title: I'm using Inotify property change.... How does INotifyPropertyChanged work? how to use it?
body: <p>I don't think this was covered in any tutorial that I've read or done.  does any know how is this 
  line works?  This is a sample code on how to use sfnumerictextbox, but its default property is object.  I need it in double or integer so Inotify came but this line is i don't get.
  <strong>viewModel.Numeric = numericBox.Value == null ? null : (double?)(numericBox.Value);</strong>
  so numeric is equal to null, so what the question mark for? then after question mark is that like if null double perhaps?</p>
  
  <p>And please touch base with Inotifypropertychange.</p>
  
  <pre><code>using System;
  using System.ComponentModel;
  using Xamarin.Forms;
  
  namespace NumericTextBoxSample
  {
  // Learn more about making custom code visible in the Xamarin.Forms previewer
  // by visiting https://aka.ms/xamarinforms-previewer
  [DesignTimeVisible(false)]
  public partial class MainPage : ContentPage
  {
      public MainPage()
      {
          InitializeComponent();
      }
  
      private void Button_Clicked(object sender, EventArgs e)
      {
          **viewModel.Numeric = numericBox.Value == null ? null : (double?)(numericBox.Value);**
  
      }
  }
  
  public class ViewModel: INotifyPropertyChanged
  {
      private double? numeric;  //checkrain
  
      public double? Numeric //rain
      {
          get { return numeric; }
          set
          {
              numeric = value;
              RaisePropertyChanged("Numeric");
          }
      }
  
      public event PropertyChangedEventHandler PropertyChanged;
  
      void RaisePropertyChanged(string property)
      {
          if (PropertyChanged != null)
          {
              PropertyChanged(this, new PropertyChangedEventArgs(property));
          }
      }
  
  }
  }
  </code></pre>