When and how do you use isolates?

Every Dart thread is an isolate with its own event loop and heap. There is no shared mutable memory, which removes data races but means you cannot pass arbitrary object graphs by reference. Small immutable messages are copied; large buffers can be transferred to avoid the copy cost.

Future<int> countPrimes(int limit) {
  return Isolate.run(() {
    var count = 0;
    for (var n = 2; n <= limit; n++) {
      var prime = true;
      for (var d = 2; d * d <= n; d++) {
        if (n % d == 0) {
          prime = false;
          break;
        }
      }
      if (prime) count++;
    }
    return count;
  });
}

• Use Isolate.run or compute for one-off CPU tasks
• Use long-lived isolates with ports for repeated work such as audio or database processing
• Send simple values or typed data; closures and native handles usually cannot cross
• Move large buffers with TransferableTypedData to avoid copying
• Background isolates need explicit setup to use platform channels

Costs and constraints

Spawning an isolate costs memory and startup time, so it is not worth it for trivial work. Sending large JSON structures between isolates adds serialisation overhead. On the web, isolates map to workers with different capabilities, so code that depends on dart:io must be isolated behind conditional imports.