private async Task RefreshViewportAsync()
{
Stream imageStream = await _googleMapsApiService.GetViewportStreamAsync(_latitude, _longitude, DefaultWidth, DefaultHeight, _zoom);
await App.Current.Dispatcher.BeginInvoke((Action) delegate()
{
var bitmapImage = new BitmapImage();
bitmapImage.BeginInit();
bitmapImage.StreamSource = imageStream;
bitmapImage.EndInit();
ViewportImageSource = bitmapImage;
});
}