Dart Flutter：Function 未执行

Question

I am new to Dart & Flutter, I am trying to test Future functionality. I wrote a short async function, in which two Future objects are nested. Problem: The function does not execute, and when I try to attribute it to a variable I get an error This expression has a type of 'void' so its value can't be used. and Only static members can be accessed in initializers. . Here is the code: [![enter image description here][1]][1]

  @override
  _ChoseLocationState createState() => _ChoseLocationState();
}

class _ChoseLocationState extends State<ChoseLocation> {
  int counter = 0;
  
  void simulateRequest() async {

    // first future holds family name
    String famNameFunc = await Future.delayed(Duration(seconds: 2), (){
      String famName = 'Shanshi';
      return famName;
    });


    // second future holds first name
    String compName = await Future.delayed(Duration(seconds: 1), (){
      String fstName = 'Yoshi';
      String compName = '$fstName - $famNameFunc';
      return compName;
    });

    print(compName);
  }

  dynamic funcex = simulateRequest();

  @override
  void initState() {
    super.initState();
    print('This is the initial state.');
  }
  
  
  @override
  Widget build(BuildContext context) {
    print('This is the build function processing.');
    return Scaffold(
        appBar: AppBar(
          backgroundColor: Colors.red,
          title: Text('Set Location'),
          centerTitle: true,
          ),
        body: RaisedButton(
          onPressed: (){
          setState(() {
            counter++;
          });
        },
          color: Colors.blue,
          child: Text('Counter is: $counter'),
        ),
      );
  }
}```

[1]: https://i.stack.imgur.com/XmvY9.png

Answer 1

Try the following:

class ChoseLocation extends StatefulWidget {
  ChoseLocation({Key key, this.title}) : super(key: key);

  final String title;

  @override
  _ChoseLocationState createState() => _ChoseLocationState();
}

class _ChoseLocationState extends State<ChoseLocation> {
  int counter = 0;
  
  void simulateRequest() async {

    // first future holds family name
    String famNameFunc = await Future.delayed(Duration(seconds: 2), (){
      String famName = 'Shanshi';
      return famName;
    });


    // second future holds first name
    String compName = await Future.delayed(Duration(seconds: 1), (){
      String fstName = 'Yoshi';
      String compName = '$fstName - $famNameFunc';
      return compName;
    });

    print(compName);
  }

//   dynamic funcex = simulateRequest();

  @override
  void initState() {
    super.initState();
    simulateRequest();
    print('This is the initial state.');
  }
  
  
  @override
  Widget build(BuildContext context) {
    print('This is the build function processing.');
    return Scaffold(
        appBar: AppBar(
          backgroundColor: Colors.red,
          title: Text('Set Location'),
          centerTitle: true,
          ),
        body: RaisedButton(
          onPressed: (){
          setState(() {
            counter++;
          });
        },
          color: Colors.blue,
          child: Text('Counter is: $counter'),
        ),
      );
  }
}

You need to call simulateRequest() inside a method example initState , and since it doesnt return anything then you cannot assign it to a variable.

Answer 2

simulateRequest has a return type of void , so if you'r trying to store this function in the variable you shouldn't put parenthesis. If you use the parenthesis you'll be running the function and assigning it's returned value which is void to the variable funcex and thats why u're getting: This expression has a type of 'void' so its value can't be used.

      @override
  void initState() {
    dynamic funcex = simulateRequest;
    funcex();
    super.initState();
    print('This is the initial state.');
  }