Flutter设置TabBar indicator宽度（爆改UnderlineTabIndicator ）

冷不防 2021-06-06 15:29 1496阅读 0赞

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][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3h1ZGFpbG9uZ19ibG9n_size_16_color_FFFFFF_t_70]

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][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3h1ZGFpbG9uZ19ibG9n_size_16_color_FFFFFF_t_70 1]

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][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3h1ZGFpbG9uZ19ibG9n_size_16_color_FFFFFF_t_70 2]

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][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3h1ZGFpbG9uZ19ibG9n_size_16_color_FFFFFF_t_70 3]

#### 修改： ####

这时候我们将整个`UnderlineTabIndicator`copy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,
                ),

[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3h1ZGFpbG9uZ19ibG9n_size_16_color_FFFFFF_t_70]: /images/20210529/a94dda367cb341f4972ab8f516b14440.png
[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3h1ZGFpbG9uZ19ibG9n_size_16_color_FFFFFF_t_70 1]: /images/20210529/0bdf6a911e5a49ad8f59c22d4c2f0cd2.png
[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3h1ZGFpbG9uZ19ibG9n_size_16_color_FFFFFF_t_70 2]: /images/20210529/916645808f394b6d8ec54254691da35d.png
[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3h1ZGFpbG9uZ19ibG9n_size_16_color_FFFFFF_t_70 3]: /images/20210529/acfe994101be422490a57e32a0258dea.png