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

void main() => runApp(new MyApp());

class MyApp extends StatefulWidget {
  @override
  MyAppState createState() {return new MyAppState();}
}
typedef OperatorFunc = double Function(double accu, double operand);

class MyAppState extends State<MyApp> {
  double accu = 0.0;
  double operand = 0.0;
  OperatorFunc queuedOperation;
  String resultString = "0.0";
  void numberPressed(int value)  {
    operand = operand * 10 + value;
    setState(() => resultString = operand.toString());
  }
  void calc(OperatorFunc operation) {
      if (operation == null) // C was pressed
      { accu = 0.0; } else
      { accu = queuedOperation != null ? queuedOperation(accu, operand) : operand; }
      queuedOperation = operation;
      operand = 0.0;
      var result = accu.toString();
      setState(() => resultString = result.toString().substring(0, min(10,result.length) ));
  }
  List<Widget>  buildNumberButtons( int count,int from, int flex) {
    return new Iterable.generate(count, (index) {
          return new Expanded(flex: flex, child: new Padding(padding: const EdgeInsets.all(1.0), child: FlatButton(onPressed: () => numberPressed(from + index), color: Colors.white, child: Text("${from + index}", style: TextStyle(fontSize: 40.0),)),),); }).toList();
  }
  Widget buildOperatorButton(String label, OperatorFunc func, int flex, {Color color = Colors.amber}){
      return Expanded(flex: flex,
              child: Padding(
                  padding: const EdgeInsets.all(1.0),
                  child: FlatButton(onPressed: () => calc(func), color: color, 
                      child: Text(label, style: TextStyle(fontSize: 40.0),)),
                      ),); }
  Widget buildRow(int numberKeyCount, int startNumber,  int numberFlex, String operationLabel, OperatorFunc operation, int operrationFlex){
      return new Expanded(child: 
                Row(crossAxisAlignment: CrossAxisAlignment.stretch,
                  children: new List.from(buildNumberButtons(numberKeyCount,startNumber ,numberFlex,))
                                    ..add(buildOperatorButton(operationLabel, operation, operrationFlex))));
  }
  @override
  Widget build(BuildContext context) {
    return new MaterialApp(      
      home: new SafeArea(
              child: new Material(color: Colors.black,
                child: Column( crossAxisAlignment: CrossAxisAlignment.end,
                   children: <Widget>[
            Expanded(child: Row(crossAxisAlignment: CrossAxisAlignment.center, mainAxisAlignment: MainAxisAlignment.end, children: [Text(resultString,textAlign: TextAlign.right,  style: TextStyle(fontSize: 50.0, color: Colors.white),)])),
            buildRow(3,7,1,"/", (accu, dividor)=> accu / dividor , 1), buildRow(3,4,1,"x", (accu, dividor)=> accu * dividor , 1), buildRow(3,1,1,"-", (accu, dividor)=> accu - dividor , 1), buildRow(1, 0,3,"+", (accu, dividor)=> accu + dividor , 1),
            Expanded(child:Row(crossAxisAlignment: CrossAxisAlignment.stretch,children: [buildOperatorButton("C", null, 1, color: Colors.grey),buildOperatorButton("=", (accu, dividor)=> accu, 3)],)) ],),),)); } }