Question

Can someone please post a working example of the PropertyChangedMessage being used? The description from the GalaSoft site states:

PropertyChangedMessage: Used to broadcast that a property changed in the sender. Fulfills the same purpose than the PropertyChanged event, but in a less tight way.

However, this doesn’t seem to work:

private bool m_value = false;
public bool Value
{
    get { return m_value ; }
    set 
    { 
        m_value = value;
        Messenger.Default.Send(new PropertyChangedMessage<bool>(m_value, true, "Value"));
    }

Answer 1

Daniel Castro commented on my question with the following question: “What do you expect from the code?”

The answer to this question prompted me to write this answer to my own question.

My expectations were, based on the badly written description for the PropertyChangedMessage class in the MVVM-Light documentation, that when I sent a PropertyChangedMessage then the RaisePropertyChanged method on the ViewModelBase class would get automatically called.

Apparently, however, it’s the other way around. When you call RaisePropertyChanged, then that method has an overload where you can set a flag which determines whether or not a PropertyChangedMessage will be sent.

However, I want the functionality that I originally expected. I want to send off a new PropertyChangedMessage that automatically causes RaisePropertyChanged to be called. Here’s how to do that.

Derive a new class from ViewModelBase with the following public NotifyPropertyChanged method which simply calls the protected RaisePropertyChanged method:

public abstract class MyViewModelBase : GalaSoft.MvvmLight.ViewModelBase
{
    public void NotifyPropertyChanged(string propertyName)
    {
        RaisePropertyChanged(propertyName);
    }
}

Then derive a new class from PropertyChangedMessage which calls the new NotifyPropertyChanged method:

public class MyPropertyChangedMessage<T> : PropertyChangedMessage<T>
{
    public MyPropertyChangedMessage(object sender, T oldValue, T newValue, string propertyName)
        : base(sender, oldValue, newValue, propertyName)
    {
        var viewModel = sender as MyViewModelBase;

        if (viewModel != null)
        {
            viewModel.NotifyPropertyChanged(propertyName);
        }
    }

    public MyPropertyChangedMessage(object sender, object target, T oldValue, T newValue, string propertyName)
        : base(sender, target, oldValue, newValue, propertyName)
    {
        var viewModel = sender as MyViewModelBase;

        if (viewModel != null)
        {
            viewModel.NotifyPropertyChanged(propertyName);
        }
    }
}

I have tested this approach and verified that I can indeed write code like the following which causes the UI to update properly:

private bool m_value = false;
public bool Value
{
    get { return m_value; }
    set
    {
        Messenger.Default.Send(new MyPropertyChangedMessage<bool>(this, m_value, value, "Value"));
        m_value = value;
    }
}