Custom CheckBox - Flutter

checkbox.dart

import 'package:flutter/material.dart';

class SCheckbox extends StatefulWidget {
  final ValueChanged<bool> onChanged;
  final bool isChecked;
  final String title;
  final double size;
  final double iconSize;
  final Color selectedColor;
  final Color selectedIconColor;
  final Color borderColor;
  final Icon checkIcon;

  const SCheckbox(
      {Key key,
      this.onChanged,
      this.isChecked,
      this.title,
      this.size,
      this.iconSize,
      this.selectedColor,
      this.selectedIconColor,
      this.borderColor,
      this.checkIcon})
      : super(key: key);

  @override
  _SCheckboxState createState() => _SCheckboxState();
}

class _SCheckboxState extends State<SCheckbox> {
  bool _isSelected = false;

  @override
  void initState() {
    _isSelected = widget.isChecked ?? false;
    super.initState();
  }

  @override
  Widget build(BuildContext context) {
    return Row(
      crossAxisAlignment: CrossAxisAlignment.start,
      children: [
        Padding(
          padding: const EdgeInsets.only(
            top: 2.5,
            right: 8,
          ),
          child: SizedBox(
            width: 24,
            height: 24,
            child: GestureDetector(
              onTap: () {
                setState(() {
                  _isSelected = !_isSelected;
                  widget.onChanged(_isSelected);
                });
              },
              child: Container(
                decoration: BoxDecoration(
                  borderRadius: BorderRadius.circular(3.0),
                  border: Border.all(
                    color: widget.borderColor ?? Colors.red,
                    width: 2,
                  ),
                ),
                child: AnimatedContainer(
                  margin: EdgeInsets.all(2),
                  duration: Duration(milliseconds: 1000),
                  curve: Curves.fastLinearToSlowEaseIn,
                  width: widget.size ?? 20,
                  height: widget.size ?? 20,
                  decoration: BoxDecoration(
                    color: _isSelected
                        ? widget.selectedColor ?? Colors.red
                        : Colors.white,
                  ),
                  child: _isSelected
                      ? Padding(
                          padding: const EdgeInsets.all(2.0),
                          child: Container(color: Colors.red),
                        )
                      : Container(),
                ),
              ),
            ),
          ),
        ),
        Expanded(
          child: Text(
            widget.title ?? "",
            style: const TextStyle(
              fontSize: 16,
              color: const Color(0xFF222222),
            ),
          ),
        )
      ],
    );
  }
}

checkbox_stack_positioned.dart

import 'package:flutter/material.dart';

class SCheckbox extends StatefulWidget {
  final ValueChanged<bool> onChanged;
  final bool isChecked;
  final String title;
  final double? size;
  final double? iconSize;
  final Color? selectedColor;
  final Color? selectedIconColor;
  final Color? borderColor;
  final Icon? checkIcon;

  const SCheckbox({
    Key? key,
    this.size,
    this.iconSize,
    this.selectedColor,
    this.selectedIconColor,
    this.borderColor,
    this.checkIcon,
    required this.onChanged,
    required this.isChecked,
    required this.title,
  }) : super(key: key);

  @override
  _SCheckboxState createState() => _SCheckboxState();
}

class _SCheckboxState extends State<SCheckbox> {
  bool _isSelected = false;

  @override
  void initState() {
    _isSelected = widget.isChecked ?? false;
    super.initState();
  }

  @override
  Widget build(BuildContext context) {
    return Stack(
      children: [
        Positioned(
          top: 7,
          left: 30,
          child: SizedBox(
            width: MediaQuery.of(context).size.width * 0.9,
            child: Text(
              widget.title ?? "",
              style: const TextStyle(
                fontSize: 16,
                color: Color(0xFF222222),
              ),
            ),
          ),
        ),
        Positioned(
          top: 5,
          left: 0.5,
          child: SizedBox(
            width: 24,
            height: 24,
            child: GestureDetector(
              onTap: () {
                setState(() {
                  _isSelected = !_isSelected;
                  widget.onChanged(_isSelected);
                });
              },
              child: Container(
                decoration: BoxDecoration(
                  borderRadius: BorderRadius.circular(3.0),
                  border: Border.all(
                    color: widget.borderColor ?? Colors.red,
                    width: 2,
                  ),
                ),
                child: AnimatedContainer(
                  margin: const EdgeInsets.all(2),
                  duration: const Duration(milliseconds: 1000),
                  curve: Curves.fastLinearToSlowEaseIn,
                  width: widget.size ?? 20,
                  height: widget.size ?? 20,
                  decoration: BoxDecoration(
                    color: _isSelected
                        ? widget.selectedColor ?? Colors.red
                        : Colors.white,
                  ),
                  child: _isSelected
                      ? Padding(
                          padding: const EdgeInsets.all(2.0),
                          child: Container(color: Colors.red),
                        )
                      : Container(),
                ),
              ),
            ),
          ),
        ),
      ],
    );
  }
}

Para usar no projeto:

SCheckbox(
    title: "Concordo com tudo!.",
    isChecked: false,
    onChanged: (bool _selectedChanged) {
      print("CHECKBOX => $_selectedChanged");
    },
)