Don't Use Function in Future Builder

Don't do this

class FutureDemoPage extends StatelessWidget {
  /// Function that will return a
  /// "string" after some time
  /// To demonstrate network call
  /// delay of [2 seconds] is used
  ///
  /// This function will behave as an
  /// asynchronous function
  Future<String> getData() {
    return Future.delayed(Duration(seconds: 2), () {
      return "I am data";
      // throw Exception("Custom Error");
    });
  }
 
  @override
  Widget build(BuildContext context) {
    return SafeArea(
      child: Scaffold(
        appBar: AppBar(
          title: Text('Future Demo Page'),
        ),
        body: FutureBuilder(
          builder: (ctx, snapshot) {
            // Checking if future is resolved or not
            if (snapshot.connectionState == ConnectionState.done) {
              // If we got an error
              if (snapshot.hasError) {
                return Center(
                  child: Text(
                    '${snapshot.error} occurred',
                    style: TextStyle(fontSize: 18),
                  ),
                );
 
                // if we got our data
              } else if (snapshot.hasData) {
                // Extracting data from snapshot object
                final data = snapshot.data as String;
                return Center(
                  child: Text(
                    '$data',
                    style: TextStyle(fontSize: 18),
                  ),
                );
              }
            }
 
            // Displaying LoadingSpinner to indicate waiting state
            return Center(
              child: CircularProgressIndicator(),
            );
          },
 
          // Future that needs to be resolved
          // inorder to display something on the Canvas
          future: getData(),
        ),
      ),
    );
  }
}

Do this

Future<String> _value;
 
  @override
  initState() {
    super.initState();
    _value = getValue();
  }

  FutureBuilder<String>(
    future: _value,
    // other arguments
  )