Completer
- Dart course
- Intermediate
- Lesson 62/116
- 5 min
Learn manual Future control with Completer.
Completer
Completer is used to complete a Future manually. Usually APIs produce Futures; but sometimes you want to create your own Future and complete it when you choose. Completer does this.
Example: var c = Completer<String>();. This produces a String Future. You give the value with c.complete('Done');. You access the Future with c.future and wait with await.
Completer lives in the dart:async library; import 'dart:async'; is required. In real apps it is used to turn callbacks into Futures.
Completer flow
The flow of using a Completer is: first create with Completer<T>(). Then give the value with complete(value). Finally access the Future with c.future and wait with await.
Example: var c = Completer<String>(); c.complete('Done'); var r = await c.future; print(r);. Output: Done. When complete is called the Future finishes at once.
This structure is used to turn event-based APIs into Futures. For example you complete a Completer when a button is clicked; you wait for the click with await.
Practice Complete a Future with Completer
import 'dart:async' is already added. void main() async is already written. Create a Completer<String>(). Give the value with c.complete('Done'). Wait with await c.future and print the result with print.
Starter code
import 'dart:async';
void main() async {
// create a Completer<String>, give a value with complete, await the future and print
}Sample solution
import 'dart:async';
void main() async {
var c = Completer<String>();
c.complete('Done');
var r = await c.future;
print(r);
}Expected output
Done
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Creating a Completer
var c = ____<String>();var c = Completer<String>();Explanation: Completer<String>() creates a manually managed Future.
2 Which method is called to complete a Completer?
var c = Completer<String>();
c.???('Done');- then
- complete Correct answer
- wait
- close
Explanation: the complete method gives the Future a value and finishes it.
3 Match the Completer parts
| Term | Meaning |
|---|---|
Completer<String>() |
Creation |
c.complete('Done') |
Completion |
c.future |
Future access |
await c.future |
Waiting |