Isolate中使用path_provider失效，DartPluginRegistrant初始化无效

问题概述 #

在运行Android 11的Pixel 2设备上，尝试在Isolate中调用path_provider的getApplicationDocumentsDirectory()时触发报错，提示"Binding has not yet been initialized"。已在主Isolate中调用WidgetsFlutterBinding.ensureInitialized()，并添加了DartPluginRegistrant.ensureInitialized()，但问题仍未解决。

环境信息 #

Flutter 3.3.1 • channel stable • https://github.com/flutter/flutter.git
Framework • revision 4f9d92fbbd (3 days ago) • 2022-09-06 17:54:53 -0700
Engine • revision 3efdf03e73
Tools • Dart 2.18.0 • DevTools 2.15.0

核心报错日志 #

E/flutter (15026): Unhandled exception:
E/flutter (15026): Binding has not yet been initialized.
E/flutter (15026): The "instance" getter on the ServicesBinding binding mixin is only available once that binding has been initialized.

原因分析 #

主Isolate中初始化的Binding不会共享给新创建的Isolate，每个Isolate都是独立的执行环境。path_provider依赖Flutter的ServicesBinding与原生平台通信，新Isolate未初始化该Binding，导致调用失败。DartPluginRegistrant.ensureInitialized()仅负责插件注册，不处理Binding初始化。

解决方案 #

在新Isolate的入口函数中，先初始化Binding，再执行插件注册和path_provider调用。

修改后的Isolate入口函数 #

Future isolateTestEntry(SendPort sendPort) async {
  // 关键：在新Isolate中独立初始化Binding
  WidgetsFlutterBinding.ensureInitialized();
  // 注册插件
  DartPluginRegistrant.ensureInitialized();
  // 调用path_provider方法
  final directory = await getApplicationDocumentsDirectory();
  debugPrint('directory path: ${directory.path}');
  Isolate.exit();
}

完整可运行代码 #

import 'dart:isolate';
import 'dart:ui';

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

void main() {
  WidgetsFlutterBinding.ensureInitialized();
  runApp(const MyApp());
}

class MyApp extends StatelessWidget {
  const MyApp({super.key});

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Isolate Test',
      theme: ThemeData(primarySwatch: Colors.blue),
      home: const HomePage(title: 'Isolate Test'),
    );
  }
}

class HomePage extends StatefulWidget {
  const HomePage({super.key, required this.title});

  final String title;

  @override
  State<HomePage> createState() => _HomePageState();
}

class _HomePageState extends State<HomePage> {

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text(widget.title),
      ),
      body: Container(),
      floatingActionButton: FloatingActionButton(
        onPressed: () => _spawnIsolate(),
        tooltip: 'Spawn',
        child: const Icon(Icons.add_circle),
      ),
    );
  }

  Future _spawnIsolate() async {
    final receivePort = ReceivePort();
    Isolate.spawn(isolateTestEntry, receivePort.sendPort);
  }
}

Future isolateTestEntry(SendPort sendPort) async {
  WidgetsFlutterBinding.ensureInitialized();
  DartPluginRegistrant.ensureInitialized();
  final directory = await getApplicationDocumentsDirectory();
  debugPrint('directory path: ${directory.path}');
  Isolate.exit();
}

关键说明 #

• 每个Isolate都需要独立初始化Binding，因为Binding是Isolate级别的资源，不跨Isolate共享。
• WidgetsFlutterBinding.ensureInitialized()会初始化包括ServicesBinding在内的所有必要Binding，足以支持path_provider的平台通信需求。
• 必须先初始化Binding，再调用DartPluginRegistrant.ensureInitialized()，确保插件注册时依赖的Binding已就绪。

内容的提问来源于stack exchange，提问作者Sound Conception