How to put the result of a function into a Text widget in Flutter?

Question

I am new in this language and I am working on a BMI(body mass index) app. As you see in the picture below:

I take the user input and calculate the result, and print out the result in console. For example:

I/flutter ( 4500): 2.25
I/flutter ( 4500): 20.0  // this is the result of BMI 
I/flutter ( 4500): you are at your ideal weight. // this is the explanation

I want to show these results in a Text widget to let user see them. But I do not know how to do it. How can I take the value of the result from a function and add it to interface?

Here is my code, and in code I pointed out where did I stuck. Main function:

import 'package:flutter/cupertino.dart';
import 'package:flutter/material.dart';
import 'package:flutter/services.dart';
import 'calculation.dart';
void main() => runApp(MyApp());

/// This is the main application widget.
class MyApp extends StatelessWidget {
  static const String _title = 'BMI';

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: _title,
      home: Scaffold(
        appBar: AppBar(title: const Text(_title),
        centerTitle: true,
        backgroundColor: Colors.amber[900],
        ),
        body: Center(
          child: MyStatefulWidget(),
        ),
      ),
    );
  }
}

enum SingingCharacter { lafayette, jefferson }

/// This is the stateful widget that the main application instantiates.
class MyStatefulWidget extends StatefulWidget {
  MyStatefulWidget({Key key}) : super(key: key);

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

/// This is the private State class that goes with MyStatefulWidget.
class _MyStatefulWidgetState extends State<MyStatefulWidget> {
  SingingCharacter _character = SingingCharacter.lafayette;
  double height=1;
  double weight=1;
  String info1="";
  Widget build(BuildContext context) {
    return Padding(
      padding: const EdgeInsets.all(10.0),

      child:Scrollbar(

      child:SingleChildScrollView(
        child:Card(
          color: Colors.amber[50],
      child:Column(

        children: <Widget>[
          Padding(
            padding: const EdgeInsets.fromLTRB(0, 30, 10, 10),
            child: Text("Sex:",
            style:TextStyle(fontSize: 24, letterSpacing: 1.0)),
          ),
          ListTile(
            title: const Text('Female',
                style:TextStyle(fontSize: 18, letterSpacing: 1.0)
            ),
            leading: Radio(
              activeColor: Colors.orange,
              value: SingingCharacter.lafayette,
              groupValue: _character,
              onChanged: (SingingCharacter value) {
                setState(() {
                  _character = value;
                });
              },
            ),
          ),
          ListTile(
            title: const Text('Male',
                style:TextStyle(fontSize: 18, letterSpacing: 1.0,)
            ),
            leading: Radio(
              activeColor: Colors.orange,
              value: SingingCharacter.jefferson,
              groupValue: _character,
              onChanged: (SingingCharacter value) {
                setState(() {
                  _character = value;
                });
              },
            ),
          ),
          SizedBox(height: 10.0),

              Text("Your height:",
              style:TextStyle(fontSize: 24, letterSpacing: 1.0)
              ),
          SizedBox(height: 10),
              Padding(
                padding: const EdgeInsets.fromLTRB(30, 0, 50, 10),
                child: TextField(
                  decoration: new InputDecoration(labelText: "Your height(cm)"),
                  keyboardType: TextInputType.number,
                  inputFormatters: <TextInputFormatter>[
                    FilteringTextInputFormatter.digitsOnly
                  ], // Only numbers can be entered
                  onSubmitted: (input1){
                    if(double.parse(input1)>0){
                    setState(() => height=double.parse(input1));
                    print(input1);
                  }
                  },
                ),
          ),
                SizedBox(height: 20),
                Text("Your weight:",
              style:TextStyle(fontSize: 24, letterSpacing: 1.0)
          ),
          SizedBox(height: 10),
                Padding(
            padding: const EdgeInsets.fromLTRB(30, 0, 50, 10),
                  child:  new TextField(
                    decoration: new InputDecoration(labelText: "Your weight(kg)"),
                    keyboardType: TextInputType.number,
                    inputFormatters: <TextInputFormatter>[
                      FilteringTextInputFormatter.digitsOnly
                    ], // Only numbers can be entered
                    onSubmitted: (input2){
                      if (double.parse(input2)>0){
           //             print(weight);
                     setState(() {
                       return weight=double.parse(input2);
                     });

                    }
                    },
                  ),),
    SizedBox(height: 10,),
    RaisedButton(
      padding:EdgeInsets.fromLTRB(20, 5, 20, 5),
    onPressed: () async{
        await Calculation(height, weight);
     //   return Calculation.info1 ??? //i don't know how to take info1 from calculation function
    },
    color: Colors.amber[900],
    child:Text(
    'Calculate',
    style:TextStyle(
    color: Colors.white,
    fontSize: 30,
      letterSpacing: 2.0,
    ),
    ),
    ),
    SizedBox(height: 20,),
    Text('Results: $height,$weight'),
    // Text('Calculation.info1'), // i do not know how to show info in a text box.

    ],
      ),
        ),
    ),
      ),
    );
    }

  }

Calculation function;

import 'package:flutter/cupertino.dart';
import 'package:flutter/material.dart';
import 'package:flutter/services.dart';
import 'dart:math';


void Calculation(height,weight) {
  double squarevalue = pow(height, 2);
  double newsquare = squarevalue / 10000;
  String info1="";
  print(newsquare);
  double value = weight / newsquare;
  print(value);
  // return value.toString();
  if (value < 18.5) {
    print("your weight is less than your ideal weight.");
//    setState(() => info1="your weight is less than your ideal weight."); //i do not know how to set
  // info1 to a new text
 //   return info1;

  }
  if (value > 25) {
    if (value > 30) {
      print("your weight is more than your ideal weight, your health is under risk.");
  //    info1="your weight is more than your ideal weight, your health is under risk.";

    }
    else {
      print("your weight is more than your ideal weight.");
  //    info1="your weight is more than your ideal weight.";
    }
  }
  else {
    print("you are at your ideal weight.");
 //   info1="you are at your ideal weight.";
  }
}

Answer 1

Instead of returning void from Calculation(height,weight) function, return String and display the string value in the Text Widget.

String Calculation(height,weight) {
....// your code with all conditions
return "you are at your ideal weight."
}

In onpressed function, update the obtained string to the state variable inside setState.

 onPressed: () async{
      String info = await Calculation(height, weight);
      setState(){
        infoDisplayedInText = info;
      }
    },