C#, WPF, MVVM and INotifyPropertyChanged

Question

I'm getting confused; I thought I understood INotifyPropertyChanged.

I have a small WPF app with a frontend MainWindow class, a viewmodel in the middle and a model at the back.

The model in my case is the Simulator class.

The SimulatorViewModel is nearly transparent and just interfaces properties between MainWindow and Simulator.

Simulator implements INotifyPropertyChanged and each property setter in Simulator calls the RaisePropertyChanged method:

private string serialNumber;
public string SerialNumber
{
    get { return serialNumber; }
    set
    {
        serialNumber = value;
        RaisePropertyChanged("SerialNumber");
    }
}

public event PropertyChangedEventHandler PropertyChanged;

public void RaisePropertyChanged(string propName)  
{  
    if (this.PropertyChanged != null)  
    {  
        this.PropertyChanged(this, new PropertyChangedEventArgs(propName));  
    }  
}    

In the xaml, I have a TextBox with binding like this:

Text="{Binding Path=SerialNumber, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}"

and the DataContext is the SimulatorViewModel (but see my comment about changing the DataContext to the model)

The ViewModel just passes the properties through:

public string SerialNumber  
{  
    get { return Simulator.SerialNumber; }  
    set { Simulator.SerialNumber = value; }  
}  

Programmatic updates to the property SerialNumber in Simulator are not propagating up to the UI although curiously the initial value, set in the Simulator constructor, is getting there.

If I break on the SerialNumber setter in the Simulator and follow into RaisePropertyChanged, I find that PropertyChanged is null and so the event is not propagating upwards to the GUI.

Which leads me to several questions:

1. What exactly should be hooking into the PropertyChanged event? I'm looking to be more specific than just "WPF". What's the link between that event and the Binding statement in the xaml?

2. Why is the initial property value getting up to the UI at startup, but not subsequently?

3. Am I right to have Simulator (the model) implementing INotifyPropertyChanged, or should it be the ViewModel doing that? If the ViewModel does it, then programmatic changes in the model don't trigger PropertyChanged, so I'm unclear on the correct pattern. I realise my ViewModel is virtually redundant, but that's due to the simplicity of the project; a more complex one would make the ViewModel concept work harder. My understanding is that the ViewModel is the place to interface my unit tests.

Answer 1

1. The problem is that you're raising PropertyChanged on your model, but your view is bound to ViewModel. So your View subscribes only to ViewModel events (not Model's) - that is why textbox is not updated - because it never receives PropertyChanged event. One of possible solutions is to listen in ViewModel for Model PropertyChanged events and raise same event on ViewModel accordingly.

2. Initial value is being propagated because your setters/getters in ViewModel are correct, the problem is in events.

3. Yes, you are correct (see my #1), you should raise PropertyChanged on your ViewModel because View is bound to it. These events will not be triggered automatically after changes in Model, so you should listen for Model.PropertyChanged in ViewModel.

Simplest dirty fix to understand the idea:

SimulatorViewModel(Simulator model)
{
    // this will re-raise Model's events on ViewModel
    // VM should implement INotifyPropertyChanged
    // method OnPropertyChanged should raise INPC for a given property
    model.PropertyChanged += (sender, args) => this.OnPropertyChanged(args.PropertyName);
}

Answer 2

1. Always make sure that the instance of the class which has been set as Data-context should implement INotifyPropertyChanged
2. In your case Simulator implements INotifyPropertyChanged but SimulatorViewModel is set as DataContext, in this case the UI will come to know only if there are changes in the SimulatorViewModel but not in the Simulator.

What you can do is that:

You can expose some events or delegates in the Simulator class and hook to some methods in SimulatorViewModel class. Then when ever values in Simulator class changes, invoke the event or the delegate that will get Execute on SimulatorViewModel, now you can get the updated values from the simulator and update the properties in SimulatorViewModel. Make sure that you use different properties in both the class