CreateThumbnail in Background

[Kevin]

Good day,

how is it possible to create the pdf thumbnail images in background without blocking the whole application?

I am generating a list of thumbnails of the 1st page of many PDFS, which results in the application UI beeing blocked.

This is what I am doing:

public ImageSource ConvertPdfToImageThumbnail(string filePath)
        {
            var tf = new ThumbnailFactory();

            using Stream stream = File.Open(filePath, FileMode.Open);
            var formatProvider = new PdfFormatProvider();
            RadFixedDocument document = formatProvider.Import(stream);

            if (document.Pages.Count > 0)
            {
                ImageSource result = tf.CreateThumbnail(document.Pages[0], document.Pages[0].Size);
                return result;
            }

            return null;
        }

 

The function is not doing anything on the UI, just loading a PDF and creating the ImageSource.

When called from a different Thread,  the line

    "ImageSource result = tf.CreateThumbnail(document.Pages[0], document.Pages[0].Size);"

is throwing this error: "the calling thread must be STA ..."

How can this task be done without blocking the UI?

Thank you for your support!

[Dimitar]

Hi Kevin,

Yes, I was able to reproduce this. The ThumnailFactory uses UI elements to render the image and this requires using the main UI thread. What you can do is to create a new thread and perform the conversion there: 

Thread thread = new Thread(() =>
{
    var image = ConvertPdfToImageThumbnail(@"..\..\SampleDoc.pdf");
    SaveImage(image);
});

thread.SetApartmentState(ApartmentState.STA);
thread.Start();

If you have any other questions, please do not hesitate to contact us.

Regards,
Dimitar
Progress Telerik

[Kevin]

Hi Dimitar,

Thank you very much, this works great!

For anyone else stumbling accross this issue: One last thing that has to be done is calling img.Freeze(); before assigning it to the view property (before the image gets assigned between the 2 threads).

Regards,

Kevin

[Dimitar]

Hi Kevin,

I am glad that this works for your case. And thanks for sharing the solution.

If you have any questions, please do not hesitate to contact us.

Regards,
Dimitar
Progress Telerik