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Question

我的Flutter应用程序在加载时崩溃

它运营着一家FutureBuilder，我相信这就是问题的根源。

我的应用程序进行API调用，并将数据返回到地图标记。

当我让FutureBuilder返回一个列表视图时，它工作得很好。

然而，当我将其更改为返回包含我的Map SDK和调用API的按钮的Stack时，它在启动时崩溃。

相关代码如下，谢谢！

class HomePage extends StatefulWidget { 
@override
_HomePageState createState() => _HomePageState();
}

class _HomePageState extends State<HomePage> {
Future<Stations> stations;
BuildContext _context;
MapMarkerExample _mapMarkerExample; 


@override
void initState() {
stations = API_Call().fetchStations(); 
super.initState();
}


@override
Widget build(BuildContext context) {
return Scaffold(
appBar: AppBar(
title: Text('Example 1'),
),

body: Container(

child: FutureBuilder<Stations>(
future: stations,

builder: (context, snapshot) {

if (snapshot.hasError) {
return Text("Error");
}

if (snapshot.connectionState == ConnectionState.done) {
return 

Stack(
children: [
HereMap(onMapCreated: _onMapCreated),

Column(
mainAxisAlignment: MainAxisAlignment.start,
children: [
Row(
mainAxisAlignment: MainAxisAlignment.spaceEvenly,
children: [
button('Stations Near Me', _anchoredMapMarkersButtonClicked),
button('Clear', _clearButtonClicked),
],
),
],
),
],
);
}

return Text("Loading");


}
) 
)
);
}

api_call.dart

class API_Call {
Future<Stations> fetchStations() async {
var client = http.Client();
final response = await client.get(
'https://transit.hereapi.com/v8/stations?in=x,-x&return=transport&apiKey=API_KEY');

if (response.statusCode == 200) {
return Stations.fromJson(jsonDecode(response.body)); 
} else {
throw Exception('Failed to load stations');
}
}
}

api_manager.dart

typedef ShowDialogFunction = void Function(String title, String message);
class MapMarkerExample{


void showAnchoredMapMarkers() { 
print('step5');

GeoCoordinates geoCoordinates = _callGeoCoordinates();
// use the coords .. to add a marker
_addCircleMapMarker(geoCoordinates, 0);
_addPOIMapMarker(geoCoordinates, 1);
print('step6');
} 

GeoCoordinates _callGeoCoordinates() {
print('step7');
var stations;
Future<Stations> fetchStations() async {
stations = await API_Call().fetchStations(); 
for (Station stations in stations) {
GeoCoordinates geoCoordinates = GeoCoordinates (stations.place.location.lat, stations.place.location.lng);
// use the coords .. to add a marker
_addCircleMapMarker(geoCoordinates, 0);
_addPOIMapMarker(geoCoordinates, 1);
} 
}
}

HereMapController _hereMapController;
List<MapMarker> _mapMarkerList = [];
MapImage _poiMapImage;
MapImage _circleMapImage;
ShowDialogFunction _showDialog;
List<MapMarker3D> _mapMarker3DList = [];

MapMarkerExample(ShowDialogFunction showDialogCallback, HereMapController hereMapController) {
_showDialog = showDialogCallback;
_hereMapController = hereMapController;
double distanceToEarthInMeters = 8000;
_hereMapController.camera.lookAtPointWithDistance(
GeoCoordinates(x, -x), distanceToEarthInMeters);
// Setting a tap handler to pick markers from map.
_setTapGestureHandler();
_showDialog("Note", "Tap markers for more.");
}


void clearMap() {
for (var mapMarker in _mapMarkerList) {
_hereMapController.mapScene.removeMapMarker(mapMarker);
}
_mapMarkerList.clear();
for (var mapMarker3D in _mapMarker3DList) {
_hereMapController.mapScene.removeMapMarker3d(mapMarker3D);
}
_mapMarker3DList.clear();
}
Future<void> _addPOIMapMarker(GeoCoordinates geoCoordinates, int drawOrder) async {
// Reuse existing MapImage for new map markers.
if (_poiMapImage == null) {
Uint8List imagePixelData = await _loadFileAsUint8List('assets/poi.png');
_poiMapImage = MapImage.withPixelDataAndImageFormat(imagePixelData, ImageFormat.png);
}
Anchor2D anchor2D = Anchor2D.withHorizontalAndVertical(0.5, 1);
MapMarker mapMarker = MapMarker.withAnchor(geoCoordinates, _poiMapImage, anchor2D);
mapMarker.drawOrder = drawOrder;
Metadata metadata = new Metadata();
metadata.setString("key_poi", "Next Departures");
mapMarker.metadata = metadata;
_hereMapController.mapScene.addMapMarker(mapMarker);
_mapMarkerList.add(mapMarker);
}
Future<void> _addCircleMapMarker(GeoCoordinates geoCoordinates, int drawOrder) async {
// Reuse existing MapImage for new map markers.
if (_circleMapImage == null) {
Uint8List imagePixelData = await _loadFileAsUint8List('assets/circle.png');
_circleMapImage = MapImage.withPixelDataAndImageFormat(imagePixelData, ImageFormat.png);
}

MapMarker mapMarker = MapMarker(geoCoordinates, _circleMapImage);
mapMarker.drawOrder = drawOrder;
_hereMapController.mapScene.addMapMarker(mapMarker);
_mapMarkerList.add(mapMarker);
}
Future<Uint8List> _loadFileAsUint8List(String assetPathToFile) async {
// The path refers to the assets directory as specified in pubspec.yaml.
ByteData fileData = await rootBundle.load(assetPathToFile);
return Uint8List.view(fileData.buffer);
}
void _setTapGestureHandler() {
_hereMapController.gestures.tapListener = TapListener.fromLambdas(lambda_onTap: (Point2D touchPoint) {
_pickMapMarker(touchPoint);
});
}
void _pickMapMarker(Point2D touchPoint) {
double radiusInPixel = 2;
_hereMapController.pickMapItems(touchPoint, radiusInPixel, (pickMapItemsResult) {
// Note that 3D map markers can't be picked yet. Only marker, polgon and polyline map items are pickable.
List<MapMarker> mapMarkerList = pickMapItemsResult.markers;
if (mapMarkerList.length == 0) {
print("No map markers found.");
return;
}
});
}
}

Answer 1

在api_manager.dart中，这看起来非常可疑，并且您没有从该函数返回任何内容，它也可以解释说future not complete的错误

Future<Stations> fetchStations() async {
stations = await API_Call().fetchStations(); 

for (Station stations in stations) {
GeoCoordinates geoCoordinates = GeoCoordinates (stations.place.location.lat, stations.place.location.lng);
// use the coords .. to add a marker
_addPOIMapMarker(geoCoordinates, 1);
}   
// GeoCoordinates geoCoordinates = stations.coordinates;
// _addPOIMapMarker(geoCoordinates, 1);

}
}

你必须从中返回一个Stations对象，在你的for循环之后尝试类似return stations;的东西，它可以解决你的问题，如果错误发生变化，这也是一个好的开始。

还将您在未来建设者中的线路更改为：

if (snapshot.connectionState == ConnectionState.done && snapshot.hasData) 

同时，删除此_setTapGestureHandler()。崩溃很可能是由内存泄漏引起的，从发布的代码来看，可以用listeners来解释。