUnderlinePainter createBoxPainter([ VoidCallback onChanged ]) { 
    return _MyUnderlinePainter(this, onChanged);
  }
}
class _MyUnderlinePainter extends BoxPainter { 
  _MyUnderlinePainter(this.decoration, VoidCallback onChanged)
      : assert(decoration != null),
        super(onChanged);
  final MyUnderlineTabIndicator decoration;
  BorderSide get borderSide => decoration.borderSide;
  EdgeInsetsGeometry get insets => decoration.insets;
  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);
  }
  @override
  void paint(Canvas canvas, Offset offset, ImageConfiguration configuration) { 
    assert(configuration != null);
    assert(configuration.size != null);
    final Rect rect = offset & configuration.size;
    final TextDirection textDirection = configuration.textDirection;
    final Rect indicator = _indicatorRectFor(rect, textDirection).deflate(borderSide.width / 2.0);
    final Paint paint = borderSide.toPaint()..strokeCap = StrokeCap.square;
    canvas.drawLine(indicator.bottomLeft, indicator.bottomRight, paint);
  }
}

使用：

Container(
              height: 46,
              child: TabBar(
                indicatorColor: MyColorRes.primaryColor,
                labelColor: MyColorRes.tvMainSelectColor,
                unselectedLabelColor: MyColorRes.tvMainUnSelectColor,
                unselectedLabelStyle: TextStyle(fontSize: 15),
                labelStyle: TextStyle(fontSize: 17),
                isScrollable: true,
                indicator: MyUnderlineTabIndicator(borderSide:  BorderSide(width: 2.0, color: MyColorRes.primaryColor)),
                controller: _tabController,
                tabs: mTabs.map((value) { 
                  return Text(value.name);
                }).toList(),
              ),
              alignment: Alignment.centerLeft,
              color: MyColorRes.bg_white,
            ),