Flutter getting data from API to build method

Question

I'm trying to accomplish what I though would be simple before I started learning flutter.

const apiKey = 'xxxxxx';
const coinApiURL = 'https://rest.coinapi.io/v1/exchangerate';

class _CurrencyState extends State<CurrencyScreen> {

void getData() async {
http.Response response = await http.get('$coinApiURL/BTC/USD?apikey=$apiKey');

String data = response.body;
double exchangeRate = jsonDecode(data)['rate'];
print(exchangeRate);
}

@override
  Widget build(BuildContext context) {
    getData(); // prints out the value
    print(exchangeRate); // error "Undefined name 'exchangeRate'"
    (...)
    child: Text($exchangeRate) // value has to be inserted here
  }

So while I can use the function getData to print the data that it contains, I can't print out the 'exchangeRate' value. Therefore I too can't insert the value into the Text widget.

Can anyone explain me in a simple words how is that working and how I can get the value of 'exchangeRate' variable and use it in the code?

That doesn't really address my issue, but I updated my question so it's easier to understand what I'm trying to accomplish — user3210787
– user3210787, Commented Sep 12, 2020 at 11:00
you cannot do it that way, you have a Future coming from getData() method so you have to use FutureBuilder as in the link i posted — pskink
– pskink, Commented Sep 12, 2020 at 11:02

Omer Gamliel · Accepted Answer · 2020-09-12 12:04:22Z

I changed your code so the Text widget is build after getData finish prosses http response data.

const apiKey = 'xxxxxx';
const coinApiURL = 'https://rest.coinapi.io/v1/exchangerate';

class _CurrencyState extends State<CurrencyScreen> {

// fetch http data and return rate value 
Future<double> getData() async {
http.Response response = await http.get('$coinApiURL/BTC/USD?apikey=$apiKey');
String data = response.body;
double exchangeRate = jsonDecode(data)['rate'];
print(exchangeRate);
// return exchangeRate value
return exchangeRate;
}

@override
  Widget build(BuildContext context) {
  return FutureBuilder<double>(
    // assign getData() to futureBuilder
    future: getData(),
    builder: (BuildContext  context, AsyncSnapshot<double> snapshot) {
      // handle getData error
      if (snapshot.hasError) {
        return Text('Error');
      }
      // handle success fetched data
      if (snapshot.hasData) {
        return Text(snapshot.data.toString());
      }
      // return progress indicator while loading data
      return Center(child: CircularProgressIndicator());
    },
     );
  }

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