Unhandled Exception: type 'String' is not a subtype of type 'double' even if a double is given

问题

I'm fetching coordinates from Firestore to fill my map with stores locations. The Google Map Marker needs a LatLng object (that takes two double as parameters) with the latitude and longitude of a point. But, for each increment, I get the following exception: type 'String' is not a subtype of type 'double' (verbose down below).

However, the points do appear on the map, which should mean that the values ARE of type double...

I used the following code to identify the latitude and longitude type:

if (store['latitude'] is double) {
  print('double!');
} else if (store['latitude'] is String) {
  print('string!');
}
print('');
if (store['longitude'] is double) {
  print('double!');
} else if (store['longitude'] is String) {
  print('string!');
}
print('');

It says "double!" each time.

Here is the code:

Firestore.instance
        .collection('points-of-sale')
        .snapshots()
        .listen((data) => data.documents.forEach((store) {
              LatLng pos = LatLng(
                 store['latitude'], // Here
                 store['longitude'] // and here
              );
              /* Adding a marker to the map */
            }));

This is the full error:

[VERBOSE-2:ui_dart_state.cc(148)] Unhandled Exception: type 'String' is not a subtype of type 'double'
#0      _StoreLocatorState._addMarker (package:leonor_greyl_app_2019_41820/pages/StoreLocatorWidget.dart:21:30)
#1      _StoreLocatorState.build.<anonymous closure>.<anonymous closure> (package:leonor_greyl_app_2019_41820/pages/StoreLocatorWidget.dart:48:15)
#2      List.forEach (dart:core-patch/growable_array.dart:278:8)
#3      _StoreLocatorState.build.<anonymous closure> (package:leonor_greyl_app_2019_41820/pages/StoreLocatorWidget.dart:47:42)
#4      _rootRunUnary (dart:async/zone.dart:1132:38)
#5      _CustomZone.runUnary (dart:async/zone.dart:1029:19)
#6      _CustomZone.runUnaryGuarded (dart:async/zone.dart:931:7)
#7      _BufferingStreamSubscription._sendData (dart:async/stream_impl.dart:336:11)
#8      _DelayedData.perform (dart:async/stream_impl.dart:591:14)
#9      _StreamImplEvents.handleNext (dart:async/stream_impl.dart:707:11)
#10     _PendingEvents.schedule.<anonymous closure> (dart:a<…>

EDIT: This is the full verbose output for parseDouble(store['latitude']):

[VERBOSE-2:ui_dart_state.cc(148)] Unhandled Exception: FormatException: Invalid double
x
#0      double.parse (dart:core-patch/double_patch.dart:110:28)
#1      _StoreLocatorState._addMarker (package:leonor_greyl_app_2019_41820/pages/StoreLocatorWidget.dart:22:66)
#2      _StoreLocatorState.build.<anonymous closure>.<anonymous closure> (package:leonor_greyl_app_2019_41820/pages/StoreLocatorWidget.dart:51:15)
#3      List.forEach (dart:core-patch/growable_array.dart:278:8)
#4      _StoreLocatorState.build.<anonymous closure> (package:leonor_greyl_app_2019_41820/pages/StoreLocatorWidget.dart:50:42)
#5      _rootRunUnary (dart:async/zone.dart:1132:38)
#6      _CustomZone.runUnary (dart:async/zone.dart:1029:19)
#7      _CustomZone.runUnaryGuarded (dart:async/zone.dart:931:7)
#8      _BufferingStreamSubscription._sendData (dart:async/stream_impl.dart:336:11)
#9      _DelayedData.perform (dart:async/stream_impl.dart:591:14)
#10     _StreamImplEvents.handleNext (dart:async/stream_impl.dart:707:11)
#11     _<…>

回答1:

Use the parseDouble function that returns null if the Object provided can not be parsed to a double. You can then check if the values of the longitude and latitude are null before creating the marker.

  Firestore.instance
      .collection('points-of-sale')
      .snapshots()
      .listen((data) => data.documents.forEach((store) {
            double latitude = parseDouble(store['latitude']);
            double longitude = parseDouble(store['longitude']);

                // Check if parser returns null for non-parsable string
            if (latitude != null && longitude != null) {
              LatLng pos = LatLng(latitude, longitude);
              /* Adding a marker to the map */
            }
          }));

parse Function

double parseDouble(dynamic value) {
  try {
    if (value is String) {
      return double.parse(value);
    } else if (value is double) {
      return value;
    } else {
      return 0.0;
    }
  } catch (e) {
    // return null if double.parse fails
    return null;
  }
}

来源：https://stackoverflow.com/questions/56253227/unhandled-exception-type-string-is-not-a-subtype-of-type-double-even-if-a-d