Find the size of photos and videos inside (Photo Library) iOS device using Photos Framework on iOS 8

Till iOS 7 we were using Assets Library Framework to get the total size of photos and videos in iOS device, look here : How to find total size for images and videos inside photo library in iOS

Can anyone please help me do the same using Photos Framework as Assets Library Framework is deprecated. Also how can I calculate the total size if there are large number of photos in a faster manner. You can see in this app : MobiShield for iPhone here if you go to More > Disk you can see that the developer is calculating the total photos size and videos size in 2 seconds. How can I achieve this ? In Objective - C.