Flutter并发执行插件flutter_isolate的使用

FlutterIsolate简介

一个Dart isolate大致相当于一个独立的执行线程。在Flutter环境中，创建(“spawning”)一个isolate允许代码在主线程之外执行，这对于运行昂贵/长时间的任务非常重要，因为这些任务可能会阻塞UI。

然而，在生成的isolate中运行的代码通常无法与Flutter插件交互。这是由于平台插件脚手架和主应用程序isolate之间的紧密集成导致的。

FlutterIsolate插件通过引入FlutterIsolate类解决了这个问题，该类是围绕创建isolate的平台API以及为在生成的isolate中运行的代码设置必要的绑定以与Flutter插件通信的封装。

FlutterIsolate API

使用方法

创建FlutterIsolate

要创建一个FlutterIsolate，调用spawn方法，并传递一个带有@pragma('vm:entry-point')装饰器的顶层或静态函数：

import 'package:flutter_isolate/flutter_isolate.dart';

@pragma('vm:entry-point')
void someFunction(String arg) { 
  print("Running in an isolate with argument : $arg");
}

class SomeWidget extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return ElevatedButton(
      child: Text('Run'),
      onPressed: () {
        FlutterIsolate.spawn(someFunction, "hello world");
      },
    );
  }
}

执行单个任务

如果你只想创建一个isolate来执行单个任务（如Flutter的compute方法），可以调用flutterCompute:

@pragma('vm:entry-point')
Future<int> expensiveWork(int arg) async {
  int result = 0;
  // 假设这里有很多计算
  for (var i = 0; i < 1000000000; i++) {
    result += i;
  }
  return result;
}

Future<int> doExpensiveWorkInBackground() async {
  return await flutterCompute(expensiveWork, 1);
}

在其他isolate中创建isolate

isolate也可以从其他isolate中创建：

import 'package:flutter_isolate/flutter_isolate.dart';

@pragma('vm:entry-point')
void isolate2(String arg) {
  Timer.periodic(Duration(seconds:1),(timer)=>print("Timer Running From Isolate 2"));
}

@pragma('vm:entry-point')
void isolate1(String arg) async  {
  final isolate = await FlutterIsolate.spawn(isolate2, "hello2");
  Timer.periodic(Duration(seconds:1),(timer)=>print("Timer Running From Isolate 1"));
}

void main() async {
  WidgetsFlutterBinding.ensureInitialized();
  final isolate = await FlutterIsolate.spawn(isolate1, "hello");
  Timer(Duration(seconds:5), (){print("Pausing Isolate 1");isolate.pause();});
  Timer(Duration(seconds:10),(){print("Resuming Isolate 1");isolate.resume();});
  Timer(Duration(seconds:20),(){print("Killing Isolate 1");isolate.kill();});
  runApp(MyApp());
}

注意事项

• 入口点必须是一个顶层函数，并且需要加上@pragma('vm:entry-point')注解。
• 类级别的方法将不会工作，并会抛出异常。
• 忘记添加@pragma('vm:entry-point')注解会导致应用在发布模式下崩溃。
• 由于FlutterIsolate是由特定于平台的’view’支持的，因此事件循环不会在没有更多’用户’工作时终止，并且需要显式地使用kill()终止FlutterIsolates。

示例项目

下面是一个完整的示例项目，展示了如何使用flutter_isolate插件来下载文件：

import 'dart:async';
import 'dart:io';

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

@pragma('vm:entry-point')
void isolate2(String arg) {
  getTemporaryDirectory().then((dir) async {
    print("isolate2 temporary directory: $dir");

    await FlutterDownloader.initialize(debug: true);
    FlutterDownloader.registerCallback(AppWidget.downloaderCallback);

    final taskId = await FlutterDownloader.enqueue(
        url:
            "https://raw.githubusercontent.com/rmawatson/flutter_isolate/master/README.md",
        savedDir: dir.path);
  });
  Timer.periodic(
      Duration(seconds: 1), (timer) => print("Timer Running From Isolate 2"));
}

@pragma('vm:entry-point')
void isolate1(String arg) async {
  await FlutterIsolate.spawn(isolate2, "hello2");

  getTemporaryDirectory().then((dir) {
    print("isolate1 temporary directory: $dir");
  });
  Timer.periodic(
      Duration(seconds: 1), (timer) => print("Timer Running From Isolate 1"));
}

@pragma('vm:entry-point')
void computeFunction(String arg) async {
  getTemporaryDirectory().then((dir) {
    print("Temporary directory in compute function : $dir with arg $arg");
  });
}

void main() async {
  WidgetsFlutterBinding.ensureInitialized();
  await FlutterDownloader.initialize(debug: true);
  runApp(MyApp());
}

class MyApp extends StatefulWidget {
  @override
  _MyAppState createState() => _MyAppState();
}

class _MyAppState extends State<MyApp> {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
        home: Scaffold(
            appBar: AppBar(
              title: const Text('Plugin example app'),
            ),
            body: AppWidget()));
  }
}

class AppWidget extends StatelessWidget {
  static void downloaderCallback(String id, int status, int progress) {
    print("progress: $progress");
  }

  Future<void> _run() async {
    print(
        "Temp directory in main isolate : ${(await getTemporaryDirectory()).path}");
    final isolate = await FlutterIsolate.spawn(isolate1, "hello");
    Timer(Duration(seconds: 5), () {
      print("Pausing Isolate 1");
      isolate.pause();
    });
    Timer(Duration(seconds: 10), () {
      print("Resuming Isolate 1");
      isolate.resume();
    });
    Timer(Duration(seconds: 20), () {
      print("Killing Isolate 1");
      isolate.kill();
    });
  }

  @override
  Widget build(BuildContext context) {
    return Column(crossAxisAlignment: CrossAxisAlignment.center, children: [
      ElevatedButton(
        child: Text('Spawn isolates'),
        onPressed: _run,
      ),
      ElevatedButton(
        child: Text('Check running isolates'),
        onPressed: () async {
          final isolates = await FlutterIsolate.runningIsolates;
          await showDialog(
              builder: (ctx) {
                return Center(
                    child: Container(
                        color: Colors.white,
                        padding: EdgeInsets.all(5),
                        child: Column(
                            children: isolates
                                    .map((i) => Text(i))
                                    .cast<Widget>()
                                    .toList() +
                                [
                                  ElevatedButton(
                                      child: Text("Close"),
                                      onPressed: () {
                                        Navigator.of(ctx).pop();
                                      })
                                ])));
              },
              context: context);
        },
      ),
      ElevatedButton(
        child: Text('Kill all running isolates'),
        onPressed: () async {
          await FlutterIsolate.killAll();
        },
      ),
      ElevatedButton(
        child: Text('Run in compute function'),
        onPressed: () async {
          await flutterCompute(computeFunction, "foo");
        },
      ),
    ]);
  }
}

这个示例展示了如何使用flutter_isolate插件来管理多个isolate，包括创建、暂停、恢复和终止isolate，以及检查当前运行的isolate。