在Flutter中，如何获取imageProvider的宽度和高度？
在下面的例子中，我需要获取imageProvider的宽度。这样我就可以计算photo_view包中PhotoView小部件的minScale。

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

class ImageViewerScreen extends StatelessWidget {

  final ImageProvider imageProvider;

  ImageViewerScreen({@required this.imageProvider});

  @override
  Widget build(BuildContext context) {
    final double _screenWidth = MediaQuery.of(context).size.width;
    final double _imageWidth =
        imageProvider.width; //Here is the question: how to get the width of this imageProvider?
    final double _minScale = _screenWidth / _imageWidth;

    return Scaffold(
      appBar: AppBar(),
      body: Container(
          child: PhotoView(
        imageProvider: imageProvider,
        minScale: _minScale,
      )),
    );
  }
}