public static perRelayCommandAsync ObservesInternalProperty(this perRelayCommandAsync command, INotifyPropertyChanged source, string propertyName)
        {
            source.PropertyChanged += (s, e) =>
            {
                if (string.Equals(e.PropertyName, propertyName, StringComparison.InvariantCultureIgnoreCase))
                {
                    command.RaiseCanExecuteChanged();
                }
            };

            return(command);
        }
 public static perRelayCommandAsync ObservesCollection(this perRelayCommandAsync command, INotifyCollectionChanged collection)
 {
     collection.CollectionChanged += (s, e) => command.RaiseCanExecuteChanged();
     return(command);
 }