public abstract class ViewModelBase : INotifyPropertyChanged

{

    private bool isbusy;

    public bool IsBusy

    {

        get

        {

            return isbusy;

        }

        set

        {

            isbusy = value;

            RaisePropertyChanged("IsBusy");

        }

    }

    public event PropertyChangedEventHandler PropertyChanged;

    protected void RaisePropertyChanged(string propertyName)

    {

        PropertyChangedEventHandler handler = PropertyChanged;

        if (handler != null)

        {

            handler(this, new PropertyChangedEventArgs(propertyName));

        }

    }

}

<extWpfTk:BusyIndicator IsBusy="{Binding IsBusy}">

        <ContentControl />

    </extWpfTk:BusyIndicator>