flutter-examples/tip_calculator/lib/main.dart

import 'package:flutter/material.dart';

void main() {
  runApp(new MaterialApp(title: 'Tip Calculator', home: new TipCalculator()));
}

class TipCalculator extends StatelessWidget {
  double billAmount = 0.0;
  double tipPercentage = 0.0;

  @override
  Widget build(BuildContext context) {
    // Create first input field
    TextField billAmountField = new TextField(
      keyboardType: TextInputType.number,
      onChanged: (String value) {
        try {
          billAmount = double.parse(value);
        } catch (exception) {
          billAmount = 0.0;
        }
      },
      decoration: new InputDecoration(labelText: "Bill amount(\$)"),
    );

    // Create another input field
    TextField tipPercentageField = new TextField(
        decoration: new InputDecoration(labelText: "Tip %", hintText: "15"),
        keyboardType: TextInputType.number,
        onChanged: (String value) {
          try {
            tipPercentage = double.parse(value);
          } catch (exception) {
            tipPercentage = 0.0;
          }
        });

    // Create button
    RaisedButton calculateButton = new RaisedButton(
        child: new Text("Calculate"),
        onPressed: () {
          // Calculate tip and total
          double calculatedTip = billAmount * tipPercentage / 100.0;
          double total = billAmount + calculatedTip;

          // Generate dialog
          AlertDialog dialog = new AlertDialog(
              content: new Text("Tip: \$$calculatedTip \n"
                  "Total: \$$total"));

          // Show dialog
          showDialog(context: context, builder: (BuildContext context) => dialog);
        });

    Container container = new Container(
        padding: const EdgeInsets.all(16.0),
        child: new Column(
            children: [billAmountField, tipPercentageField, calculateButton]));

    AppBar appBar = new AppBar(title: new Text("Tip Calculator"));

    Scaffold scaffold = new Scaffold(appBar: appBar, body: container);
    return scaffold;
  }
}