如何在颤动中加载Image.file时显示CircularProgressIndicator

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

class ImageLoader extends StatefulWidget {
  @override
  _ImageLoaderState createState() => _ImageLoaderState();
}

class _ImageLoaderState extends State<ImageLoader> {
  File _imageFile; // 用于存储选择的图片文件

  Future<void> _pickImage() async {
    // 通过图片选择器选择图片文件
    File image = await ImagePicker.pickImage(source: ImageSource.gallery);
    setState(() {
      _imageFile = image;
    });
  }
}

@override
Widget build(BuildContext context) {
  return Scaffold(
    appBar: AppBar(
      title: Text('Image Loader'),
    ),
    body: Center(
      child: Column(
        mainAxisAlignment: MainAxisAlignment.center,
        children: <Widget>[
          _imageFile == null
              ? Text('请选择一张图片')
              : Image.file(_imageFile),
          RaisedButton(
            child: Text('选择图片'),
            onPressed: _pickImage,
          ),
        ],
      ),
    ),
  );
}

_imageFile == null
  ? Text('请选择一张图片')
  : Stack(
      alignment: Alignment.center,
      children: <Widget>[
        CircularProgressIndicator(), // 显示CircularProgressIndicator
        Image.file(_imageFile),
      ],
    ),