I have below code in flutter:
getData() {
final linksStream = getLinksStream().listen((String uri) async {
return uri;
});
}
In getData method, I want to return the value of uri which is from a stream listener. Since this value is generated at a later time, I am thinking to response a Future object in getData method. But I don't know how I can pass the uri as the value of Future.
In javascript, I can simply create a promise and resolve the value uri. How can I achieve it in dart?
In your code 'return uri' is not returning from getData but returning from anonymous function which is parameter of listen.
Correct code is like:
Future<String> getData() {
final Completer<String> c = new Completer<String>();
final linksStream = getLinksStream().listen((String uri) {
c.complete(uri);
});
return c.future;
}
Try this
Future<String> getData() async{
final linksStream = await getLinksStream().toList();
return linksStream[0].toString();
}
Related
How do I set up a flutter method to return a future value that is drawn from the results of a future http post call inside the method?
The example code below is making a call to a web URL to add a new product. I want this method to return just the Id of the newly created product (i.e. 'name' inside response)
Future<String> add(Product aNewProduct) async {
var aUrl = Uri.parse(dbUrl);
http.post(aUrl,body: toBody(aNewProduct),).then((response) {
var aStr = json.decode(response.body)['name'];
return Future<String>.value(aStr);
});
}
With the code above, the parser is showing the following error/warning...
The body might complete normally, causing 'null' to be returned,
but the return type, 'FutureOr<String>', is a potentially non-nullable type.
(Documentation) Try adding either a return or a throw statement at the end.
Any suggestions on how to fix this?
You can use the await to get the value of a Future or rather your http request. After that you can simple decode it and return your desired behavior.
Future<String> add(Product aNewProduct) async {
var aUrl = Uri.parse(dbUrl);
final response = http.post(
aUrl,
body: toBody(aNewProduct),
);
return json.decode(response.body)['name'];
}
try this:
Future<String> add(Product aNewProduct) async {
var aUrl = Uri.parse(dbUrl);
var response= await http.post(aUrl,body: toBody(aNewProduct),);
if(response.statusCode==200){
var rawData = await response.stream.bytesToString();
Map data=json.decode(rawData);
return data['name'];
}else{
return '';
}
}
It is as simple as putting a return before the http.post statement
I'm trying to get the return value in my function but the output is 'Instance of Future' instead of the value of school field name in the database
#override
void initState() {
userId = _auth.currentUser!.uid;
publisherSchool =
getName(widget.postInfo['publisher-Id'], 'school').toString();
super.initState();
}
Future getName(String publisherUid, String fieldname) async {
DocumentSnapshot publisherSnapshot = await FirebaseFirestore.instance
.collection('users')
.doc(publisherUid)
.get();
print(publisherSnapshot.get(fieldname));
return publisherSnapshot.get(fieldname);
}
but whenever i'm printing the publisherSnapshop.get(fieldname) i'm getting the correct value from the database
There are 2 ways to do it, you can create a Future method and call it inside the initState like below:
#override
void initState() {
initial();
super.initState();
}
Future<void> initial() async {
userId = _auth.currentUser!.uid;
// Remember using `()` to wrap the `await` to get it result
publisherSchool = (await getName(widget.postInfo['publisher-Id'], 'school')).toString();
}
Or you can use .then to call it directly inside the initState:
#override
void initState() {
userId = _auth.currentUser!.uid;
getName(widget.postInfo['publisher-Id'], 'school').then((value) {
publisherSchool = value.toString();
});
super.initState();
}
When you declare the getName() function, specify the return type as Future<String>, and then when you call getName(), you need to await the result e.g. publisherSchool = await getName(widget.postInfo['publisher-Id'], 'school').toString();
The reason why you are not getting the correct response is because whenever you are working with Futures it takes some time to finish and return the results. Meanwhile it is fetching the result you have to make it await so that the program will continue once that future function is complete since await/then is nowhere to be found in your code hence the issues.
To solve this make this change:
Change
publisherSchool =
getName(widget.postInfo['publisher-Id'], 'school').toString();
To
getName(widget.postInfo['publisher-Id'],
'school').then((value){
publisherSchool=value.toString()});
I'm fetching data in flutter and created a helper class as seen below
import 'dart:convert';
import 'package:http/http.dart' as http;
class HttpClient {
final Map<String, String> _headers = {};
void setHeader(String key, String value) {
if (!_headers.containsKey(key)) {
_headers[key] = value;
}
}
Future<http.Response> get(String url) async {
try {
var uri = Uri.parse(url);
var response = await http.get(uri, headers: _headers);
if (response.statusCode == 200) {
dynamic result = jsonDecode(response.body);
return result;
} else {
throw Error();
}
} on Exception catch (_) {
rethrow;
}
}
}
HttpClient appClient = HttpClient();
I call the method from a class as shown below
abstract class AbstractTodoService {
dynamic getTodos() {}
}
class HttpTodoService implements AbstractTodoService {
#override
Future<Response> getTodos() async {
try {
var todos =
await appClient.get('https://jsonplaceholder.typicode.com/todos');
print(todos);
return todos;
} on Exception catch (error) {
print(error);
rethrow;
}
}
}
However, when I print todos or any string in the HttpTodoService after the await call to the HttpClient I do not see anything. However, when I print the result inside the HttpClient I see the response but does not return. When I return a normal string or map everything works normally but when I attempt to use the jsonDecoded response nothing returns.
Your jsonDecode(response.body); returns a List<dynamic> type, but your function return type is Future<http.Response>. This is why you are not getting data.
You can check runtime datatype of a variable by
print(result.runtimeType); // variable_name.runtimeType
Change function return types to Future<List<dynamic>> of get(String url) and getTodos() functions.
It might be because your function returns a future of http.Response which is actually the type of the response after you use http.get. After you use jsonDecode you should get a Map<String, dynamic> which represent the json.
From the documentation:
By looking at the dart:convert documentation, you’ll see that you can decode the JSON by calling the jsonDecode() function, with the JSON string as the method argument.
Map<String, dynamic> user = jsonDecode(jsonString);
print('Howdy, ${user['name']}!');
print('We sent the verification link to ${user['email']}.');
Unfortunately, jsonDecode() returns a Map<String, dynamic>, meaning that you do not know the types of the values until runtime. With this approach, you lose most of the statically typed language features: type safety, autocompletion and most importantly, compile-time exceptions. Your code will become instantly more error-prone.
For example, whenever you access the name or email fields, you could quickly introduce a typo. A typo that the compiler doesn’t know about since the JSON lives in a map structure.
After you use jsonDecode you should turn the map into the object you want to work with using factory fromJson method.
You can read more about is in the documentation https://flutter.dev/docs/development/data-and-backend/json
I'm trying to set langauge from a Future call. I can see that future returns an object with data(value has languageCode property and it's data) but I cannot set that data to a String variable
class Api {
String language() {
String langaugeCode;
getLocale().then((value) => langaugeCode = value.languageCode);
return langaugeCode;
}
Future<List<Product>> getProduct() async {
var response = await http.get(BASE_URL + 'language?begins-with=' + language() , headers: headers());
}
}
Future<String> language() async {
var local = await getLocale()
return local.languageCode;
}
Future<List<Product>> getProduct() async {
var lang = await language()
var response = await http.get(BASE_URL + 'language?begins-with=' + lang , headers: headers());
}
In order to set the value getLocale() returns to languageCode so it can be returned by language() you need to make language() async and await the result of language():
Future<String> language() async {
String langaugeCode;
final locale = await getLocale();
langaugeCode = locale.languageCode;
return langaugeCode;
}
The issue with the code in the question is that you get the value but only within the scope of the function passed into then(). Additionally language() is synchronous so it doesn't wait for getLocale() or its then() callback to execute before returning. This means the languageCode isn't available by the time the function returns a value.
Using this approach you'll also need to make sure that you only use language() in async functions and await it's result to get the value: await language().
I have a method inside a service class:
#override
Future<String> registerNewVoter(Object deviceAppInfo) async {
Dio dio = new Dio();
final url = API().endpointVoterUri(EndpointVoter.newVoter).toString();
final header = {'Content-type': 'application/json'};
final data = await deviceAppInfo; ///need to call the method getInfo() on the Object class which returns a future
final response =
await dio.post(url, data: data, options: Options(headers: header));
if (response.statusCode == 200) {
Map map = response.data;
final uuid = map['result']['voter_uuid'];
return uuid;
}
print(
'Request $url failed\nResponse: ${response.statusCode} ${response.statusMessage}');
throw response;
}
I'm using type Object deviceAppInfo as a parameter in the method to keep the service as pure as possible(adhering to mvvm principles). The subclass is DeviceAppInfo which has an async method called getInfo()(and where the data comes from) which is supposed to be assigned to data(see the comments in the code). I'm struggling to see how I can keep the class decoupled from DeviceAppInfo class. Any suggestions...? I'm thinking of calling a factory constructor but not sure how to implement it. Here is my DeviceAppInfo class:
class DeviceAppInfo {
DeviceAppInfo({
this.platform,
this.platformVersion,
this.appVersion,
});
final String platform;
final String platformVersion;
final String appVersion;
Map<String, dynamic> toMap() => {
'platform': this.platform,
'platform_version': this.platformVersion,
'app_version': this.appVersion,
};
Future<Map<String, dynamic>> getInfo() async {
final values = await Future.wait([
getPlatform(),
getPlatformVersion(),
getProjectVersion(),
]);
return DeviceAppInfo(
platform: values[0],
platformVersion: values[1],
appVersion: values[2],
).toMap();
}
Future<String> getPlatform() async {
try {
if (Platform.isIOS) {
return 'ios';
}
return 'android';
} on PlatformException catch (e) {
return e.toString();
}
}
Future<String> getPlatformVersion() async {
try {
final platformVersion = await GetVersion.platformVersion;
return platformVersion;
} on PlatformException catch (e) {
return e.toString();
}
}
Future<String> getProjectVersion() async {
try {
final projectVersion = await GetVersion.projectVersion;
return projectVersion;
} on PlatformException catch (e) {
return e.toString();
}
}
}
I believe that DeviceAppInfo is a clear collaborator of your service, and hiding it behind Object is simply bad engineering:
it will make your Api hard to use correctly and easy to use incorrectly
Your api is no longer self-documenting, without reading the docs or code it is impossible to use it correctly.
However, it can be discussed if it should be exposed as a parameter or provided to the constructor of your service.
Having said that, There are at least 3 options that will decouple your service from DeviceAppInfo:
Option 1: Pass in the result of getInfo() to your method
least questionable and a common form of decoupling inbound data
I am a bit sceptical if you use a Map as an input type, it is still easy to provide a map with incorrect keys
Option 2: take a function as an argument
Function a bit harder to use, it is not evident what functions accross the codebase can be used (compared to a class)
Option 3: cast to dynamic
Please dont do that
Most closely matches your goal from question
function is extremely hard to use correctly Without reading docs / code
You change compile-time errors to runtime errors
Is this what you want?
#override
Future<String> registerNewVoter(DeviceAppInfo deviceAppInfo) async {
Dio dio = new Dio();
final url = API().endpointVoterUri(EndpointVoter.newVoter).toString();
final header = {'Content-type': 'application/json'};
final data = await deviceAppInfo.getInfo();
final response =
await dio.post(url, data: data, options: Options(headers: header));
if (response.statusCode == 200) {
Map map = response.data;
final uuid = map['result']['voter_uuid'];
return uuid;
}
print(
'Request $url failed\nResponse: ${response.statusCode} ${response.statusMessage}');
throw response;
}
NOTE: I just changed the type of deviceAppInfo from Object to DeviceAppInfo