The Event Loop

Dart runs your code on a single thread per isolate. Understanding how it juggles asynchronous work explains a lot of behavior — including why a Flutter app can freeze.

One thread, two queues

Dart's event loop repeatedly does this:

1. Run the current synchronous code to completion.
2. Run all waiting microtasks (short follow-up jobs — for example, continuing after an await on an already-completed future).
3. Take the next event (a timer firing, a network response, a user tap, a Future(...)) and run its code. Then back to step 2.

import 'dart:async';

void main() {
  print('1: synchronous');
  Future(() => print('4: event queue (Future)'));
  Future.delayed(Duration.zero, () => print('5: event queue (timer)'));
  scheduleMicrotask(() => print('3: microtask'));
  print('2: synchronous');
}

1: synchronous
2: synchronous
3: microtask
4: event queue (Future)
5: event queue (timer)

All synchronous code runs first, then the microtask, then events in order.

Why this matters: never block the loop

While your code runs, nothing else happens — no other events, no screen redraws. A long synchronous loop freezes a Flutter app:

void main() {
  Future(() => print('I have to wait'));

  final stopwatch = Stopwatch()..start();
  var sum = 0;
  for (var i = 0; i < 300000000; i++) {
    sum += i;
  }
  print('Busy loop finished (${sum > 0})');
}

Busy loop finished (true)
I have to wait

The queued event can't run until the loop finishes. async/await doesn't help with this kind of work — await only frees the thread while waiting (for I/O or timers), not while computing. For heavy computation, use an isolate (a later lesson).

Practical rules

• I/O (network, files, databases) is already asynchronous in Dart — just await it.
• Keep synchronous work short, especially in Flutter, where each frame has about 16 milliseconds to render at 60 frames per second.
• Move CPU-heavy work — parsing a huge JSON file, image processing — to another isolate.