Event loop

Learn the Dart event loop model.

What is the event loop?

Dart is a single-threaded language; all code runs inside one event loop. The loop takes one task at a time, runs it to completion, then moves on.

There are two queues: the event queue (Future, I/O, timers) and the microtask queue (higher priority). Each turn drains all microtasks first, then runs one event.

Therefore long synchronous code blocks the event loop. CPU-heavy work goes to an Isolate; I/O waiting goes to a Future.

Observing the event loop

A Future, when completed, adds a task to the event queue. With await main suspends until that task is processed.

Example: print('a'); await Future.delayed(Duration(milliseconds: 1)); print('b');. 'a' prints immediately, then a 1 ms event is awaited, then 'b' prints.

This ordering is deterministic: since the event loop is single-threaded, the output is always the same.

Exercise Observe the event loop order

Make main async. Add print('a'), then await Future.delayed(Duration(milliseconds: 1)), then print('b').

Show example solution

void main() async {
  print('a');
  await Future.delayed(Duration(milliseconds: 1));
  print('b');
}

Test yourself

Which statement about the event loop is true?
- Multiple threads run in parallel
- Microtasks run after events
- Microtasks run before events
- Synchronous code does not block the loop
Show answer

Microtasks run before events — each turn drains all microtasks first, then runs one event.

Event loop queues

First ____ queue, then the event queue is processed.

Show answer

First microtask queue, then the event queue is processed.

Match the loop parts

Event loop

Single thread
Event queue

Future / I/O
Microtask

Priority task
await

Suspension point

Next lesson: Microtasks

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