How would I constantly update a string in xamarin Using MVVM

Making a clock that counts down to a persons death

i have thread that checks the time and then figures out how many weeks are left then displays them.

i thought id do this by using ViewModel.TimeLeftString = TimeLeft.ToString()

View

<Label Grid.Column="0" Grid.Row="1" VerticalOptions="CenterAndExpand" FontSize="Title" Text="{Binding TimeLeft}" BackgroundColor="Purple"/>

View Model

string timeLeft;

public string TimeLeft
{
    get => timeLeft;
    set
    {
        timeLeft = value;

        var args = new PropertyChangedEventArgs(nameof(TimeLeft));

        PropertyChanged?.Invoke(this, args);
    }
}

Model (this is being ran in a thread)

DateTime LastRecordedTime = DateTime.MinValue;
            while (true)
            {
                //read current time,
                DateTime CurrentTime = DateTime.Now;


                if (CurrentTime >= LastRecordedTime)
                {
                    LastRecordedTime = addInterval(CurrentTime, Interval, DurationOfInterval);
                    Console.WriteLine($"time has passed {LastRecordedTime.ToString("dd/MM/yyyy |                 HH:mm:ss")}");

                    //run the funkin program thing
                    ViewModel.TimeSpent = CurrentTime.ToString();
                    Console.WriteLine(CurrentTime.ToString());




                }
            }

This doesn't work though and displays nothing. any help is greatly appreciated

Based on the description you posted,I created a simple demo and achieved a similar function.

You can refer to the following code:

1.create a viewmodel (MyViewModel.cs) and implement interface INotifyPropertyChanged.

I also added Device.StartTimer to the constructor of the viewmodel to change the value of TimeLeft.Once changing the value of TimeLeft, the UI will update automatically.

public class MyViewModel: INotifyPropertyChanged 
    {

        string timeLeft;
        public string TimeLeft
        {
            set { SetProperty(ref timeLeft, value); }
            get { return timeLeft; }
        }
  
        public MyViewModel()
        {

            TimeLeft = "2023.5.18";


            Device.StartTimer(TimeSpan.FromSeconds(3), () => {

                DateTime LastRecordedTime = DateTime.MinValue;

                DateTime CurrentTime = DateTime.Now;

                if (CurrentTime >= LastRecordedTime)
                {
                    //LastRecordedTime = addInterval(CurrentTime, Interval, DurationOfInterval);
                    Console.WriteLine($"time has passed {LastRecordedTime.ToString("dd/MM/yyyy |HH:mm:ss")}");

                    //run the funkin program thing
                    TimeLeft = CurrentTime.ToString();
                }

                return true;

            });
        }

        bool SetProperty<T>(ref T storage, T value, [CallerMemberName] string propertyName = null)
        {
            if (Object.Equals(storage, value))
                return false;
            storage = value;
            OnPropertyChanged(propertyName);
            return true;
        }
        protected void OnPropertyChanged([CallerMemberName] string propertyName = null)
        {
            PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
        }
        public event PropertyChangedEventHandler PropertyChanged;
    }

2.MainPage.xaml

Here set the BindingContext for the page and bind TimeLeft to a Label.

<?xml version="1.0" encoding="utf-8" ?> 
<ContentPage xmlns="http://xamarin.com/schemas/2014/forms"
             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml" 
             xmlns:mvvmapp518="clr-namespace:MVVMApp518"
             x:Class="MVVMApp518.MainPage">

    <ContentPage.BindingContext>
        <mvvmapp518:MyViewModel></mvvmapp518:MyViewModel>
    </ContentPage.BindingContext>

    <StackLayout>
        <Label  VerticalOptions="CenterAndExpand" FontSize="Title" Text="{Binding TimeLeft}" />

        
    </StackLayout>
</ContentPage>

Note:

> ViewModel.TimeSpent = CurrentTime.ToString();

From the code you shared, I couldn't find the code that you changed the value of TimeLeft.

Besides,we also need to make sure to modify the same instance of class MyViewModel which is the BindingContext of your page.