title: How to show the progress of copying a large file in iOS?
body: <p>I am writing an iOS app. In my app, I want to copy some files from one folder to another.
  But because some files is too large, it will take a long time to finish the copy. So I want to add a progress bar to show the percentage of the copy. But I find that the file manager has no callback method to get the percentage. Does anyone have the good solution to it?</p>