Microtasks

Learn the microtask and macrotask queues.

The microtask queue

A microtask is a small task that runs before events in each event-loop turn. It is scheduled with Future.microtask or scheduleMicrotask.

Usage: small, synchronous, urgent work. For example notifying a stream listener. Long work must not go in a microtask; it blocks the loop.

Example: await Future.microtask(() => print('first')). The microtask runs and completes, then execution continues.

await Future.microtask

Future.microtask(fn) schedules fn as a microtask and returns a Future that completes with its result. With await the flow suspends until the microtask is processed.

This is useful for squeezing small synchronous work before events. Example: await Future.microtask(() => 42) — the microtask produces 42, await yields 42.

Note: putting long computation in a microtask blocks the event loop; avoid I/O and heavy CPU.

Exercise Observe microtask ordering

Make main async. Add print('a'). Add await Future.microtask(() => print('b')). Finally add print('c').

Show example solution

void main() async {
  print('a');
  await Future.microtask(() => print('b'));
  print('c');
}

Test yourself

Schedule a microtask

await Future.____(() => print('first'));

Show answer

await Future.microtask(() => print('first'));

When is a microtask processed?
- Only with await
- After events, in the next turn
- Only inside an Isolate
- Before events, in the same turn
Show answer

Before events, in the same turn — microtasks are processed before events in the same turn.
Microtask parts

Future.microtask

Microtask constructor
scheduleMicrotask

Top-level function
microtask queue

Priority queue
await

Suspension

Next lesson: Future scheduler

We Are Enhancing Your Experience

Cookie Preferences

Strictly Necessary Cookies

Analytical Cookies