使用INotifyPropertyChanged的WPF双向模式绑定不起作用

所以我面临一个棘手的问题。这是我的视图模型：

namespace Market.ViewModel
{
    public class BillingViewModel : ViewModelBase
    {
        #region Private Members
        private Customer _customer;
        private Product _products;
        private string _productId = "asd";
        RelayCommand _numClickedCommand;
        #endregion
        public Customer Customer
        {
            get { return _customer; }
            set
            {
                _customer = value;
                NotifyPropertyChanged("Customer");
            }
        }
        public Product Products
        {
            get { return _products; }
            set
            {
                _products = value;
                NotifyPropertyChanged("Products");
            }
        }
        public bool CanClick
        {
            get { return true; }
        }
        public string ProductId
        {
            get { return _productId; }
            set
            {
                _productId = value;
                NotifyPropertyChanged("ProductId");
            }
        }
        public ICommand NumClickedCommand
        {
            get
            {
                if (_numClickedCommand == null)
                {
                    _numClickedCommand = new RelayCommand(param => this.NumClicked(param.ToString()),
                        param => this.CanClick);
                }
                return _numClickedCommand;
            }
        }
        #region PrivateMethods
        private void NumClicked(string numClicked)
        {
            ProductId = ProductId+numClicked;
        }
        #endregion
    }
}

它继承了实现INotifyPropertyCanged的ViewModelBase。

public class ViewModelBase : INotifyPropertyChanged
{
    public event PropertyChangedEventHandler PropertyChanged;
    protected void NotifyPropertyChanged(string propertyName)
    {
        if (PropertyChanged != null)
        {
            PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
        }
    }
}

我的观点是：

<Window x:Class="Billing.MainWindow"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:controls="clr-namespace:Billing"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:viewModel="clr-namespace:Market.ViewModel;assembly=Market.ViewModel"
    xmlns:diag="clr-namespace:System.Diagnostics;assembly=WindowsBase"
    Title="MainWindow" Height="350" Width="525">
    <Window.Resources>
        <viewModel:BillingViewModel x:Key="ViewModel" />
    </Window.Resources>
    <Grid DataContext="{Binding Source={StaticResource ViewModel}}">
        <controls:NumPad HorizontalAlignment="Left" Margin="265,205,0,-192" VerticalAlignment="Top" Height="307" Width="242"/>
        <TextBox HorizontalAlignment="Left" Height="23" Margin="247,29,0,0" TextWrapping="Wrap" Text="{Binding ProductId, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}" VerticalAlignment="Top" Width="120"/>
    </Grid>
</Window>

NumClickedCommand用于此xaml:

<UserControl x:Class="Billing.NumPad"
         xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
         xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
         xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" 
         xmlns:d="http://schemas.microsoft.com/expression/blend/2008" 
         xmlns:viewModel="clr-namespace:Market.ViewModel;assembly=Market.ViewModel"
         mc:Ignorable="d" Height="109" Width="248">
    <UserControl.Resources>
        <viewModel:BillingViewModel x:Key="ViewModel"/>
    </UserControl.Resources>
    <Grid Height="109" VerticalAlignment="Top" DataContext="{Binding Source={StaticResource ViewModel}}">
        <Button Content="1" HorizontalAlignment="Left" Margin="10,0,0,0" VerticalAlignment="Top" Width="75" CommandParameter="1" Command="{Binding NumClickedCommand}"/>
        <Button Content="2" HorizontalAlignment="Left" Margin="90,0,0,0" VerticalAlignment="Top" Width="75" CommandParameter="2" Command="{Binding NumClickedCommand}"/>
        <Button Content="3" HorizontalAlignment="Left" Margin="170,0,0,0" VerticalAlignment="Top" Width="75" CommandParameter="3" Command="{Binding NumClickedCommand}"/>
        <Button Content="4" HorizontalAlignment="Left" Margin="10,27,0,0" VerticalAlignment="Top" Width="75" CommandParameter="4" Command="{Binding NumClickedCommand}"/>
        <Button Content="5" HorizontalAlignment="Left" Margin="90,27,0,0" VerticalAlignment="Top" Width="75" CommandParameter="5" Command="{Binding NumClickedCommand}"/>
        <Button Content="6" HorizontalAlignment="Left" Margin="170,27,0,0" VerticalAlignment="Top" Width="75" RenderTransformOrigin="1.034,2.171" CommandParameter="6" Command="{Binding NumClickedCommand}"/>
        <Button Content="7" HorizontalAlignment="Left" Margin="10,54,0,0" VerticalAlignment="Top" Width="75" CommandParameter="7" Command="{Binding NumClickedCommand}"/>
        <Button Content="8" HorizontalAlignment="Left" Margin="90,54,0,0" VerticalAlignment="Top" Width="75" CommandParameter="8" Command="{Binding NumClickedCommand}"/>
        <Button Content="9" HorizontalAlignment="Left" Margin="170,54,0,0" VerticalAlignment="Top" Width="75" CommandParameter="9" Command="{Binding NumClickedCommand}"/>
        <Button Content="0" HorizontalAlignment="Left" Margin="10,81,0,0" VerticalAlignment="Top" Width="75"  CommandParameter="0" Command="{Binding NumClickedCommand}"/>
        <Button Content="Submit" HorizontalAlignment="Left" Margin="90,81,0,0" VerticalAlignment="Top" Width="155" />
    </Grid>
</UserControl>

问题是更新视图模型中的ProductId不会反映在视图中。asd的初始值在应用程序启动时更新。控件包含一组实现ICommand接口的按钮，所有这些按钮都调用视图模型中的NumClicked（）。在调试过程中，如果我单击按钮NumClicked（），那么ProductId会被更新，NotifyPropertyChanged（）也会被调用，但UI不会更新，它保持不变。但如果我直接更新UI，即在文本框中输入一些值，则会调用PropertyChanged（），之后还会调用get来更新视图模型中的值。

我已经讨论了很多这样的问题，但无法了解到底是什么阻止了UI的更新。如有任何帮助，我们将不胜感激，如有遗漏，请务必询问。非常感谢。