This read-only property will be the image page index for the current image, or -1 if the viewer does not have a currently-open image. The first page of a file has an image page index of zero.

Note that if you call the openPage function, this property will not change to match the new image page index until the image is fully opened, which will occur just before the viewer raises the pageOpened event. Prior to that time, this property will continue to hold the value for the previously-opened image or -1 if there is no previously-opened image. If you need the new value of this property before the viewer raises the pageOpened event, use the currentState property.

Reference

ThumbnailView class
ThumbnailView Members