Listen Property Changed Events in a weak mode for Windows Phone 8.1 RT

I have the following object:

public class EventCounter : INotifyPropertyChanged
{
    private int _count = 0;
    public int Count {
        get
        {
            return _count;
        }
        set
        {
            _count = value;
            NotifyPropertyChanged("Count");
        }
    }


    public event PropertyChangedEventHandler PropertyChanged;

    public void NotifyPropertyChanged(string propertyName)
    {
        if (PropertyChanged != null)
        {
            PropertyChanged(this,
                new PropertyChangedEventArgs(propertyName));
        }
    }
}

And I want to get notified everytime the Count value is changed with with a "weak event pattern"

How can I do it?