Flutter无法将CameraPreview小部件设置为与屏幕大小匹配

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

class CameraScreen extends StatefulWidget {
  @override
  _CameraScreenState createState() => _CameraScreenState();
}

class _CameraScreenState extends State<CameraScreen> {
  CameraController _controller;
  Future<void> _initializeControllerFuture;

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

  Future<void> _initializeCamera() async {
    final cameras = await availableCameras();
    final camera = cameras.first;

    _controller = CameraController(
      camera,
      ResolutionPreset.high,
    );

    _initializeControllerFuture = _controller.initialize();
    await _initializeControllerFuture;

    setState(() {});
  }

  @override
  void dispose() {
    _controller.dispose();
    super.dispose();
  }

  @override
  Widget build(BuildContext context) {
    if (!_controller.value.isInitialized) {
      return Container();
    }

    final screenAspectRatio = MediaQuery.of(context).size.aspectRatio;
    final previewSizes = _controller.value.previewSizes;
    Size bestSize;

    double diff = double.infinity;
    for (var size in previewSizes) {
      final sizeAspectRatio = size.width / size.height;
      final newDiff = (sizeAspectRatio - screenAspectRatio).abs();
      if (newDiff < diff) {
        diff = newDiff;
        bestSize = size;
      }
    }

    return AspectRatio(
      aspectRatio: bestSize.aspectRatio,
      child: CameraPreview(_controller),
    );
  }
}