我正在创建一个使用滑块的图像编辑器应用程序，类似于Instagram，我正在使用库image/image.dart。问题是，一旦你移动滑块，它就会更新图像，但就在那时，如果你再次移动它，它就不会更新。
我已经按预期设置了所有内容，setState（）函数如flutter所要求的那样，但我不知道为什么它不会再次更新。

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

import 'package:image/image.dart' as br;
import 'package:path_provider/path_provider.dart';
import 'package:image_picker/image_picker.dart';

class ImageManager extends StatefulWidget {

  @override
  State<StatefulWidget> createState() {
    return _ImageManagerState();
  }
}

class _ImageManagerState extends State<ImageManager> {
  File imageFile;
  br.Image image;
  Image _imageWidget;

  Future<String> get _localPath async {
    final directory = await getApplicationDocumentsDirectory();

    return directory.path;
  }

  // get tmp file
  Future<File> get _localFile async {
    final path = await _localPath;
    return File('$path/tmp.jpg');
  }

  // pick image on button click
  Future<void> _pickImage(ImageSource source) async{
    File selectedFile = await ImagePicker.pickImage(source: source);
    br.Image selectedImage;
    if (selectedFile != null){
      selectedImage = br.decodeImage(selectedFile.readAsBytesSync());
      br.grayscale(selectedImage);

      selectedFile.writeAsBytesSync(br.encodeJpg(selectedImage));
    }

    setState((){
      image = selectedImage;
      imageFile = selectedFile;
      _imageWidget = Image.file(imageFile);
    });
  }

  // MAIN PROBLEM, UPDATING THE CONTRAST WILL ONLY DO IT ONCE
  Future<void> updateContrast(value) async{
    File contrastFile = imageFile;
    br.Image contrast = br.decodeImage(contrastFile.readAsBytesSync());
    contrast = br.adjustColor(contrast, contrast: value);
    contrastFile.writeAsBytesSync(br.encodeJpg(contrast));

    // Save the thumbnail as a jpg.
    File path = await _localFile;
    path.writeAsBytesSync(br.encodeJpg(contrast));
    setState(() {
      image = contrast;
      imageFile = contrastFile;
      if(path != null){
        _imageWidget = Image.file(path);
        print(value);
      }

    });
  }

  // 
  Widget _buildImage(BuildContext context){
    return Column(
      children: [
        Container(
          width: MediaQuery.of(context).size.width,
          height: MediaQuery.of(context).size.width,
          child: _imageWidget,
        ),
        Column(
          children: [
            Container(
              padding: const EdgeInsets.only(left: 8, right: 8),
              child: Column(
                children: <Widget>[
                  // contrast
                  Text("Contraste"),
                  Padding(
                    padding: const EdgeInsets.only(bottom: 4.0, top: 0.0),
                    child: Container(
                      child: Slider(
                        min: 0.0,
                        max: 1.0,
                        divisions: 100,
                        value: _contrast,
                        activeColor: Colors.blue[500],
                        inactiveColor: Colors.blue[50],
                        label: "${(_contrast *100).round()}",
                        onChanged: (value) async{
                          changeContrast(value);
                        },
                        onChangeEnd: (value) async{
                          updateContrast(value);
                        },
                      ),
                    ),
                  ),
                ],
              ),
            ),
          ],
        ),
      ]
    );
  }

我希望每次更改滑块时图像都会更新。