title: how to notify a view that a property has changed?
body: <p>i have a PopUp control whose IsOpen property is bound to a property named IsOpen in a ViewModel that realizes the INotifyPropertyChanged, i want the PopUp control to close or open automatically according to value of the IsOpen of the ViewModel,i use the MVVM patterm, the code as follow:</p>
  
  <p>code in the ViewModel:</p>
  
  <pre><code>public class MainViewModel : ViewModelBase
  {
  
      private  bool _isOpen;
      public bool IsOpen
      {
          get
          {
  
              return this._isOpen;
          }
  
          set
          {
              if (this._isOpen != value)
              {
                  this._isOpen = value;
                  RaisePropertyChanged("IsOpen");
              }
          }
      }
  </code></pre>
  
  <p>}</p>
  
  <p>code in the View:
     //The Main is an instance of the MainViewModel below</p>
  
  <p>Popup x:Name="popUp"  IsOpen="{Binding Main.IsOpen}" </p>
  
  <p>of course, there is a method in the ViewModel that used to changed the IsOpen property,i can see its change clearly,but it does not notify the view,did i miss something?thanks in advance. </p>