Event loop
- Dart course
- Advanced
- Lesson 7/83
- 5 min
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.
Practice Observe the event loop order
Make main async. Add print('a'), then await Future.delayed(Duration(milliseconds: 1)), then print('b').
Starter code
void main() {
// make async; print 'a'; wait 1 ms; print 'b'
}Sample solution
void main() async {
print('a');
await Future.delayed(Duration(milliseconds: 1));
print('b');
}Expected output
a b
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Which statement about the event loop is true?
- Multiple threads run in parallel
- Microtasks run after events
- Microtasks run before events Correct answer
- Synchronous code does not block the loop
Explanation: each turn drains all microtasks first, then runs one event.
2 Event loop queues
First ____ queue, then the event queue is processed.First microtask queue, then the event queue is processed.Explanation: the microtask queue has priority.
3 Match the loop parts
| Term | Meaning |
|---|---|
Event loop |
Single thread |
Event queue |
Future / I/O |
Microtask |
Priority task |
await |
Suspension point |