Flutter互斥锁管理插件mutex的使用

介绍

mutex 是一个用于创建锁以确保在运行关键代码段时实现互斥的库。尽管Dart使用单线程执行，但在关键代码段中使用异步操作时仍可能发生竞争条件。例如：

x = 42;
synchronousOperations(); // this does not modify x
assert(x == 42); // x will NOT have changed

y = 42; // a variable that other asynchronous code can modify
await asynchronousOperations(); // this does NOT modify y, but...
// There is NO GUARANTEE other async code didn't run and change it!
assert(y == 42 || y != 42); // WARNING: y might have changed

mutex 插件提供了两种类型的互斥锁：普通互斥锁和读写互斥锁。

普通互斥锁 (Mutex)

普通互斥锁保证在同一时间最多只有一个锁存在。如果锁已经被获取，其他尝试获取锁的操作将被阻塞，直到锁被释放。

使用示例

import 'package:mutex/mutex.dart';

void main() async {
  final m = Mutex();

  await m.acquire();
  try {
    // critical section with asynchronous code
    await Future.delayed(Duration(seconds: 1));
  } finally {
    m.release();
  }

  // 或者使用 protect 方法
  await m.protect(() async {
    // critical section
    await Future.delayed(Duration(seconds: 1));
  });

  // 如果关键代码段返回一个值
  final result = await m.protect<int>(() async {
    // critical section
    return 42;
  });
  print('result: $result');
}

读写互斥锁 (ReadWriteMutex)

读写互斥锁允许多个读锁同时存在，但最多只能有一个写锁存在。写锁和任何读锁不能同时存在。

使用示例

import 'package:mutex/mutex.dart';

void main() async {
  final m = ReadWriteMutex();

  // 获取写锁
  await m.acquireWrite();
  try {
    // critical write section with asynchronous code
    await Future.delayed(Duration(seconds: 1));
  } finally {
    m.release();
  }

  // 或者使用 protectWrite 方法
  await m.protectWrite(() async {
    // critical write section
    await Future.delayed(Duration(seconds: 1));
  });

  // 获取读锁
  await m.acquireRead();
  try {
    // critical read section with asynchronous code
    await Future.delayed(Duration(seconds: 1));
  } finally {
    m.release();
  }

  // 或者使用 protectRead 方法
  await m.protectRead(() async {
    // critical read section
    await Future.delayed(Duration(seconds: 1));
  });

  // 如果关键代码段返回一个值
  final result1 = await m.protectWrite<String>(() async {
    // critical write section
    return "write result";
  });
  print('result1: $result1');

  final result2 = await m.protectRead<int>(() async {
    // critical read section
    return 42;
  });
  print('result2: $result2');
}

完整示例

以下是一个完整的示例，展示了为什么需要使用互斥锁来防止竞争条件：

import 'dart:async';
import 'dart:math';
import 'package:mutex/mutex.dart';

const _maxDelay = 500; // milliseconds
final _random = Random();
int balance = 0;

Future<void> randomDelay() async {
  await Future<void>.delayed(
      Duration(milliseconds: _random.nextInt(_maxDelay)));
}

/// Deposit without using mutex.
Future<void> unsafeUpdate(int id, int depositAmount) async {
  await randomDelay();
  final oldBalance = balance;
  await randomDelay();
  balance = oldBalance + depositAmount;
  print('  [$id] added $depositAmount to $oldBalance -> $balance');
}

/// Deposit using mutex.
Mutex m = Mutex();
Future<void> safeUpdate(int id, int depositAmount) async {
  await randomDelay();
  await m.protect(() async {
    final oldBalance = balance;
    await randomDelay();
    balance = oldBalance + depositAmount;
    print('  [$id] added $depositAmount to $oldBalance -> $balance');
  });
}

/// Make a series of deposits and see if the final balance is correct.
Future<void> makeDeposits({bool safe = true}) async {
  print(safe ? 'Using mutex:' : 'Not using mutex:');
  const numberDeposits = 10;
  const amount = 10;
  balance = 0;
  final operations = <Future>[];
  for (var x = 0; x < numberDeposits; x++) {
    final f = (safe) ? safeUpdate(x, amount) : unsafeUpdate(x, amount);
    operations.add(f);
  }
  await Future.wait<void>(operations);
  final expected = numberDeposits * amount;
  if (balance != expected) {
    print('Error: deposits were lost (final balance $balance != $expected)');
  } else {
    print('Success: no deposits were lost');
  }
}

void main() async {
  await makeDeposits(safe: false);
  print('');
  await makeDeposits(safe: true);
}

通过这个示例，可以看到在没有使用互斥锁的情况下，由于竞争条件，最终余额可能会出错。而使用互斥锁后，可以确保所有存款操作都成功完成，最终余额正确无误。

总结

mutex 插件提供了简单且强大的互斥锁机制，可以帮助开发者避免在异步代码中出现的竞争条件问题。无论是普通互斥锁还是读写互斥锁，都能根据具体需求选择合适的锁类型来保护关键代码段。