scheduleMicrotask function

void scheduleMicrotask (void callback())

Runs a function asynchronously.

Callbacks registered through this function are always executed in order and are guaranteed to run before other asynchronous events (like Timer events, or DOM events).

Warning: it is possible to starve the DOM by registering asynchronous callbacks through this method. For example the following program runs the callbacks without ever giving the Timer callback a chance to execute:

main() {
  Timer.run(() { print("executed"); });  // Will never be executed.
  foo() {
    scheduleMicrotask(foo);  // Schedules [foo] in front of other events.
  }
  foo();
}

Other resources

  • The Event Loop and Dart: Learn how Dart handles the event queue and microtask queue, so you can write better asynchronous code with fewer surprises.

Implementation 

void scheduleMicrotask(void callback()) {
  _Zone currentZone = Zone.current;
  if (identical(_rootZone, currentZone)) {
    // No need to bind the callback. We know that the root's scheduleMicrotask
    // will be invoked in the root zone.
    _rootScheduleMicrotask(null, null, _rootZone, callback);
    return;
  }
  _ZoneFunction implementation = currentZone._scheduleMicrotask;
  if (identical(_rootZone, implementation.zone) &&
      _rootZone.inSameErrorZone(currentZone)) {
    _rootScheduleMicrotask(
        null, null, currentZone, currentZone.registerCallback(callback));
    return;
  }
  Zone.current.scheduleMicrotask(Zone.current.bindCallbackGuarded(callback));
}