scheduleMicrotask

Learn microtasks with scheduleMicrotask.

The scheduleMicrotask function

scheduleMicrotask(fn) adds fn to the microtask queue. It returns nothing; it guarantees ordering but does not wait for a result.

Unlike Future.microtask, it returns no Future. It is meant for small side-effecting work (printing, state updates).

Example: scheduleMicrotask(() => print('micro'));. Synchronous code continues after this line; the microtask is processed at the end of the turn.

microtask vs event order

In one turn all microtasks are drained first, then a single event is processed. Therefore work added via scheduleMicrotask runs before Future.delayed.

Example: scheduleMicrotask(() => print('m')); print('main'); await Future.delayed(Duration(milliseconds: 1));. Order: 'main' (sync), 'm' (microtask turn), then the delayed event completes.

The output is deterministic: 'main', 'm'.

Exercise Observe microtask vs event order

Make main async. Add scheduleMicrotask(() => print('m'));. Add print('ana');. Add await Future.delayed(Duration(milliseconds: 1));.

Show example solution

void main() async {
  scheduleMicrotask(() => print('m'));
  print('ana');
  await Future.delayed(Duration(milliseconds: 1));
}

Test yourself

What does scheduleMicrotask return?
- Future<void>
- Nothing (void)
- A value
- Stream
Show answer

Nothing (void) — scheduleMicrotask returns void, no Future to await.
Scheduling functions

scheduleMicrotask

Microtask queue
Future.microtask

Returns a Future
Future.delayed

Event queue
Future.sync

Calls immediately

Microtask call

____(() => print('m'));

Show answer

scheduleMicrotask(() => print('m'));

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