确定何时加载画布内容

Is there a way to detect when canvas content is loaded? What I'm trying is following. I need to print a page where in a widget is rendered a pdf document using the pdfjs library. Only the first pdf page have to be printed alongside with the content of the web page where the widget is placed. My approach was to get the canvas of the first pdf page which the pdfjs library created and put its content as source of an img element.Is there a way to detect when canvas content is