Future scheduler
- Dart course
- Advanced
- Lesson 9/83
- 5 min
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.
Practice Observe Future.value scheduling
Make main async. Add var f = Future.value(42);. Add print('a');. Add var v = await f; and print(v);.
Starter code
void main() {
// make async; create a Future resolving to 42; print 'a'; await and print the value
}Sample solution
void main() async {
var f = Future.value(42);
print('a');
var v = await f;
print(v);
}Expected output
a 42
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Match the Future constructors
| Term | Meaning |
|---|---|
Future.value |
Completes via microtask |
Future.sync |
Calls immediately |
Future.delayed |
Event after duration |
Future.error |
Completes with error |
2 When does Future.value resolve?
- After one microtask turn Correct answer
- Only inside an Isolate
- Only with await
- Immediately, on the same line
Explanation: Future.value resolves after one microtask turn.
3 Future.value call
var f = Future.____(42);var f = Future.value(42);Explanation: Future.value(42) creates a Future that completes with 42.