c# Bind countdown timer to textbox in windows store app

Question

I'm fairly new to C# and to programming in general.

In a previous question "C# Reset a countdown timer-DispatcherTimer-" I got help to reset my timer. Then I tried to make my code more elegant and tried to create a separate class for the timer and update the countdown text block through databinding instead of hardcoding the text property in this line in timer_Tick():

  Countdown.Text = (int)(duration - sw.Elapsed).TotalSeconds + " second(s)

My problem is that the binding fails. I still struggle with MVVM. Here is my code:

CountDownTimer.cs

class CountDownTimer : DispatcherTimer
{
    public System.Diagnostics.Stopwatch sw { get; set; }

    static readonly TimeSpan duration = TimeSpan.FromSeconds(60);

    private int _seconds;

    public int Seconds
    {
        get { return _seconds; }
        set { _seconds = value; NotifyPropertyChanged("Seconds"); }
    }

    private string _timeElapsed;

    public string TimeElapsed
    {
        get { return _timeElapsed; }
        set { _timeElapsed = value; NotifyPropertyChanged("TimeElapsed"); }
    }

    public event PropertyChangedEventHandler PropertyChanged;

    protected void NotifyPropertyChanged(string propertyName)
    {
        if (PropertyChanged != null)
        {
            PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
        }
    }


    public void timer_Tick(object sender, object e)
    {
        if (sw.Elapsed <= duration)
        {
            Seconds = (int)(duration - sw.Elapsed).TotalSeconds;
            TimeElapsed = String.Format("{0} second(s)", Seconds);
        }
        else
        {
            TimeElapsed = "Times Up";
            this.Stop();
        }
    }
}

EquationView.xaml

 <StackPanel x:Name="timePanel" Orientation="Horizontal" Visibility="Collapsed">
            <TextBlock Text="Time Left: " Height="auto" 
                       Margin="20,10,5,10" FontSize="26"/>
            <TextBlock x:Name="countdown" Text="{Binding TimeElapsed}" 
                       Margin="20,10,20,10" Width="200"  
                       Height="auto" FontSize="26"/>
 </StackPanel>

EquationView.xaml.cs

public sealed partial class EquationView : Page
{

    //code

    private void startButton_Click(object sender, RoutedEventArgs e)
    {
        //more code

        // If level == difficult enable timer
        if (Level == PlayerModel.LevelEnum.Difficult)
        {
            // timer commands
            timer.sw = System.Diagnostics.Stopwatch.StartNew();
            timer.Interval = new TimeSpan(0, 0, 0, 1);
            timer.Tick += timer.timer_Tick;
            timer.Start();

            countdown.DataContext = timer;

     //more code

    } //end of method

    // much more code

} //end of class EquationView

I inserted the line countdown.Text = timer.TimeElapsed; to try to figure out what was off and it gave me a System.NullReferenceException. Then I changed it to timer.Seconds.ToString() the first time it showed 0 but after that it returns 56 or 57.

p.s. I retyped the property changed method from my BindableBase class because I don't want to deal with multiple inheritance right now.

Answer 1

I changed the countdown timer seeing this question: How do I display changing time within a TextBlock?. I'd seen before I started building the timer but it helped me more now.

CountDownTimer.cs

public class CountDownTimer :  BindableBase
{
    System.Diagnostics.Stopwatch sw;

    static readonly TimeSpan duration = TimeSpan.FromSeconds(60);

    private DispatcherTimer timer;

    public CountDownTimer() 
    {
        timer = new DispatcherTimer();
        sw = System.Diagnostics.Stopwatch.StartNew();
        timer.Interval = new TimeSpan(0, 0, 0, 1);
        timer.Tick += timer_Tick;
    }

    private int? _seconds;

    public int? Seconds
    {
        get { return _seconds; }
        set { _seconds = value; NotifyPropertyChanged("Seconds"); }
    }

    private string _timeElapsed;

    public string TimeElapsed
    {
        get { return _timeElapsed; }
        set { _timeElapsed = value; NotifyPropertyChanged("TimeElapsed"); }
    }


    public void timer_Tick(object sender, object e)
    {
        if (sw.Elapsed < duration)
        {
            Seconds = (int)(duration - sw.Elapsed).TotalSeconds;
            TimeElapsed = String.Format("{0} second(s)", Seconds);
        }
        else
        {
            TimeElapsed = "Times Up";
            timer.Stop();
        }
    }

   public void StartCountDown()
    {
        sw.Start();
        timer.Start();
    }

    public void StopCountDown()
    {
        timer.Stop();
        sw.Stop();
    }
}