How to find difference between two dates in flutter?

Question

I am trying out a flutter application which can calculate the difference between two dates and display it in days.

But for some reason, I couldn't get the logic correct. I created enough variables to hold the current date and another variable to hold the old date, But still, I'm missing something.

I used FLutter_datetime_picker package.

import 'package:flutter/cupertino.dart';
import 'package:flutter/material.dart';
import 'package:flutter_datetime_picker/flutter_datetime_picker.dart';

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData.dark(),
      home: MyHomePage(),
    );
  }
}

class MyHomePage extends StatefulWidget {
  MyHomePage({Key key}) : super(key: key);

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

class _MyHomePageState extends State<MyHomePage> {
  @override
  Widget build(BuildContext context) {
    final currentDate = DateTime.now();
    final oldDate = DateTime.tryParse();
    final difference = currentDate.difference(oldDate).inDays;
    return Scaffold(
      body: Row(
        children: <Widget>[
          Center(
            child: RaisedButton(
                child: Text("data"),
                onPressed: () => DatePicker.showDatePicker(context,
                        showTitleActions: true,
                        currentTime: currentDate,
                        minTime: DateTime(1990, 1, 1), onConfirm: (date) {
                      setState(() {
                        date = oldDate;
                      });
                      print(difference);
                    })),
          ),
          Text(difference.toString())
        ],
      ),
    );
  }
}

Answer 1

You can calculate the difference when you select new date in onConfirm method like this:

class _MyHomePageState extends State<MyHomePage> {
  DateTime currentDate = DateTime.now();
  String difference = "";

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: Row(
        children: <Widget>[
          Center(
            child: RaisedButton(
              child: Text("data"),
              onPressed: () => DatePicker.showDatePicker(
                context,
                showTitleActions: true,
                currentTime: currentDate,
                minTime: DateTime(1990, 1, 1),
                onConfirm: (date) {
                  setState(() {
                    difference = "${currentDate.difference(date).inDays}";
                  });
                  print(difference);
                },
              ),
            ),
          ),
          Text(difference),
        ],
      ),
    );
  }
}

Answer 2

You should put the variables outside of the build() method, because when you call setState() the build() method is called again.

You could initialize variables inside the initState() method, or in this case, you could just declare them with initialization.