Swift: Load NSData from URL after webpage finished loading

I have an NSData instance from a URL. The point is I have a class that takes NSData as a parameter and displays the HTML code of the webpage.

When you view the webpage in the browser, it first loads. After that a progress bar appears within the webpage. After the progress bar completes, the webpage displays additional content.

What I want is for NSData to wait for the progress bar to finish in order to load the additional content. Sort of delaying the initialization of NSData.

Any idea how could I achieve that?