Bad state: Stream has already been listened to Flutter error - ios

Question

I am calling an api. I am getting a streamed response after sending the request. But i cannot parse the response and convert it to String/JSON. This is where I am calling the api.
static Future<String> callDeviceListFetchApi() async {
Completer completer = new Completer();
String jsonResponse;
String url = Constants.BASE_URL + Constants.DEVICE_REGISTER_URL;
var client = new http.Client();
var request = new http.Request('GET', Uri.parse(url));
request.headers[HttpHeaders.CONTENT_TYPE] = 'application/json';
request.headers[HttpHeaders.AUTHORIZATION] = '<auth code>';
await client.send(request).then((response) {
response.stream.bytesToString().then((value) {
print(value.toString());
jsonResponse = value.toString();
completer.complete(jsonResponse);
});
}).catchError((error) {
print(error.toString());
});
return completer.future;
}
I am getting the error,
Bad state: Stream has already been listened to Flutter error. Any idea why this is happening?

Answer 1

There's a couple of things wrong with your code. I think you have a slight misunderstanding about how Async and Futures work in dart - you should re-read the docs and this tutorial (part 1 and part 2).
Basically, the problem is that you were returning a 'Future' from an async function. If you return a future from an async function, it has issues (I don't know why the analyzer doesn't catch that).
Future<String> callDeviceListFetchApi() async {
Completer completer = new Completer();
String url = "<url>";
var client = new http.Client();
var request = new http.Request('GET', Uri.parse(url));
request.headers[HttpHeaders.CONTENT_TYPE] = 'application/json';
request.headers[HttpHeaders.AUTHORIZATION] =
'<auth string>';
var response = await client.send(request);
String jsonResponse;
try {
var value = await response.stream.bytesToString();
print(value.toString());
jsonResponse = value.toString();
} catch (error) {
print(error.toString());
}
return completer.complete(jsonResponse);
}
Or not async:
Future<String> callDeviceListFetchApiNotAsync() {
String url = "<url>";
var client = new http.Client();
var request = new http.Request('GET', Uri.parse(url));
request.headers[HttpHeaders.CONTENT_TYPE] = 'application/json';
request.headers[HttpHeaders.AUTHORIZATION] =
'<auth string>';
Completer completer = new Completer();
return client.send(request).then((response) {
return response.stream.bytesToString();
}).then((value) {
print(value.toString());
return value.toString();
}).catchError((error) {
print(error.toString());
// if you use catchError, whatever you return from it
// is the value you'll get wherever you resolve the future.
return null;
});
}
But unless you're trying to do something I'm not seeing, there's a way easier way to do this (assuming all you want to do is get a string from a server):
Future<String> getList() async {
var response = await http.get("<url>", headers: {
HttpHeaders.CONTENT_TYPE: 'application/json',
HttpHeaders.AUTHORIZATION: '<auth string>',
});
if (response.statusCode == 200) {
return response.body;
} else {
throw Error();
}
}