Putting it together
- Dart course
- Intermediate
- Lesson 116/116
- 6 min
Combine all layers into a working app.
Putting it together
In the final step you combine everything and emit tasks through a Stream. A Stream is an asynchronous data flow.
You turn a list into a Stream with Stream<Task>.fromIterable(tasks). You listen with await for (var t in stream).
For this main must now be async: void main() async { ... }. This is required to use await.
Async main and Stream
Typical usage: void main() async { var stream = Stream<Task>.fromIterable(tasks); await for (var t in stream) { print(t.title); } }.
Stream.fromIterable emits each list element in order. await for waits for each value and processes it; the loop runs until the Stream ends.
This pattern combines file + collections + Stream; it gives the skeleton of a real console app.
Practice Print tasks with a Stream
Make main async. Define var tasks = <Task>[Task('Code write'), Task('Test write')];. Create var file = File('tasks.txt'); and write with file.writeAsStringSync(tasks.map((t) => t.title).join('\n'));. Build a Stream with var stream = Stream<Task>.fromIterable(tasks); and print each title with await for (var t in stream) { print(t.title); }.
Starter code
import 'dart:io';
abstract class Trackable {
bool get isDone;
}
class Task implements Trackable {
final String title;
bool done;
Task(this.title, {this.done = false});
@override
bool get isDone => done;
}
void main() {
// create a tasks list, write to a file, print with a Stream
}Sample solution
import 'dart:io';
abstract class Trackable {
bool get isDone;
}
class Task implements Trackable {
final String title;
bool done;
Task(this.title, {this.done = false});
@override
bool get isDone => done;
}
void main() async {
var tasks = <Task>[Task('Code write'), Task('Test write')];
var file = File('tasks.txt');
file.writeAsStringSync(tasks.map((t) => t.title).join('\n'));
var stream = Stream<Task>.fromIterable(tasks);
await for (var t in stream) {
print(t.title);
}
}Expected output
Code write Test write
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Listening to a Stream
await for (var t in ____) { print(t.title); }await for (var t in stream) { print(t.title); }Explanation: await for listens to stream.
2 Stream parts
| Term | Meaning |
|---|---|
Stream.fromIterable |
List to Stream |
await for |
Listen loop |
async |
Async main |
yield |
Emit a value |
3 How is a Stream listened to?
- for (var t in stream)
- while (stream)
- await for (var t in stream) Correct answer
- stream.next()
Explanation: await for is the way to listen to a Stream.