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packages/mutex/example/example.dart

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+// Mutex example.
+//
+// This example demonstrates why a mutex is needed.
+
+import 'dart:async';
+import 'dart:math';
+import 'package:mutex/mutex.dart';
+
+//----------------------------------------------------------------
+// Random asynchronous delays to try and simulate race conditions.
+
+const _maxDelay = 500; // milliseconds
+
+final _random = Random();
+
+Future<void> randomDelay() async {
+  await Future<void>.delayed(
+      Duration(milliseconds: _random.nextInt(_maxDelay)));
+}
+
+//----------------------------------------------------------------
+/// Account balance.
+///
+/// The classical example of a race condition is when a bank account is updated
+/// by different simultaneous operations.
+
+int balance = 0;
+
+//----------------------------------------------------------------
+/// Deposit without using mutex.
+
+Future<void> unsafeUpdate(int id, int depositAmount) async {
+  // Random delay before updating starts
+  await randomDelay();
+
+  // Add the deposit to the balance. But this operation is not atomic if
+  // there are asynchronous operations in it (as simulated by the randomDelay).
+
+  final oldBalance = balance;
+  await randomDelay();
+  balance = oldBalance + depositAmount;
+
+  print('  [$id] added $depositAmount to $oldBalance -> $balance');
+}
+
+//----------------------------------------------------------------
+/// Deposit using mutex.
+
+Mutex m = Mutex();
+
+Future<void> safeUpdate(int id, int depositAmount) async {
+  // Random delay before updating starts
+  await randomDelay();
+
+  // Acquire the mutex before running the critical section of code
+
+  await m.protect(() async {
+    // critical section
+
+    // This is the same as the unsafe update. But since it is performed only
+    // when the mutex is acquired, it is safe: no other safe update can happen
+    // until this mutex is released.
+
+    final oldBalance = balance;
+    await randomDelay();
+    balance = oldBalance + depositAmount;
+
+    // end of critical section
+
+    print('  [$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;
+
+  // Create a set of operations, each attempting to deposit the same amount
+  // into the account.
+
+  final operations = <Future<void>>[];
+  for (var x = 0; x < numberDeposits; x++) {
+    final f = (safe) ? safeUpdate(x, amount) : unsafeUpdate(x, amount);
+    operations.add(f);
+  }
+
+  // Wait for all the deposit operations to finish
+
+  await Future.wait<void>(operations);
+
+  // Check if all of the operations succeeded
+
+  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);
+}