How do you create and use custom Isolate in Dart?

To create a custom isolate, I typically start by defining a top-level function that contains the heavy computation logic. Then, I use Isolate.spawn() to launch a new isolate and pass a SendPort to communicate between the main isolate and the new one. For example, I once had a use case where I had to process a large JSON file before displaying analytics in Flutter. Parsing that file on the main isolate caused noticeable UI lag, so I moved the parsing logic to a background isolate.

Here’s a simplified version of that implementation:

import 'dart:isolate';
import 'dart:convert';

void parseJson(SendPort sendPort) {
  final receivePort = ReceivePort();
  sendPort.send(receivePort.sendPort);

  receivePort.listen((message) {
    final jsonString = message[0] as String;
    final replyPort = message[1] as SendPort;
    final data = jsonDecode(jsonString);
    replyPort.send(data);
  });
}

void main() async {
  final receivePort = ReceivePort();
  final isolate = await Isolate.spawn(parseJson, receivePort.sendPort);
  
  SendPort? isolateSendPort;
  receivePort.listen((message) {
    if (message is SendPort) {
      isolateSendPort = message;
      final responsePort = ReceivePort();
      isolateSendPort!.send(["{\"key\": \"value\"}", responsePort.sendPort]);
      responsePort.listen((result) {
        print(result);
      });
    }
  });
}

In this example, the main isolate spawns another isolate to handle JSON parsing. The two communicate via ports. The UI remains completely responsive while the data is being processed.

A challenge I faced while working with custom isolates was data transfer overhead — since isolates don’t share memory, large objects like image data or long strings can take time to transfer. To overcome that, I used TransferableTypedData, which allows efficient binary data transfer without copying.

Another challenge was managing isolate lifecycle, especially in Flutter widgets. If the user navigates away from the screen, I have to ensure the isolate is properly killed using isolate.kill() to avoid memory leaks.

In terms of limitations, isolates are great for CPU-bound work but not ideal for I/O-bound operations like network calls, because async-await already handles those efficiently within a single isolate.

An alternative for simpler one-time tasks is Flutter’s compute() function, which internally spawns an isolate and returns the result. I generally use compute() for small, quick background tasks, and Isolate.spawn() when I need more control, like continuous communication or long-running background processing.

So, overall, using custom isolates helped me improve performance and maintain a smooth UI, especially in data-heavy applications.