Future scheduler

Learn the relation between Future and the scheduler.

The Future scheduler

Futures are scheduler objects. Future.value(x) completes with x as a microtask; Future.sync(fn) calls fn immediately and yields its result; Future.delayed(d, fn) schedules fn as an event after duration d.

When a Future completes, it adds a task to the event queue that resumes awaiting code. Even without await the Future is scheduled — there's just no one waiting for its result.

Example: var f = Future.value(42); schedules the Future immediately; await f then yields the result.

Future.value scheduling

Future.value(x) completes by scheduling a microtask. So right after creating the Future you cannot read its result in the same turn; at least one microtask turn must pass.

Example: var f = Future.value(42); print('a'); var v = await f; print(v);. Order: 'a' prints (sync), then await waits for the microtask, the microtask yields 42, v=42, 42 prints.

This ordering is deterministic: 'a' first, 42 second.

Exercise Observe Future.value scheduling

Make main async. Add var f = Future.value(42);. Add print('a');. Add var v = await f; and print(v);.

Show example solution

void main() async {
  var f = Future.value(42);
  print('a');
  var v = await f;
  print(v);
}

Test yourself

Match the Future constructors

Future.value

Completes via microtask
Future.sync

Calls immediately
Future.delayed

Event after duration
Future.error

Completes with error
When does Future.value resolve?
- After one microtask turn
- Only inside an Isolate
- Only with await
- Immediately, on the same line
Show answer

After one microtask turn — Future.value resolves after one microtask turn.

Future.value call

var f = Future.____(42);

Show answer

var f = Future.value(42);

Next lesson: scheduleMicrotask

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