如何使用 flutter 将 Base64 字符串解码为图像文件

Question

我实际上是在尝试将图像文件转换为 Base64 并将 Base64 字符串解码为 Dart/Flutter 中的图像文件。 实际上，我的应用程序使用相机插件拍照，然后将其作为 Base64 字符串保存在数据库中（因为我不知道任何其他方式，但可能有更相关的方式来存储多张图像？）。

我用这段代码编码：

            final path = join(
              (await getTemporaryDirectory()).path,
              '${DateTime.now()}.png',
            );
            await _controller.takePicture(path);
            final bytes = File(path).readAsBytesSync();
            String img64 = base64Encode(bytes);

            Navigator.push(
              context,
              MaterialPageRoute(
                builder: (context) => DisplayPictureScreen(imageAnalysed: img64),
              ),
            );

并用这个解码：

class DisplayPictureScreen extends StatelessWidget {
  final String imageAnalysed;
  const DisplayPictureScreen({Key key, this.imageAnalysed}) : super(key: key);

  @override
  Widget build(BuildContext context) {
    final decodedBytes = base64Decode(imageAnalysed);
    var fileImg= File("testImage.png");
    fileImg..writeAsBytesSync(decodedBytes);

    return Scaffold(
      appBar: AppBar(title: Text('Display the Picture')),
      body: Image.file(fileImg),
    );
  }
}

但是有这个错误：

The following FileSystemException was thrown building DisplayPictureScreen(dirty):
Cannot open file, path = 'testImage.png' (OS Error: Read-only file system, errno = 30)

如何解决我的问题并从 Base64 字符串中获取图像？

谢谢您的帮助 ！

Answer 1

您可以在下面复制粘贴运行完整代码
您可以使用path_provider获取getApplicationDocumentsDirectory并使用bool isLoading来控制写入文件完成时的显示图像

代码片段

 void writeFile() async {
    final decodedBytes = base64Decode(widget.imageAnalysed);
    final directory = await getApplicationDocumentsDirectory();
    fileImg = File('${directory.path}/testImage.png');
    print(fileImg.path);
    fileImg.writeAsBytesSync(List.from(decodedBytes));

    setState(() {
      isLoading = false;
    });
  }

  @override
  void initState() {
    WidgetsBinding.instance.addPostFrameCallback((_){
      writeFile();
    });

 ...      
 @override
  Widget build(BuildContext context) {
    return isLoading ? CircularProgressIndicator() : Image.file(fileImg);
  }

工作演示

完整代码

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

class DisplayPictureScreen extends StatefulWidget {
  final String imageAnalysed;
  const DisplayPictureScreen({Key key, this.imageAnalysed}) : super(key: key);

  @override
  _DisplayPictureScreenState createState() => _DisplayPictureScreenState();
}

class _DisplayPictureScreenState extends State<DisplayPictureScreen> {
  File fileImg;
  bool isLoading = true;

  void writeFile() async {
    final decodedBytes = base64Decode(widget.imageAnalysed);
    final directory = await getApplicationDocumentsDirectory();
    fileImg = File('${directory.path}/testImage.png');
    print(fileImg.path);
    fileImg.writeAsBytesSync(List.from(decodedBytes));

    setState(() {
      isLoading = false;
    });
  }

  @override
  void initState() {
    WidgetsBinding.instance.addPostFrameCallback((_){
      writeFile();
    });

    super.initState();
  }

  @override
  Widget build(BuildContext context) {
    return isLoading ? CircularProgressIndicator() : Image.file(fileImg);
  }
}

void main() {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
        visualDensity: VisualDensity.adaptivePlatformDensity,
      ),
      home: MyHomePage(title: 'Flutter Demo Home Page'),
    );
  }
}

class MyHomePage extends StatefulWidget {
  MyHomePage({Key key, this.title}) : super(key: key);

  final String title;

  @override
  _MyHomePageState createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  int _counter = 0;
  String base64String =
      '''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''';
  void _incrementCounter() {
    setState(() {
      _counter++;
    });
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text(widget.title),
      ),
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            DisplayPictureScreen(imageAnalysed: base64String),
            Text(
              'You have pushed the button this many times:',
            ),
            Text(
              '$_counter',
              style: Theme.of(context).textTheme.headline4,
            ),
          ],
        ),
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: _incrementCounter,
        tooltip: 'Increment',
        child: Icon(Icons.add),
      ),
    );
  }
}