How can I use Writablebitmap in ScheduledTask in windows phone 8? - windows-phone-8

Question

I want to update my live tile by creating and saving a writable bitmap image in scheduled task agent.my code works inside the app but in background task, it breaks into the debugger
I'm using this simple code:
protected override void OnInvoke(ScheduledTask task)
{
#if DEBUG_AGENT
ScheduledActionService.LaunchForTest(task.Name, TimeSpan.FromSeconds(30));
#endif
CreateWideTile();
NotifyComplete();
}
and my code to create and save image is this:
private void CreateWideTile()
{
int width = 691;
int height = 336;
string imagename = "WideBackground";
WriteableBitmap b = new WriteableBitmap(width, height);
var canvas = new Grid();
canvas.Width = b.PixelWidth;
canvas.Height = b.PixelHeight;
var background = new Canvas();
background.Height = b.PixelHeight;
background.Width = b.PixelWidth;
//Created background color as Accent color
SolidColorBrush backColor = new SolidColorBrush(Colors.Red);
background.Background = backColor;
var textBlock = new TextBlock();
textBlock.Text = "Example text";
textBlock.FontWeight = FontWeights.Normal;
textBlock.TextAlignment = TextAlignment.Left;
textBlock.Margin = new Thickness(20, 20, 0, 0);
textBlock.TextWrapping = TextWrapping.Wrap;
textBlock.Foreground = new SolidColorBrush(Colors.White); //color of the text on the Tile
textBlock.FontSize = 30;
canvas.Children.Add(textBlock);
b.Render(background, null);
b.Render(canvas, null);
b.Invalidate(); //Draw bitmap
//Save bitmap as jpeg file in Isolated Storage
using (IsolatedStorageFile isf = IsolatedStorageFile.GetUserStoreForApplication())
{
using (IsolatedStorageFileStream imageStream = new IsolatedStorageFileStream("/Shared/ShellContent/" + imagename + ".jpg", System.IO.FileMode.Create, isf))
{
b.SaveJpeg(imageStream, b.PixelWidth, b.PixelHeight, 0, 100);
}
}
}
when I test the app, after executing background task, an error comes that point to Debugger.Break();
private static void UnhandledException(object sender, ApplicationUnhandledExceptionEventArgs e)
{
if (Debugger.IsAttached)
{
// An unhandled exception has occurred; break into the debugger
Debugger.Break();
}
}
I don't understand the reason. my code works in foreground app but not in background ...
any solution ????

Answer 1

You should use dispatcher, because WriteableBitmap.Render should be used in the UI thread:
protected override void OnInvoke(ScheduledTask task)
{
Deployment.Current.Dispatcher.BeginInvoke( () =>
{
CreateWideTile();
NotifyComplete();
} );
}