How to Concatenate Strings & fetch data flutter - flutter

Question

I have problem in api calling i getting a geolocation of current place,i am passing the latitude&longitude
but i having a problem is i need to pass the latitude&longitude to certain format like this lat_11.3054724$75.8744252 so i can't try to concatinate the $ sign along with it,Also i am not getting any data when i pass latitude&longitude data i cannot use in api it throws
unhandled Exception: NoSuchMethodError: The getter 'latitude' was called on null.
E/flutter (27500): Receiver: null
E/flutter (27500): Tried calling: latitude
But i can print the data to Text but not pass to api
Code
Future<String> getMainbanner() async {
var latitude=_currentPosition.latitude.toString();
var longitude=_currentPosition.longitude.toString();
var response = await http.post(Urls.HOME_BANNER,
headers: {"Content-Type": "application/json"},
body: json.encode({
"banner_type": "Main_Banner",
"location": "lat_"+latitude+'$'+longitude,
}),);
Map<String, dynamic> value = json.decode(response.body);
if (response.statusCode == 200) {
var resp = response.body;
Map<String, dynamic> value = json.decode(resp);
var message = value['msg'];
var banner =value['bannerapp'][0];
for (int i = 0; i < banner.length; i++) {
var data = banner[i];
print("Data:"+data);
}
}
else
{
CustomDialogs().showErrorAlert(context, "Main Banner Image NotFound");
}
}
Code for fetching Current location
_getCurrentLocation() {
final Geolocator geolocator = Geolocator()..forceAndroidLocationManager;
geolocator
.getCurrentPosition(desiredAccuracy: LocationAccuracy.best)
.then((Position position) {
setState(() {
_currentPosition = position;
});
}).catchError((e) {
print(e);
});
}

Answer 1

Edit: The problem is, that _currentLocation is still null when you call _currentLocation.latitude, because _getcurrentLocation() needs some time to set it. There are different approaches to make it work, depending on how your architecture looks.
Change _getCurrentLocation() to an async function
Future<void> _getCurrentLocation() async {
final Geolocator geolocator = Geolocator()..forceAndroidLocationManager;
try {
final position = await geolocator.getCurrentPosition(desiredAccuracy: LocationAccuracy.best);
setState(() {
_currentPosition = position;
});
} catch(e) {
print(e);
}
}
And await on it inside getMainbanner
Future<String> getMainbanner() async {
await _getCurrentLocation();
var latitude=_currentPosition.latitude.toString();
var longitude=_currentPosition.longitude.toString();
var response = await http.post(Urls.HOME_BANNER,
headers: {"Content-Type": "application/json"},
body: json.encode({
"banner_type": "Main_Banner",
"location": "lat_"+latitude+'$'+longitude,
}),);
Map<String, dynamic> value = json.decode(response.body);
if (response.statusCode == 200) {
var resp = response.body;
Map<String, dynamic> value = json.decode(resp);
var message = value['msg'];
var banner =value['bannerapp'][0];
for (int i = 0; i < banner.length; i++) {
var data = banner[i];
print("Data:"+data);
}
}
else
{
CustomDialogs().showErrorAlert(context, "Main Banner Image NotFound");
}
}
The problem is the $. $ is a special character in dart strings for interpolation so you have to add an escape \ before it.
See here:
https://dart.dev/guides/language/language-tour#strings
And you can make use of this string interpolation to build your string:
var latitude=_currentPosition.latitude;
var longitude=_currentPosition.longitude;
....
"location":"lat_$latitude\$$longitude"
...
Also, you don't need the toString() for latitude and longitude