onLocationChanged

Track location continuously with onLocationChanged.

The onLocationChanged stream

onLocationChanged is a stream that emits new location values as the device moves. It returns a Stream<LocationData> and is listened to with listen. It is used instead of getLocation for continuous tracking.

location.onLocationChanged.listen((data) {
  print(data.latitude);
});

Canceling the subscription

The listen method returns a StreamSubscription. When continuous listening is no longer needed, you cancel the subscription with subscription.cancel(). Otherwise the stream stays open and causes a memory leak.

final sub = location.onLocationChanged.listen((data) {});
// ...
await sub.cancel();

Exercise Listen to the location stream

Create a Location instance, listen to the onLocationChanged stream with listen and print the incoming latitude. Wait 10 milliseconds.

Show example solution

class LocationData {
  final double latitude;
  LocationData(this.latitude);
}

class Location {
  Stream<LocationData> get onLocationChanged => Stream.value(LocationData(41.0));
}

void main() async {
  final location = Location();
  location.onLocationChanged.listen((data) {
    print(data.latitude);
  });
  await Future.delayed(Duration(milliseconds: 10));
}

Test yourself

When does onLocationChanged emit a new value?
- When the app closes
- Only when a button is pressed
- Continuously as the device position changes
- Only once, at startup
Show answer

Continuously as the device position changes — onLocationChanged emits new location values continuously as the device moves; it does not fire once or on button press.

Listen to the stream

location.____.listen((data) {
  print(data.latitude);
});

Show answer

location.onLocationChanged.listen((data) {
  print(data.latitude);
});

onLocationChanged parts

onLocationChanged

Location change stream
listen

Listens to the stream
StreamSubscription

Subscription object
cancel

Cancels the subscription

Next lesson: Summary - Location

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