In some case (for example: a swipe back gesture...),
if the viewController call -viewWillDisappear before the progress created,
and then, the progressView was created, and loaded to less than 100%,
at last, the viewController was disappear by calling -viewDidDisappear, the progressView will be left in the navigationBar.
So, it's necessary to clear progressView in viewDidDisappear.
In some case (for example: a swipe back gesture...), if the
viewControllercall-viewWillDisappearbefore the progress created, and then, the progressView was created, and loaded to less than 100%, at last, theviewControllerwas disappear by calling-viewDidDisappear, theprogressViewwill be left in thenavigationBar. So, it's necessary to clearprogressViewinviewDidDisappear.