ButtonLoader

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

void main() => runApp(new MyApp());

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return new MaterialApp(
      title: 'Button Loader',
      theme: new ThemeData(
        primarySwatch: Colors.green,
      ),
      home: new MyHomePage(title: 'Button Loader'),
    );
  }
}

class MyHomePage extends StatefulWidget {
  MyHomePage({Key key, this.title}) : super(key: key);

  final String title;

  @override
  _MyHomePageState createState() => new _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> with TickerProviderStateMixin {
  int myState = 0;

  @override
  Widget build(BuildContext context) {
    return new Scaffold(
      appBar: new AppBar(
      centerTitle: true ,
        title: new Text(widget.title),
      ),
      body: new Center(
        child: new Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            new Padding(
              padding: const EdgeInsets.all(16.0),
              child: new MaterialButton(
                child: ButtonTextAndIcon(),
                onPressed: () {
                  setState(() {
                    if (myState == 0) {
                      controllerButton();
                    }
                  });
                },
                elevation: 4.0,
                height: 48.0,
                color: Colors.red,
                splashColor: Colors.white, 
             
             ),
            )
          ],
        ),
      ),
    );
  }

  Widget ButtonTextAndIcon() {
    if (myState == 0) {
      return new Text(
        "Login",
        style: const TextStyle(
          color: Colors.black,
          fontSize: 16.0,
        ),
      );
    } else if (myState == 1) {
      return CircularProgressIndicator(
        valueColor: AlwaysStoppedAnimation<Color>(Colors.green),
      );
    } else {
      return Icon(Icons.check, color: Colors.black);
    }
  }

  void controllerButton() {
    setState(() {
      myState = 1;
    });

    Timer(Duration(milliseconds: 4000), () {
        myState = 2;
    });
  }
}