ImageTools only creates PNG successfully under certain conditions - vb.net

Question

I'm trying to generate images using ImageTools, and my code works and successfully creates images...but only if I have user input before trying to create the images!
If I try to generate the images in the New sub, for example, the images are created, but they only contain the textbox control from my canvas and not the image (my control consists of text + image). So the image is being created...but it's only rendering partial content.
If I put a button on my page and generate my images from the button click even handler the images are generated correctly.
So what am I doing wrong here? And how can I get my images to generate without user input (i.e. when the app launches).
I get the exact same results using WriteableBitmap in place of ImageTools, FWIW.
I create stackpanels with a canvas and my text/image elements, then use the standard code to render the images to files in isolated storage. Since it all works perfectly after user input I don't know which parts of the code to provide...I'm basically using unmodified sample code.
Code parts (this is all in my MainPage.XAML.VB):
Public Sub New()
InitializeComponent()
' some code commented out while debugging - not relevant here
SetupHubTiles() ' this is the method that sets up the images (see below)
End Sub
The SetupHubTiles method makes several calls to the following method:
Public Sub CreateHubTile(background As StackPanel, tileImage As String, tiletoupdate As HubTile)
Dim isoStoreTileImage = String.Format("isostore:{0}", tileImage)
'Create a bitmapImage to IsolatedStorage.
Using store As IsolatedStorageFile = IsolatedStorageFile.GetUserStoreForApplication()
'Tile image's Height * Width are 173 * 173.
Dim bitmap = New WriteableBitmap(173, 173)
'Render a bitmap from StackPanel.
bitmap.Render(background, New TranslateTransform())
Dim stream = store.CreateFile(tileImage)
bitmap.Invalidate()
bitmap.SaveJpeg(stream, 173, 173, 0, 100)
stream.Close()
End Using
SetHubTileImage(tileImage, tiletoupdate) ' this is what sets a control on the MainPage to display the generated image
End Sub
And finally the button click handler (which I just implemented because the code I'm using works fine in another app, but that app always gets user input before creating images, so I figured it was the only difference between the two apps)
Private Sub StartButton_Click(sender As System.Object, e As System.Windows.RoutedEventArgs) Handles StartButton.Click
SetupHubTiles()
End Sub
As you can see the code being executed is identical, but I get a different result when I run it directly in my contructor compared to running it from a button click handler.
The goal is for these images to be generated at runtime (without any interaction from the user) to be used in the UI.
I've tried a few different methods of doing this, but I always get the same results - an image is generated with just text when there should be text + image. I am using the same method in other apps with the only difference being those other apps are not creating the images as soon as the app launches, which may be the problem.
It also doesn't seem to make a difference if I change the location/type of controls that I am using to build my images.

Answer 1

Based on your comment, try to invoke your method either later in the loading sequence or use:
Dispatcher.BeginInvoke(() => { GenerateImages(); });
This will queue the function up to be run on the next UI thread tick which should be after any pending layout work which is queued up.
The fact that you are calling this function right at the end of the constructor has no consequence (other than the fact that the variables representing different objects have been initialized). It's all happening in the same UI thread tick, before Layout is kicked off so there is nothing in the StackPanel for you to capture.
To fix this, add your code to the Loaded event of the Page and still wrap it in a Dispatcher.BeginInvoke call so that it is guaranteed to happen after all of your Controls have rendered (at least their first pass, not withstanding any content that is loaded after the startup sequence).