Sample loader

main.dart

import 'dart:async';

import 'package:flutter/material.dart';

void main() => runApp(PeekUp());

class PeekUp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Peek Up by FTC',
      home: GeekLoader(),
    );
  }
}

class GeekLoader extends StatefulWidget {
  @override
  _GeekLoaderState createState() => _GeekLoaderState();
}

class _GeekLoaderState extends State<GeekLoader> {
  bool isLoading = false;

  void runFuture() async {
    setState(() {
      isLoading = true;
    });
    Timer(Duration(seconds: 5), () {
      setState(() {
        isLoading = false;
      });
    });
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
        backgroundColor: Colors.white,
        body: Container(
          constraints: BoxConstraints.expand(),
          child: Column(
            crossAxisAlignment: CrossAxisAlignment.center,
            mainAxisAlignment: MainAxisAlignment.center,
            children: <Widget>[
              Placeholder(fallbackHeight: 50),
              if (isLoading)
                CircularProgressIndicator(),
              Placeholder(fallbackHeight: 50),
              FlatButton(
                onPressed: () {
                  runFuture();
                },
                child: Text('run a future'),
              )
            ],
          ),
        ));
  }
}