BackgroundWorker _backTask = new BackgroundWorker();
            _backTask.DoWork += _backTask_DoWork;
            _backTask.RunWorkerCompleted += _backTask_RunWorkerCompleted;
            _backTask.RunWorkerAsync();



------------------------------

 void _backTask_DoWork(object sender, DoWorkEventArgs e)
        {
            Thread.Sleep(5000);
        }
        void _backTask_RunWorkerCompleted(object sender, RunWorkerCompletedEventArgs e)
        {
            MessageBox.Show(LayoutRoot.ActualWidth.ToString() + "  " + LayoutRoot.ActualHeight.ToString());
        }