What is difference between Isolates and Event Loops in Flutter?

By default, Dart is a single-threaded language. So, to perform multi-threaded or heavy operations, we rely on Isolates and Event Loops to handle the concurrency.

Event Loop

The event loop is one of Dart’s core mechanisms for handling asynchronous operations. It processes one event at a time, in specific order while also continuously monitoring for new events. The event loop maintains the following queues to decide the order in which to queue the tasks.

1. MicroTask Queue

MicroTask Queue has a higher priority than Event Queue. It’s mainly used for short and internal asynchronous operations. All the operations are performed in FIFO (First In, First Out) order, after which event queue operations are performed.

// Adding to MicroTask queue
scheduleMicrotask(() {
    print('This executes before any Events');
});

// Using Future.microtask
Future.microtask(() {
    // High-priority short operations
});

2. Event Queue

Event Queue is used for external events and longer operations. It is used to manage system events, I/O operations, and timers. It also follows FIFO order and is executed after MicroTask Queue operations.

// Adding to Event queue
Future(() {
    print('This executes after all MicroTasks');
});

// Timer events
Timer(Duration(seconds: 1), () {
    // This goes to Event queue
});

Execution order for Event Loop queues

void main() {
    print('Start'); // 1
    
    Future(() {
        print('Event queue 1'); // 4
    });
    
    scheduleMicrotask(() {
        print('Microtask 1'); // 3
    });
    
    print('End'); // 2
}

The synchronous operations 1 and 2 are executed immediately. Then control executes the MicroTask event and returns to check if there are any other MicroTask pending events. Since there are none, in this case, control goes to execute the Event Queue operation at the end.

Isolates

Isolates are Dart’s answer for concurrent processing operations. Each isolate has it’s own memory heap and runs independently with its own event loop. However, this causes additional issues such as communication to an isolate can happen only via message passing and sharing state with other isolates isn’t possible.

import 'dart:isolate';

void main() async {
    // Creating a simple isolate
    var result = await Isolate.run(() {
        // This code runs in a separate isolate
        return computeIntensiveTask();
    });
}

void computeIntensiveTask() {
    // Heavy computation here
}

Key differences between Isolates and Event loops

Memory and State Management

Execution

Communication

Practical Application

Isolates

• CPU-Intensive computations
• Parallel processing requirements
• Long-running background tasks

Event Loops

• UI updates and animations
• Network requests
• File I/O operations

This separation of concern between Isolates and Event loops helps Dart provide concurrent execution and efficient asynchronous programming capabilities. You can check the official flutter documentation here. Understand Flutter Async Operation to understand async functionality in detail.