MVVM update view from model

At the moment I am teaching my self WPF, with the MVVM model. And I'm getting it :bigyello:.
But one thing I don't understand. If there is a simple view:

Code:

---

                                <ProgressBar
                                Height="18"
                                Canvas.Top="89"
                                Canvas.Left="14"
                                Width="621"
                        Value="{Binding ProgressPercentage, Mode=TwoWay}"/>

---

And I have ViewModel that implements the INotifyPropertyChanged

Code:

---

public class ViewModelClass: INotifyPropertyChanged
{
private ModelClass mc;
public int ProgressPercentage
{
        get { return this.mc.ProgressPercentage;}
        set
        {
                if (this.mc.ProgressPercentage != value)
                {
                        this.zah.ProgressPercentage = value;
                        InvokePropertyChanged ("ProgressPercentage");
                }
        }
}
public event PropertyChangedEventHandler PropertyChanged;
private void InvokePropertyChanged(string propertyName)
 {
        var e = new PropertyChangedEventArgs(propertyName);
        PropertyChangedEventHandler changed = PropertyChanged;
        if (changed != null) changed(this, e);
 }
}

---

And there is a simple model:

Code:

---

public class ModelClass
{
public int ProgressPercentage { get;set;}
public void DoSomething()
{
    this.ProgressPercentage ++;
}
}

---

The model update it's %Progress, but the ViewModel and the View don't know.
If the update takes place in the ViewModel it works, because the INotifyPropertyChanged. And the Model shouldn't implement that interface (I think, because it is UI-stuff that shouldn't be in the model).
Can anyone explain how this should work?

Firstly, there is no law against the Model implementing INotifyPropertyChanged and the View binding to the model directly. When you have something as simple as that model (I assume this is a slightly contrived example, but still) there is very little harm in combining the Model and ViewModel.

In a more complicated example, you would use all the normal techniques to update the ViewModel from the Model: have the model call the ViewModel directly, have the Model raise an event that the ViewModel is subscribed to, etc...

This is very simple example (still learning). I want to know how it should be done "full MVVM". A work-around can be found easy