Function returns an instance of Future<String> instead of String

Question

I have a streambuilder, within, I want to calculate the distance for each different card of the streambuilder. It calculates the distance if I print it in the terminal, but on the screen it shows: Instance of future < String >. Is there a fix for this? I tried making a variable and run set state, but the value has to be different for each card of the streambuilder.

Future<String> distanceInMeters({double uLat, double uLong}) async {
  if (uLat == null && uLong == null) {
    return "";
  } else {
    double distanceInMeters =
        await Geolocator().distanceBetween(uLat, uLong, 52.3546274, 4.8285838);
    String distance = distanceInMeters.toString();
    return "$distance";
  }
}

can we see the code where you call this method ?

Steve
– Steve

2019-08-05 20:09:33 +00:00
Commented Aug 5, 2019 at 20:09 — Steve
– Steve, Commented Aug 5, 2019 at 20:09

creativecreatorormaybenot · Accepted Answer · 2019-08-05 20:11:02Z

1

You will need to use a FutureBuilder. If you are not familar with this widget, you can follow the link to the documentation, which describes the function of the widget extensively.

In practice, it would look something like this:

FutureBuilder(
    future: distanceInmeters(uLat: uLat, uLong: uLong),
    builder: (context, AsyncSnapshot<String> snapshot) {
      if (snapshot.hasError) return Text('${snapshot.error}');
      if (snapshot.hasData) return Text('${snapshot.data}');
      return const CircularProgressIndicator();
    },
  )

The reason you need this is that you are returning your String value from an asynchronous function and thus you need to have some logic that waits to extract the result from the Future.

answered Aug 5, 2019 at 20:11

creativecreatorormaybenot

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4 Comments

Karel Debedts Over a year ago

Tested the code, but got the text: Futurebuilder <String>

creativecreatorormaybenot Over a year ago

@KarelDebedts You need to use this as a widget and not pass it as a string.

creativecreatorormaybenot Over a year ago

@KarelDebedts How would I know? If you want me to investigate that, you should share the place you are using the widget (probably ask another question).

ombiro Over a year ago

I am in the exact scenario, below are my codes: Container(child: distanceText(),) -- where the widget is called @creativecreatorormaybenot. Future<String> getDistance( lat, long, lat1, long1) async { return geolocator.distanceBetween(lat, long, lat1, long1).toString(); } Widget distanceText() => FutureBuilder( future: getDistance( lat, long, lat1, long1), builder: (context, AsyncSnapshot<String> snapshot) { if (snapshot.hasData) return Text('${snapshot.data}'); }, ); getting on text: Instance of 'Future<double> not text

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