TabBar with badge that update color on swap - Demo included (Flutter)

custom_tab_bar.dart

// Copyright 2021, Thea Choem, All rights reserved.

import 'package:badges/badges.dart';
import 'package:flutter/material.dart';

class CustomTabBarItem {
  final String label;
  final String? value;

  CustomTabBarItem({
    required this.label,
    this.value,
  });
}

class CustomTabBar extends StatelessWidget implements PreferredSizeWidget {
  const CustomTabBar({
    Key? key,
    this.controller,
    required this.items,
    this.isScrollable = true,
    this.height = kToolbarHeight,
    this.onTap,
    this.padding,
  }) : super(key: key);

  /// if `TabController` isn't provided, make sure you have
  /// wraped your widget with `DefaultTabController`
  final TabController? controller;
  final List<CustomTabBarItem> items;
  final double height;
  final bool isScrollable;
  final ValueChanged<int>? onTap;
  final EdgeInsetsGeometry? padding;

  @override
  Widget build(BuildContext context) {
    final TabController? tabController = controller ?? DefaultTabController.of(context);
    return Theme(
      data: Theme.of(context).copyWith(
        splashColor: Colors.transparent,
        highlightColor: Colors.transparent,
      ),
      child: Container(
        width: double.infinity,
        padding: padding,
        child: TabBar(
          isScrollable: isScrollable,
          controller: tabController,
          onTap: onTap,
          tabs: List.generate(
            items.length,
            (itemIndex) {
              var item = items[itemIndex];
              return GestureDetector(
                onTap: () {
                  tabController?.animateTo(itemIndex);
                  if (onTap != null) onTap!(itemIndex);
                },
                child: Row(
                  mainAxisAlignment: MainAxisAlignment.center,
                  children: [
                    Text(item.label),
                    if (item.value != null && item.value!.isNotEmpty)
                      _buildValueContainer(
                        controller: tabController!,
                        index: itemIndex,
                        item: item,
                      ),
                  ],
                ),
              );
            },
          ),
        ),
      ),
    );
  }

  // this method is used to build the
  Widget _buildValueContainer({
    required TabController controller,
    int? index,
    CustomTabBarItem? item,
  }) {
    return AnimatedBuilder(
      animation: controller.animation!,
      builder: (context, child) {
        var unselectedColor = Theme.of(context).unselectedWidgetColor;
        var selectedColor = Theme.of(context).colorScheme.primary;

        Color? color = unselectedColor;

        final double offset = controller.offset;

        final bool isCurrentChild = index == controller.index;
        final int currentIndex = controller.index;

        bool dragToRight = offset > 0;

        if (dragToRight) {
          bool inScope = index! <= currentIndex + 1 && index >= currentIndex;

          if (inScope) {
            color = isCurrentChild
                ? Color.lerp(selectedColor, unselectedColor, offset)!
                : Color.lerp(unselectedColor, selectedColor, offset)!;
          }
        } else {
          bool inScope = index! <= currentIndex && index >= currentIndex - 1;

          if (inScope) {
            color = isCurrentChild
                ? Color.lerp(selectedColor, unselectedColor, -offset)!
                : Color.lerp(unselectedColor, selectedColor, -offset)!;
          }
        }

        var themeData = Theme.of(context);
        var textStyle = themeData.textTheme.overline?.copyWith(color: themeData.backgroundColor);
        var containerSize = 16.0;

        return Container(
          height: containerSize,
          margin: const EdgeInsets.only(left: 4.0),
          child: Badge(
            alignment: Alignment.topRight,
            animationType: BadgeAnimationType.scale,
            shape: BadgeShape.square,
            showBadge: !(item?.value == "0" || item?.value == null),
            animationDuration: Duration(milliseconds: 200),
            badgeColor: Colors.transparent,
            elevation: 0.0,
            padding: EdgeInsets.zero,
            badgeContent: Container(
              height: containerSize,
              alignment: Alignment.center,
              padding: const EdgeInsets.symmetric(horizontal: 4),
              decoration: BoxDecoration(borderRadius: BorderRadius.circular(containerSize), color: color),
              child: Text(
                item?.value ?? "",
                style: textStyle,
                maxLines: 1,
                overflow: TextOverflow.ellipsis,
              ),
            ),
          ),
        );
      },
    );
  }

  @override
  Size get preferredSize => Size.fromHeight(height);
}