从View转换为ViewModel的变量值未在View上显示

Question

我正在学习WPF MVVM，我遇到了一个问题。我每隔200ms计算一个类中的变量值，然后我将它发送到触发PropertyChanged事件的ViewModel，但该值没有出现在View上。当我将此代码复制到ViewModel构造函数的计算变量值时，一切正常。你能给我一些提示我做错了吗？

我计算值的类：

public class CalculatingSecondsTask
{
    public string seconds { get; set; }

    public CalculatingSecondsTask()
    {
        Task.Run(async () =>
        {
            int i = 0;

            while (true)
            {
                await Task.Delay(200);
                seconds = (i++).ToString();

                UpdateSecondsInViewModel(seconds);                    
            }

        });
    }

    public void UpdateSecondsInViewModel(string secs)
    {
        ViewModel test = new ViewModel();
        test.Seconds_To_Start = secs;
    }

我的ViewModel：

public class ViewModel : INotifyPropertyChanged
{
    private string mSeconds_To_Start;          
    public event PropertyChangedEventHandler PropertyChanged;        

    private void OnPropertyChanged(string propertyName)
    {
        PropertyChangedEventHandler handler = PropertyChanged;
        if (propertyName == null)
            throw new ArgumentNullException("propertyExpression");

        if (handler != null)
        {
            handler(this, new PropertyChangedEventArgs(propertyName));
        }       
    }        

    public string Seconds_To_Start
    {
        get
        {                
            return mSeconds_To_Start;
        }

        set
        {              
            mSeconds_To_Start = value;
            OnPropertyChanged(nameof(Seconds_To_Start));                                                
        }
    }

我的观点就像：

<Window x:Class="Program.Views.StartWindow"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
    xmlns:local="clr-namespace:Program.Views"
    xmlns:ViewModel="clr-namespace:Program.ViewModels"
    mc:Ignorable="d"
    Title="Program" Height="110" Width="400">
<Window.DataContext>
    <ViewModel:ViewModel/>
</Window.DataContext>

    <Grid>             

        <!-- Text -->
        <TextBlock x:Name="Seconds_To_Start"  HorizontalAlignment="Center">                    
            <Run Text="{Binding Seconds_To_Start}" />
            <Run Text="seconds" />
        </TextBlock>

    </Grid>
</Window>