aplicacion_movil_syp/lib/memorias_calculo/widgets/cargando.dart

import 'dart:async';
import 'package:flutter/material.dart';
import 'package:google_fonts/google_fonts.dart';

class Cargando extends StatefulWidget {
  final String text;

  const Cargando({
    Key? key,
    required this.text
  }) : super (key: key);

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

class _CargandoState extends State<Cargando> {
  late Timer _timer;
  int _start = 3;

  void _startTimer(){
    const oneSec = Duration(
      seconds: 1,
    );

    _timer = Timer.periodic(
        oneSec,
            (timer){
          if(_start == 0){
            setState((){
              _start = 3;
            });
          }else if(_start == 2){
            setState((){
              _start--;
            });
          }else{
            setState(() => _start--);
          }
        }
    );
  }

  @override
  void dispose() {
    super.dispose();
    _timer.cancel();
  }

  @override
  void initState() {
    super.initState();
    _startTimer();
  }

  @override
  Widget build(BuildContext context) {
    return AnimatedCrossFade(
      firstChild: Text(
        widget.text,
        textAlign: TextAlign.center,
        style: GoogleFonts.robotoSlab(
          fontWeight: FontWeight.bold,
          color: const Color(0xFFB40404),
          fontSize: 16.0,
        ),
      ),
      secondChild: Text(
        widget.text,
        textAlign: TextAlign.center,
        style: GoogleFonts.robotoSlab(
          fontWeight: FontWeight.bold,
          color: const Color(0x55B40404),
          fontSize: 16.0,
        ),
      ),
      duration: const Duration(
        seconds: 2,
      ),
      crossFadeState: _start > 0 && _start <= 2 ? CrossFadeState.showFirst : CrossFadeState.showSecond,
    );
  }
}