Flutter dynamically update date and time

Question

I am new to flutter and this is my first app. I am trying to make a to do list app and want to display the time left for each task in the subtitle. I have a listview and in each element I want the have the subtitle display the minute, counting downwards towards 0. Can anyone help me with this ? Thanks!

Code : -

class toDoListState extends State<toDoList>
{
  List<String> tasks = [];
  List<String> completedTasks = [];
  List<String> descriptions = [];
  List<bool> importance = [];
  List<String> time2completion = [];
  List<DateTime> time = [];
  
    Widget buildToDoList()
    {
        return new ListView.builder
        (
            itemBuilder: (context, index)
            {
              if(time2completion.length > 0 && index < time2completion.length && time2completion[index] != "none")
              {
                  if(time2completion[index] == "30 minutes")
                  {
                      time[index] = DateTime.now().add(Duration(minutes: 30));
                  }
                  else if(time2completion[index] == "1 hour")
                  {
                      time[index] = DateTime.now().add(Duration(hours: 1));
                  }
                  else if(time2completion[index] == "12 hours")
                  {
                      time[index] = DateTime.now().add(Duration(hours: 12));
                  }
                  else if(time2completion[index] == "1 day")
                  {
                      time[index] = DateTime.now().add(Duration(days: 1));
                  }
              }
                if(index < tasks.length)
                {
                    return row(tasks[index], descriptions[index], index);
                }
            },
        );
    }

    Widget row(String task, String description, int index)
    {                  
        return Dismissible(
        key: UniqueKey(),
        background: Container(color: Colors.red, child: Align(alignment: Alignment.center, child: Text('DELETE', textAlign: TextAlign.center, style: TextStyle(color: Colors.white, fontSize: 18),))),
        direction: DismissDirection.horizontal,
        onDismissed: (direction) {
        setState(() {
          tasks.removeAt(index);
          if(completedTasks.contains(task))
          {
              completedTasks.removeAt(index);
          }
          descriptions.removeAt(index);
          importance.removeAt(index);
        });
          Scaffold.of(context).showSnackBar(SnackBar(content: Text(task+" dismissed")));
        },
        child: CheckboxListTile(
          controlAffinity: ListTileControlAffinity.leading,
          title: Text(task, style: (completedTasks.contains(task)) ? TextStyle(decoration: TextDecoration.lineThrough) : TextStyle(),),
          subtitle: Text((time[index].difference(DateTime.now()).toString())),
          value: completedTasks.contains(task),
          onChanged: (bool value) {
           setState(() {
              if(!completedTasks.contains(task))
              {
                  completedTasks.add(task);
              }
              else
              {
                  completedTasks.remove(task);
              }
           });
          },
        ));
    }
}

Answer 1

You can use a timer to calculate the time differences every minute.

Timer.periodic(
  Duration(minutes: 1),
  (Timer t) => setState(() {
    // your calculation here
  }),
);

The following will create a timer object in your stateful widget and dispose of it when you navigate away from the view:

Timer _timer;

@override
void initState() {
    _timer = Timer.periodic(
        Duration(minutes: 1),
        (Timer t) => setState(() {
            // your calculation here
        }),
    );

    super.initState();
}

@override
void dispose() {
    _timer.cancel();
    super.dispose();
}

Answer 2

You can achieve this by following: Suppose you already have saved date in your todo and have already a date available in form of DateTime object here in my example I am assuming savedDateTime which can be achieved either by assigning it using .hour , .sec or parsing it from string.

Now what you do is to find what time is left that is differnce

//already assumed saved date as a DateTime object that is savedDateTime
  // it contains our saved date of note
  //now
 final currentDateTime = DateTime.now();
 final difference = currentDateTime.difference(savedDateTime);

  
 difference in Seconds,Hours,Minutes,Days is given by 
 print(difference.inSeconds);
 print(difference.inHours);
 print(difference.inMinutes);
 print(difference.inDays);