Flutter Isolate解决耗时任务导致卡死

先来对比一下在Flutter的ui主线程下直接计算一个耗时任务的情况：

import 'package:flutter/material.dart';

void main() {
  runApp(const MaterialApp(
    home: H(),
  ));
}

class H extends StatefulWidget {
  const H({super.key});

  @override
  State createState() => _HState();
}

class _HState extends State {
  int _count = 0;

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: const Text("耗时计算"),
      ),
      body: Center(
        child: Text("$_count"),
      ),
      floatingActionButton: FloatingActionButton(
          child: const Text("start"),
          onPressed: () {
            _count = countPrimes(100000000);
            setState(() {});
          }),
    );
  }
}

// 计算质数个数
int countPrimes(int n) {
  List isPrime = List.filled(n + 1, true);
  isPrime[0] = isPrime[1] = false;

  for (int i = 2; i * i <= n; i++) {
    if (isPrime[i]) {
      for (int j = i * i; j <= n; j += i) {
        isPrime[j] = false;
      }
    }
  }
  return isPrime.where((prime) => prime).length;
}

发现点击按钮后，直接卡死，现在换成异步执行:

import 'package:flutter/material.dart';

void main() {
  runApp(const MaterialApp(
    home: H(),
  ));
}

class H extends StatefulWidget {
  const H({super.key});

  @override
  State createState() => _HState();
}

class _HState extends State {
  int _count = 0;

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: const Text("耗时计算"),
      ),
      body: Center(
        child: Text("$_count"),
      ),
      floatingActionButton: FloatingActionButton(
          child: const Text("start"),
          onPressed: () async {
            _count = await countPrimes(100000000);
            setState(() {});
          }),
    );
  }
}

// 计算质数个数
Future countPrimes(int n) async {
  List isPrime = List.filled(n + 1, true);
  isPrime[0] = isPrime[1] = false;

  for (int i = 2; i * i <= n; i++) {
    if (isPrime[i]) {
      for (int j = i * i; j <= n; j += i) {
        isPrime[j] = false;
      }
    }
  }
  return isPrime.where((prime) => prime).length;
}

发现仍旧会卡死，因为计算过程还是发生在ui线程中，现在使用isolate进行对比：

import 'dart:isolate';

import 'package:flutter/material.dart';

void main() {
  runApp(const MaterialApp(
    home: H(),
  ));
}

class H extends StatefulWidget {
  const H({super.key});

  @override
  State createState() => _HState();
}

class _HState extends State {
  int _count = 0;
  ReceivePort? _receivePort;

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: const Text("耗时计算"),
      ),
      body: Center(
        child: Text("$_count"),
      ),
      floatingActionButton: FloatingActionButton(
          child: const Text("start"),
          onPressed: () {
            _receivePort = ReceivePort();

            Isolate.spawn(countPrimes, [100000000, _receivePort!.sendPort]);
            _receivePort!.listen((message) {
              setState(() {
                _count = message;
              });
            });
          }),
    );
  }
}

// 计算质数个数
void countPrimes(List args) {
  int n = args[0];
  SendPort sendPort = args[1];
  List isPrime = List.filled(n + 1, true);
  isPrime[0] = isPrime[1] = false;

  for (int i = 2; i * i <= n; i++) {
    if (isPrime[i]) {
      for (int j = i * i; j <= n; j += i) {
        isPrime[j] = false;
      }
    }
  }
  sendPort.send(isPrime.where((prime) => prime).length);
}

发现问题得到解决