custom_switch.dart

  @override
  Widget build(BuildContext context) {
    return AnimatedBuilder(
      animation: _animationController,
      builder: (context, child) {
        return Container(
          width: 70.0,
          height: 35.0,
          decoration: BoxDecoration(
              borderRadius: BorderRadius.circular(20.0),
              color: _circleAnimation.value == Alignment.centerLeft
                  ? Colors.grey
                  : widget.activeColor),
          child: Padding(
            padding: const EdgeInsets.only(
                top: 4.0, bottom: 4.0, right: 4.0, left: 4.0),
            child: Row(
              mainAxisAlignment: MainAxisAlignment.spaceBetween,
              children: <Widget>[
                _circleAnimation.value == Alignment.centerRight
                    ? Padding(
                        padding: const EdgeInsets.only(left: 4.0, right: 4.0),
                        child: Text(
                          'On',
                          style: TextStyle(
                              color: Colors.white70,
                              fontWeight: FontWeight.w900,
                              fontSize: 16.0),
                        ),
                      )
                    : Container(),
                Align(
                  alignment: _circleAnimation.value,
                  child: Container(
                    width: 25.0,
                    height: 25.0,
                    decoration: BoxDecoration(
                        shape: BoxShape.circle, color: Colors.white),
                  ),
                ),
                _circleAnimation.value == Alignment.centerLeft
                    ? Padding(
                        padding: const EdgeInsets.only(left: 4.0, right: 5.0),
                        child: Text(
                          'Off',
                          style: TextStyle(
                              color: Colors.white70,
                              fontWeight: FontWeight.w900,
                              fontSize: 16.0),
                        ),
                      )
                    : Container(),
              ],
            ),
          ),
        );
      },
    );
  }