Passing a List of objects between Dart and JavaScript — convert List<dynamic> to List<GeocodeResult>

Question

I'm trying to access the Google Maps Geocoding JavaScript API from Dart.

I have this code, which uses the js package to translate to Dart:

@JS('google.maps')
library google_maps;

import 'package:js/js.dart';

@JS()
class Geocoder {
  external Geocoder();
  external geocode(GeocodeOptions options, void Function(List<GeocodeResult> results, String status) callback);
}

@JS()
@anonymous
class GeocodeResult {
  // ...
}

... And I'm calling it from my application like this:

geocoder.geocode(
  GeocodeOptions(
    address: address,
    region: region,
  ),
  allowInterop((results, status) {
    // ???
  }),
);

The problem is with the type of the results list in the callback function:

void Function(List<GeocodeResult> results, String status)

With the type set to GeocodeResult , I get an error:

Expected a value of type List<GeocodeResult> but got one of type List<dynamic>

But if I change the type to dynamic , then my callback function in dart receives a List<NativeJavaScriptObject> .

I don't know what to do with a NativeJavascriptObject !

• it's not a Map , so I can't use square brackets — result['geometry']
• if I try to use List.cast<GeocodeResult>() then I get an error: dart.notNull() is not a Function
• it's a dart object, so I can't just access arbitrary properties using dot notation — result.geometry

Any and all suggestions welcome, thank you.

Answer 1

Okay, I've got it working by converting to JSON in JavaScript and then parsing the JSON into a Map in dart:

geocoder.geocode(
  GeocodeOptions(
    address: address,
    region: region,
  ),
  allowInterop((results, status) {
    final list = List<Map<String, dynamic>>();
    results.forEach((jsObject) {
      //Convert NativeJavascriptObject to Map by encoding and decoding JSON
      final json = stringify(jsObject);
      final map = Map<String, dynamic>.from(jsonDecode(json));
      list.add(map);
    });
  }),
);

This uses stringify() , below, which calls JSON.stringify() from JavaScript.

@JS('JSON.stringify')
external String stringify(Object obj);