Completer

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.

Exercise 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.

Show example solution

import 'dart:async';

void main() async {
  var c = Completer<String>();
  c.complete('Done');
  var r = await c.future;
  print(r);
}

Test yourself

Creating a Completer

var c = ____<String>();

Show answer

var c = Completer<String>();

Which method is called to complete a Completer?
```
var c = Completer<String>();
c.???('Done');
```
- then
- complete
- wait
- close
Show answer

complete — the complete method gives the Future a value and finishes it.
Match the Completer parts

Completer<String>()

Creation
c.complete('Done')

Completion
c.future

Future access
await c.future

Waiting

Next lesson: Summary - Async

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